Funny Front Ends
Discussion
I'm posting this in here so that it doesn't get bogged down in th FWD vs RWD bunfight in GG. SO to quote the conversation so far.
To set the scene
To set the scene
dilbert said:
I wouldn't want to be inflammatory, I don't know anything about bikes at all really. I assume that you're saying that there are no bikes with powered steered wheeels, and this is evidence to support your position. Wasn't the Bimota Tesi, two wheel drive?
I don't know if it was any good or not, but they do exist I think.
Edited to add;
Nah, hang that. Done a bit of research and it doesn't look like it is.
>> Edited by dern on Friday 2nd December 18:48
I've included a picture. I don't think getting power to it would be too much of a problem. I guess the front swinging arm was the reason I thought it might be two wheel drive. Judge for yourselves, but I don't think getting the power there would be too much of a problem. The question over the benefit is different. I don't know anything about bikes, so I'll avoid that one.
If you wanted a FWD bike you'd be best off looking here first!
Edited to add;
Now I've thought about it a bit more, what I've realised is that if you did make the front hub assembly powered, it would suffer torque steer in the same way that a FWD car does.
It must be one of those sacred geometry things!
Further edited to add;
I just got your post Dern, we're in agreement about the torque steer!
>> Edited by dilbert on Friday 2nd December 20:00
Pigeon said:
I've included a picture. I don't think getting power to it would be too much of a problem. I guess the front swinging arm was the reason I thought it might be two wheel drive. Judge for yourselves, but I don't think getting the power there would be too much of a problem.
Blimey, that's a pretty unconventional front end.
Unfortunately true. Telescopic forks are sort of like the FWD of motorcycles - cheap shit used because it's cheap, not because it's better; alternatives not explored because of the staggering conservatism of motorcycle buyers, which means the motorcycle of 2005 is even less advanced compared to that of 1905 than the equivalently-dated cars.
See www.tonyfoale.com/Articles/Steer/STEER.htm
I have a mate who used to have a Foale QL and can confirm that the guy is right...dern said:
I think you may well be right but I'm also pretty sure the torque reaction would be detrimental to handling.
Yep
See www.tonyfoale.com/Articles/2WD_2WS/2WD_2WS.htm
There does exist a Yamaha dirt bike with 2WD, using hydraulic drive to the front wheel - it's not mechanically difficult, but it's not useful on the road. The dirt bike situation is different because of the traction consideration. AIUI the Yamaha is set up so that the front wheel doesn't actually receive significant drive unless the rear is slipping.
xm5er said:
I've included a picture. I don't think getting power to it would be too much of a problem. I guess the front swinging arm was the reason I thought it might be two wheel drive. Judge for yourselves, but I don't think getting the power there would be too much of a problem.
Blimey, that's a pretty unconventional front end.
Unfortunately true. Telescopic forks are sort of like the FWD of motorcycles - cheap shit used because it's cheap, not because it's better; alternatives not explored because of the staggering conservatism of motorcycle buyers, which means the motorcycle of 2005 is even less advanced compared to that of 1905 than the equivalently-dated cars.
See www.tonyfoale.com/Articles/Steer/STEER.htm
I have a mate who used to have a Foale QL and can confirm that the guy is right...dern said:
I think you may well be right but I'm also pretty sure the torque reaction would be detrimental to handling.
Yep
See www.tonyfoale.com/Articles/2WD_2WS/2WD_2WS.htm
There does exist a Yamaha dirt bike with 2WD, using hydraulic drive to the front wheel - it's not mechanically difficult, but it's not useful on the road. The dirt bike situation is different because of the traction consideration. AIUI the Yamaha is set up so that the front wheel doesn't actually receive significant drive unless the rear is slipping.
The tesi didnt drive the front wheel.
Tony Foal has been banging on about his designs for as long as I've been alive, the reason they arent successful has nothing to do with conservative bikers (come on, how is a bike for example, that weighs 170kg and knocks out 180bhp in any way conservative). It has everything to do with an assumption that changing geometry under braking is detrimental to handling, it isnt its beneficial. All the other, so called, deficiencies of forks were ironed out by materials scientists in the 80s. Honda threw millions at the elf GP bikes and they had no success.
Oh and BTW Hasbeen, nicely put.
