Brake Balancing
Discussion
GarryM said:
I can't quite see how chassis flex would cause the tank slapper you experienced. It sounds more like weight transfer as mentioned earlier in this thread:
RobM77 said:
If you decelerate in a bend, the front will get more grip than the rear and you will start to oversteer; and if you accelerate the opposite will happen...
When you say "the front will get more grip" surely if its gripping then more grip is not an issue.
I am certainly no expert when it comes to bikes although i passed my test at 19 and have quite a few bikes but not in the last 10 years.
When i had my little expearience i had a mate behind on his bike and said he thought i was going to get spat off the bike.
All i did was ease off the gas, i was no were near the limit of grip so i think the rear wheel moved forward a fraction causeing the whole bike to become less rigid and so it flexed.
The tank slapper started and got worse but as i was trying to slow down without braking and the curve became less the slapping motion eased and i got back on the gas and chased after my other mate who had pulled away by about 400 yards.
Oh how we laughed afterwards but i was shitting myself at the time and looking for the softest place to land, the lad behind said he really did think i was going to get thrown off.
A more rigid frame and rear swinging arm lessen the chances of a tank slapper with a bike but i don`t think oversteer on a car can be cured by taking this route.
If the front of the vehicle is slowing down better than the rear of the vehicle then the rear will try to come round on you, somehow.
Now i`m stating the bleeding obvious, sorry i`l get my coat.
leosayer said:
ABS would still always beat the driver in the wet though, even back when it was a new technology.
Not true, on all the ABS cars I've owned, I can always stop a good deal shorter if I avoid activating the ABS in the wet or dry.
Try it yourself,
Am I completely wrong in thinking that the only benefit of ABS is the ability to STEER while braking?
My experience, wet or dry roads, very limited track work ( couple of hours at Millbrook) showed NO reduction of braking distance in my X Type...Green Stuff pads fitted.
BOF
RobM77 said:
You asked about disc brakes on cars. A lot of cars don't really need disc brakes at the rear, you are quite right. The Toyota Avensis was fitted with drums at the back right up to the recent redesign a couple of years ago. Most cars in the 1980s operated discs at the front, and drums at the back. Nowadays, with increased grip available from tyres and various other reasons, braking has improved to the point where a smaller disc on the rear can be used to great effect. (there is also some marketing involved here!)
I thought rear discs was more to do with making 4 channel abs easier to implement. You need 4 channel to get electronic brake distribution and all the stability control stuff.
iirc cars with front discs rear drums had a sort of two channel abs where you had one front and the opposite rear linked in one circuit. Willing to be shot down in flames.
On the question of front rear balance its very important in off road motor sport especially where mixed surfaces are involved.
On tarmac the bias needs to be at the front for reasons explained by others very clearly.
On gravel the bias needs to be more towards the rear which makes the driver able to unsettle the car to allow turn in before you reach the bend and thus arrive at the bend pointing in the right direction. See profile pic.
If the corner tightens it is easy to let the tail slide a bit more, if it opens out then straighten up. I'm taking about driving on stages blind without practice and accurate notes, often notes were banned. In those circumstances sideways = safe. No doubt brucie had a good demo from Pentii.
Also on a gravel or packed/loose snow surface the quickest way to slow down is get the car slightly sideways so each wheel ploughs its own furrow, building up a wedge infron of the tyre and thus increase resistance.
In the case of the RS2000 in profile pic this had a bias pedal box, one master cylinder for front circuit, one for back circuit. The pedal pushed at a point along a threaded bar which joins the two master cylinders. The end of the threaded bar had a bowden cable attached connected to a knob on the dash. Thus it was possible to adjust bias during the stage according to surface change. I presume similar set ups on race cars? Maybe its all electronic now??
In fact because that car was Gp1, it had drum rears as discs were not homologated, so you had to be careful not to wind too much rear bias to avoid brake fade and getting into a downward spiral. In fact used to wind the bias right off the rear n road sections to give the rear brakes as little work to do as possible, except on the gravel stages.
Sorry I'll hang my anorak up now.
FiF
>> Edited by Flat in Fifth on Thursday 13th October 18:36
I can think of two other benefits:
1) For the everyday person in an emergency when they might panic, ABS allows them to just stamp on the brake pedal and wait. Foolproof! Providing you use enough brake pressure, because research shows that few people do...
2) As far as I know, ABS allows brakes on each of the four wheels to be modulated independantly. For instance, you might have wet mud under the left hand wheels, but dry tarmac on the right.
1) For the everyday person in an emergency when they might panic, ABS allows them to just stamp on the brake pedal and wait. Foolproof! Providing you use enough brake pressure, because research shows that few people do...
2) As far as I know, ABS allows brakes on each of the four wheels to be modulated independantly. For instance, you might have wet mud under the left hand wheels, but dry tarmac on the right.
bmw114 said:
... his adventure ...
It may feel as if the frame is flexing (and no doubt frame flex does have some effect on bike handling) but I believe the effects you're describing are caused by changes in the lateral stiffness at the front and rear of the car as the weight and acceleration varies.
