Effect of Track Width on Suspension Geometry
Discussion
What effect would widening the track of a vehicle have on the effectiveness of the suspension? Let me clarify.
Say a suspension was set up(ie attachment points, A-Arm length, ect) specifically for a certain wheelbase and track width. What if you moved the entire suspension (attachment points and all) say 2 inches outward on each side, would that have a negative impact on handling?
No, this is not just putting spacers on a wheel and calling it a day, this is actually moving the entire assembly out a couple of inches (retaining the same attachment points, just moved out parallel to the ground from where they were). This would increase your track width by 4" overall. Would you have to start from scratch at that point, or would anything at all need to be changed?
Say a suspension was set up(ie attachment points, A-Arm length, ect) specifically for a certain wheelbase and track width. What if you moved the entire suspension (attachment points and all) say 2 inches outward on each side, would that have a negative impact on handling?
No, this is not just putting spacers on a wheel and calling it a day, this is actually moving the entire assembly out a couple of inches (retaining the same attachment points, just moved out parallel to the ground from where they were). This would increase your track width by 4" overall. Would you have to start from scratch at that point, or would anything at all need to be changed?
It think it would have a positive effect because the track is wider by a total of 4". it would be a similar effect to leaving it standard position but lowering the centre of gravity slightly.
it's the same basic principle of standing with your legs further apart, your more stable.
Of course I could be totally wrong and no doubt someone will come along with a big gun and put me out of my misery soon
it's the same basic principle of standing with your legs further apart, your more stable.
Of course I could be totally wrong and no doubt someone will come along with a big gun and put me out of my misery soon
How long is a piece of string? We would need a LOT more detail to completely analyse the potential effects.
As Jimbob said, it will definitely reduce lateral weight transfer, which is broadly speaking a Good Thing, but, there may be other side-effects to contend with:
As Jimbob said, it will definitely reduce lateral weight transfer, which is broadly speaking a Good Thing, but, there may be other side-effects to contend with:
- Depending on the suspension geometry, it could effect both static and dynamic location of the geometric roll centre. For an unequal length, non-parallel double wishbone arrangement with above-ground roll centre, as Nick said, it would tend to increase the height of the roll centre (which would reduce roll but increase jacking effect), but other geometries could differ... or (eg. some beam axle arrangements, or trailing arms, or parallel equal length wishbones) not be affected at all.
- With different front/rear geometries, you could get different effects on the static and dynamic roll centre location front and rear, therefore the roll axis inclination and movement could be affected... which in turn will alter the dynamic weight transfer characteristics (which are a key factor in understeer/oversteer balance).
- Whilst the lateral weight transfer (dictated by track) would reduce, the longitudinal weight transfer (dictated by wheelbase) would remain the same, therefore the ratio of longitudinal to lateral weight transfer would change, affecting transitional handling. This is why cars with a short wheelbase relative to their track tend to be less dynamically stable/more responsive than those with a long wheelbase relative to their track.
- Ackermann steering geometry would be affected. If the car has positive Ackermann, the amount would be reduced by increasing the track relative to the wheelbase (lines projected along the steering arms would intersect further to the rear).
- Note that you'd need to extend the steering rack length (not the trackrod lengths), otherwise you'd end up with some degree of bump steer being introduced.
- The purpose of a properly designed chassis is to 'join the dots' between the points where the suspension loads are fed into the sprung mass. If the chassis has been designed properly in the first place, therefore, moving the suspension pickups outboard may mean that you are cantilevering the loads out from the structure that had been carefully designed to accommodate them. You might have some structural issues to resolve...
LastPost, awesome response, that was pretty much what I was looking for. You're knowledge of the dynamics of suspension is impressive.
The aforementioned vehicle has unequal length A-Arms front and rear.
If the roll center was raised, would lowering the suspension slightly lower the roll center? I am not referring to the center of gravity. I know that as you lower a car, the roll center of the car is lowered to a greater extent than what the car was lowered.