Pigeon said:
Tony Foal has been banging on about his designs for as long as I've been alive, the reason they arent successful has nothing to do with conservative bikers (come on, how is a bike for example, that weighs 170kg and knocks out 180bhp in any way conservative).
It is highly conservative, because it is basically the same design as a 1905 bike but with a bloody big engine in it. In the same time, aerospace technology has gone from a rickety crate to something capable of leaving the Solar System entirely. Motorcycle design is highly influenced by fashion - see the BMW "Telelever" front end which was deliberately engineered to look like a normal tele fork front end because the market wouldn't accept anything radically different for reasons having nothing to do with engineering.xm5er said:
It has everything to do with an assumption that changing geometry under braking is detrimental to handling, it isnt its beneficial.
Not at all, a suspension design which allows dive under braking means the suspension design is compromised by the need to allow for the dive. The ride height has to be higher to allow ground clearance to be maintained, and spring rates have to be set undesirably high to keep dive from getting out of hand - this last tends to pass unnoticed because of the misconception common to both car and bike worlds that hard springing is a requirement for good handling - it isn't, it's a fudge to keep suboptimal suspension geometries around the sweet spot.
If you disagree, there is no reason why a hub-centre front end could not be designed to allow the same dive as tele forks.
Furthermore, the Japanese seem to agree that dive is undesirable, but have found from experience that the only methods available to mitigate it with tele forks - such as linking damper stiffness to brake line pressure - are even more undesirable...
Furthermore the ability to incorporate anti-dive is only one of the advantages of a hub-centre design.xm5er said:
All the other, so called, deficiencies of forks were ironed out by materials scientists in the 80s.
Again, not so. What happened was that the tendency to lateral flex was reduced by such methods as ceasing to fit silly skinny little forks as on 70s bikes, moving to "upside down" forks and manufacturing them to tighter tolerances, in which materials played a part. At the same time, frame design was taken by the scruff of the neck and given a good shake, the increased stiffness of frames being a very significant factor in the improvement of bike handling.
If I had welding facilities I would definitely be making a hub-centre front end for my MZ. In this case, the forks are actually not bad, being considerably larger than is customary for a 250. But there would be several other advantages. By moving the attachment point for the front suspension from the steering head to somewhere down in front of the engine, it would be possible to replace pretty much the whole of the current spine frame structure with a small cradle frame around the engine. This would make a much stiffer connection between the two suspension mounting points than at present. It would also save a considerable amount of mass, and not only mass but high-up mass so the C of G would be significantly lower... the small cradle arrangement would have to maintain rigidity over a much smaller distance than the spine frame, and with a more favourable distribution of forces, so would weigh little; the suspension unit could be underslung, instead of in the forks, again giving reductions in total mass, unsprung mass, and steered mass; the headstock would be nothing but a pivot for the handlebars, no longer needing to take steering and suspension forces, nor brake forces magnified by the leverage of forks, so would need much less in the way of supporting structure. Furthermore, the loss of the frame spine would allow the fuel tank to be mounted under the seat in the area currently occupied by the vertical section of the spine, which would give a further reduction in C of G height. Not to mention that I would design it so as to maintain its attitude under braking
I hope this makes sense so far.
>> Edited by xm5er on Saturday 3rd December 22:56
Pigeon said:
Tony Foal has been banging on about his designs for as long as I've been alive, the reason they arent successful has nothing to do with conservative bikers (come on, how is a bike for example, that weighs 170kg and knocks out 180bhp in any way conservative).
It is highly conservative, because it is basically the same design as a 1905 bike but with a bloody big engine in it. In the same time, aerospace technology has gone from a rickety crate to something capable of leaving the Solar System entirely. Motorcycle design is highly influenced by fashion - see the BMW "Telelever" front end which was deliberately engineered to look like a normal tele fork front end because the market wouldn't accept anything radically different for reasons having nothing to do with engineering.
I need to correct you here, if you go back through the history of bikes, particularly pre war, you will see a huge variety of diferent suspension designs, the telecopic fork became dominant post war due to its success in racing and adoption by the British bike industry. The BMW telelever was designed to be used by the GS series of paris dakar replicas which means that a tesi style hub front end was not possible due to ground clearance and turning circle issues.