The underlying factors are relatively simple (primarily, the total available grip goes up as the weight on a wheel is increased, and the amount of grip available to push the car sideways is reduced if you are using some of the grip to accelerate/brake the car). However, the resulting behaviour is surprisingly complex.
Great pic, FiF! And yes, Pentti gave us the full demo of the Scandinavian flick, setting the car up the "wrong" way when approaching the bend, using rear biased brakes to initiate the slide, then flicking it back to be fully sideways pointing into the bend while still 20 yds short of the conventional turn in point. He used a 4wd Cosworth Sierra Saphire. Of course, he made it look completely natural but it was the accuracy that impressed. When we tried it, it was all wheel twirling, lurid slides and missed apexes. When he did it it was just so unhurried, controlled and inch perfect. Ah well...! 
At the risk of extending this thread, has anyone experienced brake assist? As I understand it, this is meant to compensate for the point Rob makes that research shows most people don't stand on the pedal hard enough in an emergency stop. I can see the logic but I can't say I'm keen on the idea of yet another system taking over control of the brakes. What happens to the driver who thinks he's missed his turning and brakes hard? Does brake asist take over and what then happens to following traffic? Anyone any experience?
At the risk of extending this thread, has anyone experienced brake assist? As I understand it, this is meant to compensate for the point Rob makes that research shows most people don't stand on the pedal hard enough in an emergency stop. I can see the logic but I can't say I'm keen on the idea of yet another system taking over control of the brakes. What happens to the driver who thinks he's missed his turning and brakes hard? Does brake asist take over and what then happens to following traffic? Anyone any experience?
Rob,
""1) For the everyday person in an emergency when they might panic, ABS allows them to just stamp on the brake pedal and wait. Foolproof! Providing you use enough brake pressure, because research shows that few people do...""
Where is the advantage, if the stopping distance is not shorter?
BOF.
""1) For the everyday person in an emergency when they might panic, ABS allows them to just stamp on the brake pedal and wait. Foolproof! Providing you use enough brake pressure, because research shows that few people do...""
Where is the advantage, if the stopping distance is not shorter?
BOF.
bmw114 said:
All i did was ease off the gas, i was no were near the limit of grip so i think the rear wheel moved forward a fraction causeing the whole bike to become less rigid and so it flexed.
The tank slapper started and got worse but as i was trying to slow down without braking and the curve became less the slapping motion eased and i got back on the gas and chased after my other mate who had pulled away by about 400 yards.
A more rigid frame and rear swinging arm lessen the chances of a tank slapper with a bike but i don`t think oversteer on a car can be cured by taking this route.
My brother rides an R1, and he says that the fork geometry plays a big part in how likely you are to get a tank slapper. Hitting a bump at the wrong time can cause one apparantly, especially if the front is loaded under deceleration, or when the front wheel touches down too hard after a wheelie.
He says the way to stop it is to wind the power on and shift weight back to unload the front suspension.
BOF said:
Rob,
""1) For the everyday person in an emergency when they might panic, ABS allows them to just stamp on the brake pedal and wait. Foolproof! Providing you use enough brake pressure, because research shows that few people do...""
Where is the advantage, if the stopping distance is not shorter?
BOF.
It allows you to retain steering while under braking, as the wheels do not lock. Without ABS, you would need to come off the brakes to turn.
BOF said:
r988 said:
ABS would still always beat the driver in the wet though, even back when it was a new technology.
Not true, on all the ABS cars I've owned, I can always stop a good deal shorter if I avoid activating the ABS in the wet or dry.
Try it yourself,
Am I completely wrong in thinking that the only benefit of ABS is the ability to STEER while braking?
My experience, wet or dry roads, very limited track work ( couple of hours at Millbrook) showed NO reduction of braking distance in my X Type...Green Stuff pads fitted.
BOF
You can steer and also you don't need to be skilled in modulating brake pressure to prevent lock up (optimum braking is just before lockup). You can steer with you doing the braking too, but it takes skill and you can't vary the pressure to indiviudal wheels.
ABS does negatively impact optimal braking slightly. The new Porsche cup cars have found extra lap time by removing the ABS allowing very precise threshold braking when in the hands of an expert on a consistent grip surface. Frankly the ABS system they removed is the best I've ever experienced and for road use infinitely better than the alternative. Even on track its likely better for most people. I flat spotted a lot of tyres in my Elise and Noble
bmw114 said:
I can't quite see how chassis flex would cause the tank slapper you experienced. It sounds more like weight transfer as mentioned earlier in this thread:
RobM77 said:
If you decelerate in a bend, the front will get more grip than the rear and you will start to oversteer; and if you accelerate the opposite will happen...
When you say "the front will get more grip" surely if its gripping then more grip is not an issue...
Interesting info’ from Tank Slapper about the front fork geo’ causing problems.
bmw114, I’m only considering general principles as they would affect a car and applying that to your situation on the bike. Although the physics will be the same, the specific reactions of a bike compared to a car may be quite different for all I know. Certainly, chassis flex at such critical times would be “a very bad thing” so may be that tips the balance between keeping everything in shape and losing it. Forgive me if my explanation below is just a lesson in the bleeding obvious!