The rear track would also be increased, however I was thinking the rear could be done with wider wheels with a little less offset. Though it would be almost as easy (but not quite) to move the rear out as well. To fine tune the oversteer/understeer tendencies could you not adjust it with a anti-sway bar and even shock/spring adjustment?
Could this be corrected by changing the lengths of the control arms, or would the attachment points need to be changed as well? I was looking at some suspension software that helps calculate correct lengths/angles ect of the suspension. This is for the most part hypothetical, but was really looking at widening my SLC.
I still have yet to fully understand Ackerman steering geometry (will have to do more reading on that one).
The aforementioned vehicle has unequal length A-Arms front and rear.
If the roll center was raised, would lowering the suspension slightly lower the roll center? I am not referring to the center of gravity. I know that as you lower a car, the roll center of the car is lowered to a greater extent than what the car was lowered.
The rear track would also be increased, however I was thinking the rear could be done with wider wheels with a little less offset. Though it would be almost as easy (but not quite) to move the rear out as well. To fine tune the oversteer/understeer tendencies could you not adjust it with a anti-sway bar and even shock/spring adjustment?
Could this be corrected by changing the lengths of the control arms, or would the attachment points need to be changed as well? I was looking at some suspension software that helps calculate correct lengths/angles ect of the suspension. This is for the most part hypothetical, but was really looking at widening my SLC.
I still have yet to fully understand Ackerman steering geometry (will have to do more reading on that one).
Edited by guitaraholic on Sunday 7th April 06:02
guitaraholic said:
If the roll center was raised, would lowering the suspension slightly lower the roll center?
Not in a straightforward manner, no. You need to appreciate that even if you lower both inboard and outboard pickups (and the latter would require you to create a new upright, with the stub axle position raised relative to the top and bottom ball joints, which in turn would f
k up aspects of the steering geometry like steering axis offset and caster trail), you wouldn't be lowering the tyre contact patch, which stays stubbornly exactly where it was, on the ground plane. And the geometric roll centre is established by projecting lines along your wishbones then back to the tyre contact patch - static location of the RC is where this line croses the centreline of the car.Lowering the static location of the geometric roll centre therefore isn't simply a matter of lowering the suspension pickups by an inch will result in the roll centre being an inch lower... and dynamic location of the roll centre (fixed by the intersection of those lines projected to the tyre contact patches) is even less predictable.
guitaraholic said:
To fine tune the oversteer/understeer tendencies could you not adjust it with a anti-sway bar and even shock/spring adjustment?
Yes, everything can be tuned out to some extent, if you know what you're doing.The big problems would come if your changes had affected dynamic location of the geometric roll centre. Most race car designers take great pains to ensure that the geometric roll centre is very accurately located (within a few millimetres) as the suspension moves, because if the roll centre leaps about all over the place as the suspension moves when you're negotiating a bend, then the diagonal weight transfer (and hence the load at each tyre contact patch) leaps about with it... which is a Very Bad Thing.
guitaraholic said:
Could this be corrected by changing the lengths of the control arms, or would the attachment points need to be changed as well?
As you're perhaps beginning to appreciate by now, everything is inter-related to everything else, with suspension design. So no, you couldn't expect to just change the length of the control arms and not the attachment points. The virtual swing axle length (which influences both camber change and roll centre location) would increase if you simply lengthened the A-arms.For what it's worth, my personal opinion (though others might disagree) is that Ackermann is the least of your worries - it's not that important, in the overall scheme of things.
guitaraholic said:
This is for the most part hypothetical, but was really looking at widening my SLC.
As in Mercedes Benz SLC? Then I assume your prime motivation is aesthetics rather than dynamics, in which case none of the above really matters. If you're just interested in something that looks good cruising the boulevards of Florida, the fact that you've screwed up the handling to some extent probably doesn't make that much difference. And it's not like the SLC is the sharpest tool in the shed to start off with...
Good points.