Interesting that you should bring up aerospace, go and take a look at what holds up the front end of most aircraft and has done since WW2 (incase you cant be arsed, its a telescopic tube)
Pigeon said:
It has everything to do with an assumption that changing geometry under braking is detrimental to handling, it isnt its beneficial.
Not at all, a suspension design which allows dive under braking means the suspension design is compromised by the need to allow for the dive. The ride height has to be higher to allow ground clearance to be maintained, and spring rates have to be set undesirably high to keep dive from getting out of hand - this last tends to pass unnoticed because of the misconception common to both car and bike worlds that hard springing is a requirement for good handling - it isn't, it's a fudge to keep suboptimal suspension geometries around the sweet spot.
If you disagree, there is no reason why a hub-centre front end could not be designed to allow the same dive as tele forks.
Now lets get to some fundamentals, there are two reason why you get dive when you brake, one is due to the horizontal displacement of an angled front fork (in other words it moves backwards as well as upwards), this is desirable in all suspension design as it gives compliancy when the wheel hits a bump, on a road bike design this is more than desirable its essential. Anti dive geometry works by removing this compliancy and in some cases reversing it (ie the wheel hub wants to move forward on relation to the contact patch, when you brake, this is how you can make it rise under braking). The second reason for dive is the couple caused by the bike and riders mass acting through the CoG. You can only eliminate this by introducing an element of antidive/prolift, which as discussed above, means zero compliancy.
Now coming back to geometry, with forks when you brake, you get dive which reduces the castor angle and the trail length thus reducing stability and making it easier to turn the bike fast. When hard on the power you get the reverse, everyones a winner.
Pigeon said:
Furthermore, the Japanese seem to agree that dive is undesirable, but have found from experience that the only methods available to mitigate it with tele forks - such as linking damper stiffness to brake line pressure - are even more undesirable...
A bad idea poorly executed (in the 80's, I know I had one), as I said earlier fork design came on in leaps and bounds in the 80s.
Pigeon said:
All the other, so called, deficiencies of forks were ironed out by materials scientists in the 80s.
Again, not so. What happened was that the tendency to lateral flex was reduced by such methods as ceasing to fit silly skinny little forks as on 70s bikes, moving to "upside down" forks and manufacturing them to tighter tolerances, in which materials played a part. At the same time, frame design was taken by the scruff of the neck and given a good shake, the increased stiffness of frames being a very significant factor in the improvement of bike handling.
I was really referring to use of teflons, light alloys etc, but you reinforce my point.
Pigeon said:
If I had welding facilities I would definitely be making a hub-centre front end for my MZ. In this case, the forks are actually not bad, being considerably larger than is customary for a 250. But there would be several other advantages. By moving the attachment point for the front suspension from the steering head to somewhere down in front of the engine, it would be possible to replace pretty much the whole of the current spine frame structure with a small cradle frame around the engine. This would make a much stiffer connection between the two suspension mounting points than at present. It would also save a considerable amount of mass, and not only mass but high-up mass so the C of G would be significantly lower... the small cradle arrangement would have to maintain rigidity over a much smaller distance than the spine frame, and with a more favourable distribution of forces, so would weigh little; the suspension unit could be underslung, instead of in the forks, again giving reductions in total mass, unsprung mass, and steered mass; the headstock would be nothing but a pivot for the handlebars, no longer needing to take steering and suspension forces, nor brake forces magnified by the leverage of forks, so would need much less in the way of supporting structure. Furthermore, the loss of the frame spine would allow the fuel tank to be mounted under the seat in the area currently occupied by the vertical section of the spine, which would give a further reduction in C of G height. Not to mention that I would design it so as to maintain its attitude under braking
I hope this makes sense so far.
[/quote]
An interesting project but I seriously suggest that you go back to your basic assumptions before wrecking a good bike.
Why do I know all this, well I had the pleasure of my lecturers ripping the arse out of one of my theses at University.
>> Edited by xm5er on Saturday 3rd December 23:45
>> Edited by xm5er on Saturday 3rd December 23:46
>> Edited by xm5er on Saturday 3rd December 23:47
Edited because for some bleedin reason Norton doesn't like the bloody preview pop up
>> Edited by xm5er on Saturday 3rd December 23:48
xm5er said:
I need to correct you here, if you go back through the history of bikes, particularly pre war, you will see a huge variety of diferent suspension designs, the telecopic fork became dominant post war due to its success in racing and adoption by the British bike industry.