By lifting off (in the bend in your case), the engine slows the bike acting as a brake via the back wheel. Weight is then transferred to the front. As you say, you were not on the limit of grip (although doing 90mph in a bend has to be putting the tyres under a little bit of stress
) however prior to lifting, front and rear wheels were in balance. After lifting, the front now has more weight/grip and the rear has less weight/grip added to which the rear wheel is trying to brake. The rear wheel now has far less grip available to resist the cornering force. The result is that it starts to slip wide (not necessarily in a violent slide initially) and if this is not anticipated and corrected with the steering, you get into the situation that you experienced. GarryM said:
Interesting info’ from Tank Slapper about the front fork geo’ causing problems.
More interesting info from Tony Foale
BOF said:
Rob,
""1) For the everyday person in an emergency when they might panic, ABS allows them to just stamp on the brake pedal and wait. Foolproof! Providing you use enough brake pressure, because research shows that few people do...""
Where is the advantage, if the stopping distance is not shorter?
BOF.
It is a shorter distance than locking all the brakes up though, surely? Correct me if I am wrong - I don't actually know the figures.
But of course, it does allow even an idiot to still steer around an obstacle.
Interesting thread ...
"And finally if 80% braking on the front and 20% braking on the back is the best way of braking surely dics brakes on the rear of a average car are unnessesary, unless its for cost or weight reasons. "
.. for performance cars brake cooling plays a major part and drum brakes overheat very quickly compared to vented or even unvented discs.
"And finally if 80% braking on the front and 20% braking on the back is the best way of braking surely dics brakes on the rear of a average car are unnessesary, unless its for cost or weight reasons. "
.. for performance cars brake cooling plays a major part and drum brakes overheat very quickly compared to vented or even unvented discs.
Well, I finally have that hire car I've been waiting for so I did a quick experiment at lunchtime on an straight empty road in the middle of nowhere. 
We tried a stop from 50mph on a dry road with and without ABS and they were identical. I was quite pleased at my threshold braking, and I don't think I could have stopped the car much quicker really.
I wasn't so sure that they were identical, as we used a broken branch as a landmark for the first stop and then it turned out there were two broken branches! My friend was convinced that he could identify the branch we saw the second time as the same one we saw the first time. Hardly scientific I'm afraid.
I will try a repeat test this evening and report back.
I should add that the car is very new, and the ABS was very impressive, giving chirrups from the tyres all the way down to a standstill. Early ABS systems that I have sampled have been much less effective, getting nowhere near the limit of grip.
We tried a stop from 50mph on a dry road with and without ABS and they were identical. I was quite pleased at my threshold braking, and I don't think I could have stopped the car much quicker really.
I wasn't so sure that they were identical, as we used a broken branch as a landmark for the first stop and then it turned out there were two broken branches! My friend was convinced that he could identify the branch we saw the second time as the same one we saw the first time. Hardly scientific I'm afraid.
I will try a repeat test this evening and report back.
I should add that the car is very new, and the ABS was very impressive, giving chirrups from the tyres all the way down to a standstill. Early ABS systems that I have sampled have been much less effective, getting nowhere near the limit of grip.
Drum breaks have just as much power as disc breaks, but may not have it for as many applications. As with all breaks, it depends on the cooling. My first open wheeler was a Lotus 20 formula junior. Weight 925 LBS, about 100 BHP, & "Allfin" drum breakes all round. These were normal drums with huge alloy fins sweated on to them. It was about a 1960 car & I was racing it in 1965, against later cars, many of which had discs all round. I could out break any of the cars I raced against, up to & including F1 cars. It took a lot of setting up to get them right, but once done, they would handle 1000 racing miles with out attention, & I think they had a little less drag than discs. After a year I replaced the Lotus with a late model Brabham, with discs all round, & although it had a higher cornering speed than the Lotus, I could never break quite as late with it as I had with the Lotus. Motor racing was a fairly "Hairy" activity in Australia in the 60s. We FJ boys often found ourselves making up the grid, with F1s & F2s, some of which might be 10 years old, & not much faster than us in lap times. Because of this we found most of the carnage was occuring in front of us, & often, just in front of us. Now I know the that the physics do not agree with me, but I found, after a few emergency stops, that the thing slowed down much quicker when I got my self sideways. Well, not quite side ways, but about 50%. At that angle, with the clutch in, the breaks locked & steering into the slide, you do have a lot of directional control. I can't explain how to do it, as I have always found that its all over, & my hands & feet have done it before the brain [what brain] can catch up. You must be able to lock the breaks, particularly the rear breaks, & I'm bloody sure that its better if the car is not trying to do some thing else at the same time. And thats why, as a reactionary old B I do not want a car with a string of letters after its name. I will do with out ABS EBA EBD EFI ASC & even fly by wire thank you.
>> Edited by hasbeen on Friday 11th November 16:41
>> Edited by hasbeen on Friday 11th November 16:41
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