LOL,, sorry I didn't specify. SLC as in Superlite Coupe. With full access to the monocoque, it's not to hard to add new extending brackets and attachment points to the chassis. If fact, I was looking at the design, and it could be made reversible (for the front at least).
this one is mine, it's sitting in storage till I get back from overseas
here is a finished example.
Here is a shot of the front suspension (again, this one is not mine)
LOL,, sorry I didn't specify. SLC as in Superlite Coupe. With full access to the monocoque, it's not to hard to add new extending brackets and attachment points to the chassis. If fact, I was looking at the design, and it could be made reversible (for the front at least).
this one is mine, it's sitting in storage till I get back from overseas
here is a finished example.
Here is a shot of the front suspension (again, this one is not mine)
guitaraholic said:
With full access to the monocoque, it's not to hard to add new extending brackets and attachment points to the chassis.
Careful!I'm not familiar with the RCR monocoque, but as a general rule the loads from suspension pickups need to be taken out into the skins of monocoques in pure shear (or something very close to it). Cantilevering the pickup points a long way out on big brackets could be a really bad idea.
How are the existing brackets mounted to the monocoque?
It looks as though they pass through the outer skin - are there CNC machined subframes/bulkheads inside the tub, or hardpoints within a sandwich skin construction, or just brackets bolted directly to the skin?
What is your motivation for widening the car anyway - it seems like an awful lot of effort for questionable gain?
It looks as though they pass through the outer skin - are there CNC machined subframes/bulkheads inside the tub, or hardpoints within a sandwich skin construction, or just brackets bolted directly to the skin?
What is your motivation for widening the car anyway - it seems like an awful lot of effort for questionable gain?
when I get a chance in a few months I'll look at how mine are fastened. It looks like at least the attachment points for the UCA are just bolted on. I'll have to look for how the LCA are. That's the bad thing about being overseas, I can only let my mind wander, and do nothing about it for the moment.
http://staceydavid.com/sites/default/files/imageca...
http://staceydavid.com/sites/default/files/imageca...
@TheLastPost
Wouldn't the OP be better off fitting longer UCA and LCA? that would widen the track without moving the mount points/having to worry about them. In turn if the suspension was stiffened slightly aswell wouldn't it make for a more stable setup?
would jacking be reduced this way?
Wouldn't the OP be better off fitting longer UCA and LCA? that would widen the track without moving the mount points/having to worry about them. In turn if the suspension was stiffened slightly aswell wouldn't it make for a more stable setup?
would jacking be reduced this way?
The OP would (IMO) be better off:
a) Doing the job properly and re-designing the geometry from scratch (with whatever pick-up locations and control arm lengths are necessary to give suitable geometry), rather than trying to make bodged adaptations that retain either the existing arms, or the existing pick-up points, or better still;
b) Forgetting the whole idea unless he has a clear idea of why he thinks a wider track is required.
Clearly, the car's designer will have made an informed decision in selecting the wheelbase/track ratio (and other aspects of the geometry), so it would be as well to attempt to fully understand that decision (discuss with the manufacturer/designer?) before launching into major alterations of the design.
a) Doing the job properly and re-designing the geometry from scratch (with whatever pick-up locations and control arm lengths are necessary to give suitable geometry), rather than trying to make bodged adaptations that retain either the existing arms, or the existing pick-up points, or better still;
b) Forgetting the whole idea unless he has a clear idea of why he thinks a wider track is required.
Clearly, the car's designer will have made an informed decision in selecting the wheelbase/track ratio (and other aspects of the geometry), so it would be as well to attempt to fully understand that decision (discuss with the manufacturer/designer?) before launching into major alterations of the design.
Widening the track will be a two fold benefit.
1. Wider track will (if geometry is correct) should give better handling. Though from what I've been told it already out corners almost everything on the street.
2. Yes aesthetics are part of it. The car is too flat on the sides, widening the front and rear will give a nice contour and more depth. There are some other changes I am looking at as well.
I have been looking up a few suspension design programs (eg SusProg3D). I will more than likely be fabricating new control arms. Actually, going with chomoly should lighten it up a little bit as well vs the aluminum ones supplied.