AIUI it was not very possible to mass-produce forks of adequate quality until that point, so everyone had their own ideas - a lot of which were also compromised due to the materials available, though to a less crippling degree. Bit like the way FWD cars were few, low-powered and with poor steering lock until it became possible to mass-produce decent CV joints. Scooters, it is interesting to note, still show a variety of front ends (including hub-centre) in common use.
xm5er said:
The BMW telelever was designed to be used by the GS series of paris dakar replicas which means that a tesi style hub front end was not possible due to ground clearance and turning circle issues.
I didn't realise that had been a design goal - thought it was designed for the performance road bikes and used on the GS because it was there. I agree that for that application a hub-centre front end is not likely to be a good solution.
xm5er said:
Interesting that you should bring up aerospace, go and take a look at what holds up the front end of most aircraft and has done since WW2 (incase you cant be arsed, its a telescopic tube)
I have looked at them to see if the steering mechanism offered any useful ideas that could be adapted to motorcycle use
It doesn't suffer from the problem of the tube deflecting causing the contact patch to move in relation to the steering axis, because the tube defines the steering axis. No doubt it comes in useful when the pilot's getting his knee down on the taxi run 
xm5er said:
Now lets get to some fundamentals, there are two reason why you get dive when you brake, one is due to the horizontal displacement of an angled front fork (in other words it moves backwards as well as upwards), this is desirable in all suspension design as it gives compliancy when the wheel hits a bump, on a road bike design this is more than desirable its essential.
Anti dive geometry works by removing this compliancy and in some cases reversing it (ie the wheel hub wants to move forward on relation to the contact patch, when you brake, this is how you can make it rise under braking). The second reason for dive is the couple caused by the bike and riders mass acting through the CoG. You can only eliminate this by introducing an element of antidive/prolift, which as discussed above, means zero compliancy.
On a dirt bike I'd agree that it is useful to allow the wheel to move backward when hitting a bump. On a road bike I'm inclined not to. The bumps you hit on a road bike are small in relation to the height of the tyre and are hit at significant speed. The first effect is to compress the tyre, the compressed tyre then exerts a force vertically upwards on the wheel. There is not a large force applied in the horizontal direction.
A fairly extreme example is to compare wheeling a wheelbarrow with a big balloon tyre across a rough but hard surface such as a badly damaged concrete slab, with wheeling one with a solid tyre. While the balloon tyre is more lossy so is more effort to push on the smooth, on the rough it greatly reduces the horizontal loads applied to the wheelbarrow and you don't get the great jolts back up your arms that you do with a solid tyre.
Furthermore, if you have anti-dive geometry the suspension can be made softer as it no longer has to resist braking loads through the springing.
xm5er said:
Now coming back to geometry, with forks when you brake, you get dive which reduces the castor angle and the trail length thus reducing stability and making it easier to turn the bike fast. When hard on the power you get the reverse, everyones a winner.
Well, I guess riding style comes into it here. I prefer not to turn under braking, but to come off the brakes shortly before arriving at the corner, allow the suspension to return to neutral, then turn in under a touch of throttle, opening up as I exit.
This is where it is useful that anti-dive and wheel-not-moving-backwards-on-bump are not fundamental characteristics of a hub-centre design. You can design them in if you want them and design "pro-dive" like forks if you prefer that.
Indeed, if you were prepared to accept that it would be a bit of a heavy bastard you could build a bike with multiple suspension attachment points, to make it possible to experiment with different dive characteristics on the same bike.
xm5er said:
All the other, so called, deficiencies of forks were ironed out by materials scientists in the 80s.
Again, not so. What happened was that the tendency to lateral flex was reduced by such methods as ceasing to fit silly skinny little forks as on 70s bikes, moving to "upside down" forks and manufacturing them to tighter tolerances, in which materials played a part. At the same time, frame design was taken by the scruff of the neck and given a good shake, the increased stiffness of frames being a very significant factor in the improvement of bike handling.
I was really referring to use of teflons, light alloys etc, but you reinforce my point.