From what I have been told the current SLC's suspension is within 3% of that of the Ford GT's. I was thinking if I were to widen the track 4-5", then maybe copying the setup of the Enzo would work. Though getting the measurements might be a bit of a trick. Enzo has (wheelbase 104"), (width 80").
1. Wider track will (if geometry is correct) should give better handling. Though from what I've been told it already out corners almost everything on the street.
2. Yes aesthetics are part of it. The car is too flat on the sides, widening the front and rear will give a nice contour and more depth. There are some other changes I am looking at as well.
I have been looking up a few suspension design programs (eg SusProg3D). I will more than likely be fabricating new control arms. Actually, going with chomoly should lighten it up a little bit as well vs the aluminum ones supplied.
From what I have been told the current SLC's suspension is within 3% of that of the Ford GT's. I was thinking if I were to widen the track 4-5", then maybe copying the setup of the Enzo would work. Though getting the measurements might be a bit of a trick. Enzo has (wheelbase 104"), (width 80").
You are correct, though was implying it to better corner grip. Yes, handling has to do more than with overall lateral grip. Handling also involves turn in, oversteer, understeer, responsiveness, ect ect. What would you recommend as a good suspension design software? I know there are guys that can hand jam it, but that's a special mind. I would much rather the software help design the optimal geometry.
guitaraholic said:
You are correct, though was implying it to better corner grip. Yes, handling has to do more than with overall lateral grip. Handling also involves turn in, oversteer, understeer, responsiveness, etc. ect.
And if we get down to basics, part of the fundamentals of that has to do with the ratio of track width to wheelbase.Yes, increasing track width reduces lateral weight transfer. But then increasing wheelbase reduces longitudinal weight transfer, which can be seen as an equally good thing.
So why do wheelbase:track ratios fall within such a narrow band in practice?
Although I don't believe in a specific 'magic number' related to the 'Golden Section', as some people have suggested, I think that experience has taught us that the optimum compromise between grip and the ability for human senses and responses to balance the transitional weight transfer of a car at the limit of its capabilities lies within this fairly narrow bandwidth. By widening the track independently of the wheelbase, I think you risk straying toward the Lancia Stratos end of the spectrum and creating what we in the trade refer to as 'a right twitchy b
d'. 
In other words, you may well be increasing the potential peak grip, but reducing the driver's capability to exploit it.
guitaraholic said:
What would you recommend as a good suspension design software?
Personally I use Susprog3D, but only as a basic tool, only in conjunction with my own spreadsheets to calculate weight transfer, and only because I can't justify the cost of anything more sophisticated. At the lower end of the price range, I prefer it to the the Mitchell software (WinGeo3) though, which I used to use in the past. If money is less of an object, you might want to consider the Lotus software or OptimumG packages.
...but the cheaper software is only really any good for basic kinematics, whereas I'm of the personal opinion that weight transfer is far more important than wheel geometry.
I did look at Susprog3D. I guess the only way to do it is dive in and play with the software a bit.
The SLC has a pretty decent wheelbase of 105", and a track I am guessing around 61-62" for the front, with the rear being a little less due to the wider tires (I'll have to measure it when I get back)
As a comparison the Enzo has a 104" wheelbase and a F65"/R65" track.
Figure I would be adding about 3-4" of track with the increased width, it would be close to the same ratio of that of the Enzo, with almost the exact same weight distribution and height center of gravity.
This is just a question, but could you take out a little bit of the twitchiness by running a little bit of toe in on the rear wheels?
I did a little bit of a look up on the Stratos. Holy Chit Batman, the thing had a 86" wheelbase and was 69" wide. Yeah, that's go kart territory. Did some calculations after finding the track of the Lancia. Ratio is around 1.45, yeah, that's pretty nutts!!!!
The SLC has a pretty decent wheelbase of 105", and a track I am guessing around 61-62" for the front, with the rear being a little less due to the wider tires (I'll have to measure it when I get back)
As a comparison the Enzo has a 104" wheelbase and a F65"/R65" track.