Er, no, that wasn't what I meant
I was saying that the forks themselves improved in quality, and that that and the improvement in frame quality were significant factors in the improvement in handling. But there was no change regarding the undesirable effects the use of a fork design has on the rest of the bike - notably the need to resist huge loads at a point which is high up and a long way from the rear swingarm pivot, resulting in the need for a heavy frame with a lot of high-up mass. xm5er said:
If I had welding facilities I would definitely be making a hub-centre front end for my MZ...
An interesting project but I seriously suggest that you go back to your basic assumptions before wrecking a good bike.
I can get a spare frame for the hassle of collecting it, so I'm not too worried there
I would like to do it purely for the comparison between the two different designs on the same bike, even if the comparison does turn out to be unfavourable (though I continue to be reasonably confident that it will be favourable; my mate who had the Foale QL was sufficiently impressed that he is now planning a similar conversion for a Guzzi). xm5er said:
Why do I know all this, well I had the pleasure of my lecturers ripping the arse out of one of my theses at University.
Sounds interesting... what was this about?
Forks work well enough for me.
Can't say I could pull a decent wheelie with FWD on a bike so it probably won't get my vote.
As for bikes with 2WD see the WR450F 2-TRAC on:
www.yamaha-motor.co.uk/products/information/extra_2trac.jsp
Can't say I could pull a decent wheelie with FWD on a bike so it probably won't get my vote.
As for bikes with 2WD see the WR450F 2-TRAC on:
www.yamaha-motor.co.uk/products/information/extra_2trac.jsp
Hmmm.....
I think that's the conclusion I came to on a different thread.
On that thread, I think I described that I didn't think that the engineering to achieve drive on the front wheel would be too difficult.
It seems that the motorcyclists among us don't like "car people" talkng about bikes on a thread about front and rear wheel drive.
I'll remember not to think about bikes in future!
People have accuse me of thread hijacking in the past, but I'm afraid that this must constitute total unmitigated thread rape!
I think that's the conclusion I came to on a different thread.
On that thread, I think I described that I didn't think that the engineering to achieve drive on the front wheel would be too difficult.
It seems that the motorcyclists among us don't like "car people" talkng about bikes on a thread about front and rear wheel drive.
I'll remember not to think about bikes in future!
People have accuse me of thread hijacking in the past, but I'm afraid that this must constitute total unmitigated thread rape!
dilbert said:
Hmmm
It seems that the motorcyclists among us don't like "car people" talkng about bikes on a thread about front and rear wheel drive.
I'll remember not to think about bikes in future!
People have accuse me of thread hijacking in the past, but I'm afraid that this must constitute total unmitigated thread rape!
Nah mate, I just moved so that we could have a propers debate without the FWD/RWD slanging match.
Pigeon, I'll reply tomorrow old chap.
>> Edited by xm5er on Monday 5th December 23:00
Pigeon said:
I have looked at them to see if the steering mechanism offered any useful ideas that could be adapted to motorcycle use
You could use a scissor mechanism as a fairly direct connection for steering; I have definitely seen this done on a bike
Pigeon said:
Now lets get to some fundamentals, there are two reason why you get dive when you brake, one is due to the horizontal displacement of an angled front fork (in other words it moves backwards as well as upwards), this is desirable in all suspension design as it gives compliancy when the wheel hits a bump, on a road bike design this is more than desirable its essential.
Anti dive geometry works by removing this compliancy and in some cases reversing it (ie the wheel hub wants to move forward on relation to the contact patch, when you brake, this is how you can make it rise under braking). The second reason for dive is the couple caused by the bike and riders mass acting through the CoG. You can only eliminate this by introducing an element of antidive/prolift, which as discussed above, means zero compliancy.
On a dirt bike I'd agree that it is useful to allow the wheel to move backward when hitting a bump. On a road bike I'm inclined not to. The bumps you hit on a road bike are small in relation to the height of the tyre and are hit at significant speed. The first effect is to compress the tyre, the compressed tyre then exerts a force vertically upwards on the wheel. There is not a large force applied in the horizontal direction.
The horizontal component of the impact force is large enough not to ignore. You wont find many car designs that have zero horizontal/longitudinal movement plus they have the added advantage of being able to use rubber bushes for additional compliancy.
Pigeon said:
A fairly extreme example is to compare wheeling a wheelbarrow with a big balloon tyre across a rough but hard surface such as a badly damaged concrete slab, with wheeling one with a solid tyre. While the balloon tyre is more lossy so is more effort to push on the smooth, on the rough it greatly reduces the horizontal loads applied to the wheelbarrow and you don't get the great jolts back up your arms that you do with a solid tyre.
I appreciate what you are saying, however a bike travelling at 30mph hitting a pothole with a low profile 17inch tyre is not a wheelbarrow. I seriously suggest that you build in some more compliancy that this into your design
Pigeon said:
Well, I guess riding style comes into it here. I prefer not to turn under braking, but to come off the brakes shortly before arriving at the corner, allow the suspension to return to neutral, then turn in under a touch of throttle, opening up as I exit.
Fair comment, its more important in the racing fraternity.
Pigeon said:
This is where it is useful that anti-dive and wheel-not-moving-backwards-on-bump are not fundamental characteristics of a hub-centre design. You can design them in if you want them and design "pro-dive" like forks if you prefer that.
Indeed, if you were prepared to accept that it would be a bit of a heavy bastard you could build a bike with multiple suspension attachment points, to make it possible to experiment with different dive characteristics on the same bike.
Adjustable rose joints and eccentric adjusters on the frame attachment points could give you some room for manoeuvre without making it too heavy.
Pigeon said:
Er, no, that wasn't what I meant
Indeed the introduction of big alloy frames tended to negate a lot of Tony Foals criticisms of conventional bikes. BTW that diagram of his with the bendy forks in a turn is just plain bollox, look at the back end of a sports bike and all the bracing required to stop the swing arm from bending. You will need to mirror that at the front to match the bending characteristics of the rear.
Pigeon said:[/quote]
If I had welding facilities I would definitely be making a hub-centre front end for my MZ...
An interesting project but I seriously suggest that you go back to your basic assumptions before wrecking a good bike.
I can get a spare frame for the hassle of collecting it, so I'm not too worried therexm5er said:
Why do I know all this, well I had the pleasure of my lecturers ripping the arse out of one of my theses at University.
Sounds interesting... what was this about?
I did a design exercise for the redesign of a motocrosser front end, unfortunately I pitched it out in recent house move but I remember enough of the brutal cross examination I received to always examine first assumptions carefully.
xm5er said:
I have looked at them to see if the steering mechanism offered any useful ideas that could be adapted to motorcycle use
You could use a scissor mechanism as a fairly direct connection for steering; I have definitely seen this done on a bike
I've half an idea that I have, too, only I'm buggered if I can remember where. I concluded it was not very useful for a swingarm-based front end, though; because the hub end moves in an arc, you can only get the axes of the two bits at either end of the scissor mechanism to line up with the suspension in one position. Load the bike up a bit, and the axes are no longer aligned, so trying to turn the steering results in trying to move the two ends of the scissor mechanism into different planes, which isn't on.
xm5er said:
The horizontal component of the impact force is large enough not to ignore. You wont find many car designs that have zero horizontal/longitudinal movement plus they have the added advantage of being able to use rubber bushes for additional compliancy.
Huh? All the car front ends I've ever seen don't allow any fore-and-aft movement apart from give in the bushes. You have a lower wishbone with two widely-separated mounting points on the body, or you have a tie-bar of some kind from the lower arm to a point forward on the body, or some other feature which is specifically designed to locate the wheel accurately in the fore-and-aft sense. If the tie bar is not connected to the lower arm because some dickwit garage monkey hasn't fitted the rubbers, things get extremely hairy
xm5er said:
I appreciate what you are saying, however a bike travelling at 30mph hitting a pothole with a low profile 17inch tyre is not a wheelbarrow. I seriously suggest that you build in some more compliancy that this into your design
Sure, the wheelbarrow example was a deliberately extreme example chosen because there is a very large effect which is directly noticeable to human senses. But I remain to be convinced that a bump which is much smaller than the tyre height - as the vast majority of them are - gives rise to problematical levels of horizontal force. Furthermore, a hub-centre front end has the swingarm more or less in a horizontal line with the axle, and it is much less of a problem to cope with such forces than it is when they are fed in at the headstock multiplied by the tremendous lever arm of a fork.