Figure I would be adding about 3-4" of track with the increased width, it would be close to the same ratio of that of the Enzo, with almost the exact same weight distribution and height center of gravity.
This is just a question, but could you take out a little bit of the twitchiness by running a little bit of toe in on the rear wheels?
I did a little bit of a look up on the Stratos. Holy Chit Batman, the thing had a 86" wheelbase and was 69" wide. Yeah, that's go kart territory. Did some calculations after finding the track of the Lancia. Ratio is around 1.45, yeah, that's pretty nutts!!!!
Edited by guitaraholic on Sunday 7th April 16:53
Agreed, your wheelbase:track ratio wouldn't be too extreme even with a bit of additional track: that 'Golden Section' magic number that I said I didn't believe in, but which happens to be close to what experience has proved to be ballpark optimum is 1.618... you're at 1.69 at the moment, would be 1.59 with an extra 4" of track, and the Enzo is 1.6.
But still - and I appreciate that I'm not going to dissuade you whatever I say - I'm not hearing what I'd consider to be a sound reason reason for completely re-engineering an otherwise satisfactory suspension system.
If it ain't broke, don't fix it - particularly if you run the risk of breaking lots of other things in the process!
You should be running toe-in on the rear wheels anyway - most IRS's will feel diabolical without it - but toe is simply there to take the slack out of the system and only really affects straight line stability and initial turn-in. It doesn't have a big influence on either stability or balance once the chassis has adopted a 'set' in a corner.
Eta: the Stratos isn't quite as extreme in its wheelbase:track ratio as you suggest, by the way - I think you may be confusing track and overall width. The Stratos' mean track is 56.9", which with its wheelbase of 85.8 gives it a ratio of 1.51. It also suffered from quite a high, rearward-biased CoG but, even so, that illustrates what a narrow bandwidth there is for 'typical' wheelbase:track ratios?
But still - and I appreciate that I'm not going to dissuade you whatever I say - I'm not hearing what I'd consider to be a sound reason reason for completely re-engineering an otherwise satisfactory suspension system.
If it ain't broke, don't fix it - particularly if you run the risk of breaking lots of other things in the process!
You should be running toe-in on the rear wheels anyway - most IRS's will feel diabolical without it - but toe is simply there to take the slack out of the system and only really affects straight line stability and initial turn-in. It doesn't have a big influence on either stability or balance once the chassis has adopted a 'set' in a corner.
Eta: the Stratos isn't quite as extreme in its wheelbase:track ratio as you suggest, by the way - I think you may be confusing track and overall width. The Stratos' mean track is 56.9", which with its wheelbase of 85.8 gives it a ratio of 1.51. It also suffered from quite a high, rearward-biased CoG but, even so, that illustrates what a narrow bandwidth there is for 'typical' wheelbase:track ratios?
I got the info from this site: Though after reading, the left column was a customized Stratos, the measurements below (a little narrower, are the real Stratos, which is 1.49, but still pretty short.
http://www.rallycars.com/Cars/Lancia/Stratos1.html
Length: 3710mm
Width: 1750mm
Height: 1114mm
Wheelbase: 2180mm
Front track: 1433mm
Rear track: 1457mm
Laden weight: 980Kg
Fuel capacity: 2 tanks totaling 80lt
And yes, I have definitely taken your words to heart. I may or may not go through with this, just looking at, if I do, to do it correctly. I'm still overseas for another 6-8 months, so it's just dreaming for now.
http://www.rallycars.com/Cars/Lancia/Stratos1.html
Length: 3710mm
Width: 1750mm
Height: 1114mm
Wheelbase: 2180mm
Front track: 1433mm
Rear track: 1457mm
Laden weight: 980Kg
Fuel capacity: 2 tanks totaling 80lt
And yes, I have definitely taken your words to heart. I may or may not go through with this, just looking at, if I do, to do it correctly. I'm still overseas for another 6-8 months, so it's just dreaming for now.
Edited by guitaraholic on Sunday 7th April 18:11
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