It has to be said that my mate's experience with his Foale QL was such that he feels no need to introduce rearwards wheel movement on bumps into his Guzzi front end design.
xm5er said:
Indeed the introduction of big alloy frames tended to negate a lot of Tony Foals criticisms of conventional bikes.
Yes, I guess I would be better off describing the majority of modern bike frames as "steam locomotive" rather than "bicycle"
xm5er said:
BTW that diagram of his with the bendy forks in a turn is just plain bollox, look at the back end of a sports bike and all the bracing required to stop the swing arm from bending.
Er, I don't quite see the connection here... how does the need to keep the back end stiff relate to a diagram of a front end which is not stiff?
xm5er said:
You will need to mirror that at the front to match the bending characteristics of the rear.
Er, no, I'll be trying to keep front end, back end and the bit connecting the two all as stiff as possible... designing a vehicle where the chassis members are deliberately allowed to bend is a
nightmare which I have no intention of getting into This will probably mean double-sided swingarms; the more favourable configuration means there's not a lot in it weight-wise and they're also a lot easier to fabricate on a DIY basis. xm5er said:
I did a design exercise for the redesign of a motocrosser front end, unfortunately I pitched it out in recent house move but I remember enough of the brutal cross examination I received to always examine first assumptions carefully.
Oh, right. What kind of configuration did you come up with? It has to be said that for a motocrosser I'd have a hard job arguing against tele forks myself - things like suspension travel, steering lock and clearance make anything else a bit awkward...
Pigeon said:
I've half an idea that I have, too, only I'm buggered if I can remember where. I concluded it was not very useful for a swingarm-based front end, though; because the hub end moves in an arc, you can only get the axes of the two bits at either end of the scissor mechanism to line up with the suspension in one position. Load the bike up a bit, and the axes are no longer aligned, so trying to turn the steering results in trying to move the two ends of the scissor mechanism into different planes, which isn't on.
The Britten racer from New Zealand used it, here’s an interesting article for you http://splashmedia.co.nz/users/britten/art3.html#bri covers a lot of different types of design. My design used a Britten style girder (similar to the Hossak in Foals article) with unequal length wishbones and a scissor link, I can’t for the life of me remember how I got round the problems you described but I’m sure it wasn’t that difficult. Personally, I think this style of design hold the most promise for road bikes as it gets around one of the inherent problems of swing arm style funnies, steering lock/ground clearance.
Pigeon said:
Huh? All the car front ends I've ever seen don't allow any fore-and-aft movement apart from give in the bushes. You have a lower wishbone with two widely-separated mounting points on the body, or you have a tie-bar of some kind from the lower arm to a point forward on the body, or some other feature which is specifically designed to locate the wheel accurately in the fore-and-aft sense. If the tie bar is not connected to the lower arm because some dickwit garage monkey hasn't fitted the rubbers, things get extremely hairy
All the car designs I’ve examined (admittedly not that many), show the front mounting point to be slightly higher than the rear, giving a slight backwards diagonal cant to the movement of the wheel. Hmmm, on second thoughts that could be because it’s been mostly Macpherson struts I’ve been looking at. My personal opinion is that a bit of compliancy wont go amiss.
Pigeon said:
Indeed the introduction of big alloy frames tended to negate a lot of Tony Foals criticisms of conventional bikes.
Yes, I guess I would be better off describing the majority of modern bike frames as "steam locomotive" rather than "bicycle"xm5er said:
BTW that diagram of his with the bendy forks in a turn is just plain bollox, look at the back end of a sports bike and all the bracing required to stop the swing arm from bending.
Er, I don't quite see the connection here... how does the need to keep the back end stiff relate to a diagram of a front end which is not stiff?xm5er said:
You will need to mirror that at the front to match the bending characteristics of the rear.
Er, no, I'll be trying to keep front end, back end and the bit connecting the two all as stiff as possible... designing a vehicle where the chassis members are deliberately allowed to bend is a
Bringing that lot together, the point I am making is Foals diagram is a silly attempt to make forks look far worse than they are (in fact he misses a major reason for twisting of the front forks which makes me question his engineering abilities in a big way), the point being that heavily braced rear swing arms show that it is also quite difficult to prevent wheel deflection due to twisting of the swing arm. However I am willing to concede that the double wishbone/long upright designs are better at dealing with this than the tesi/elf designs.
Pigeon said:
I did a design exercise for the redesign of a motocrosser front end, unfortunately I pitched it out in recent house move but I remember enough of the brutal cross examination I received to always examine first assumptions carefully.
Oh, right. What kind of configuration did you come up with? It has to be said that for a motocrosser I'd have a hard job arguing against tele forks myself - things like suspension travel, steering lock and clearance make anything else a bit awkward...
Tell me about it.
The area you have got the most work to do is in the geometry of the whole suspension mechanism; for example, if you introduce an antidive element into the wishbone geometry, when the suspension compresses under turning load, you will get forward movement of the front wheel coupled with a lengthening of trail and an increase in castor angle. This is going to have a very strange effect on handling. Get yourself some cardboard and some drawing pins and have a muck about to see what happens as the suspension compresses. Personally I would try to keep the top and bottom wishbones as parallel and equal lengthed as possible.
If you do go ahead with it, stay in touch I’d be interested to see your progress and help out if I can.
xm5er said:
The Britten racer from New Zealand used it, here’s an interesting article for you http://splashmedia.co.nz/users/britten/art3.html#bri covers a lot of different types of design.
Cheers, it is interesting... unfortunately it doesn't give any details on how the steering works, and nor do other articles I can find on Google. Most of them are more interested in the engine
xm5er said:
My design used a Britten style girder (similar to the Hossak in Foals article) with unequal length wishbones and a scissor link, I can’t for the life of me remember how I got round the problems you described but I’m sure it wasn’t that difficult. Personally, I think this style of design hold the most promise for road bikes as it gets around one of the inherent problems of swing arm style funnies, steering lock/ground clearance.
It also makes it a lot easier to sort out a steering linkage, due to the pivot being near the handlebars making the path for the linkage a lot shorter. Makes it much easier to keep it all rigid...
xm5er said:
All the car designs I’ve examined (admittedly not that many), show the front mounting point to be slightly higher than the rear, giving a slight backwards diagonal cant to the movement of the wheel. Hmmm, on second thoughts that could be because it’s been mostly Macpherson struts I’ve been looking at. My personal opinion is that a bit of compliancy wont go amiss.
A slight backwards cant is not uncommon, especially on McPhersons as you say, since the strut defines the steering axis... but it's only a couple of degrees, so it's far less significant than the angle of tele forks. Double-wishbone type designs are generally more or less flat, although there are some where the wishbone pivoting axes are not parallel, so the wheel moves in a fore-and-aft curve.
xm5er said:
Bringing that lot together, the point I am making is Foals diagram is a silly attempt to make forks look far worse than they are (in fact he misses a major reason for twisting of the front forks which makes me question his engineering abilities in a big way), the point being that heavily braced rear swing arms show that it is also quite difficult to prevent wheel deflection due to twisting of the swing arm. However I am willing to concede that the double wishbone/long upright designs are better at dealing with this than the tesi/elf designs.
Oh, I get you now. Yes, swingarms do bend... however since the steering axis moves with the wheel instead of remaining fixed to the bike, it has less effect than the bending of forks.
That article does very much skate over the subject of forks - to me it reads as if he can't really be arsed to talk about them much.
xm5er said:
The area you have got the most work to do is in the geometry of the whole suspension mechanism; for example, if you introduce an antidive element into the wishbone geometry, when the suspension compresses under turning load, you will get forward movement of the front wheel coupled with a lengthening of trail and an increase in castor angle. This is going to have a very strange effect on handling. Get yourself some cardboard and some drawing pins and have a muck about to see what happens as the suspension compresses. Personally I would try to keep the top and bottom wishbones as parallel and equal lengthed as possible.
No disagreement there. Plenty of variables to juggle with to arrive at the best compromise...
xm5er said:Cheers.
If you do go ahead with it, stay in touch I’d be interested to see your progress and help out if I can.
I've got other projects which are calling for prior attention, but it remains on the cards for as long as I have the MZ - which will be until it falls to bits Gassing Station | Biker Banter | Top of Page | What's New | My Stuff