Locking wheels under braking
Discussion
Reading the post "lost back end" prompted me to pen this. I surprised myself a little yesterday by lighting up the rear wheels but more so after locking the rear wheels when braking. Steve’s book refers to a "deceleration valve" (I think) which is supposed to ensure the front wheels lock up first. It was not a dangerous situation because no-one was near me at the time but one rear wheel obviously locked first as the back end shifted sideways. Have others experienced this or do my brakes need an overhaul?
Is there such a valve? Sounds too high tech for a TVR, and against the general drivers car ethos methinks.
The reason why the rear disc are bigger than the fronts is supposed to be to pull the rear down and minimize rear lock-ups. Maybe thats what "the book" refers to.
Edited by philshort on Wednesday 28th November 17:52
The reason why the rear disc are bigger than the fronts is supposed to be to pull the rear down and minimize rear lock-ups. Maybe thats what "the book" refers to.
Edited by philshort on Wednesday 28th November 17:52
quote:
Is there such a valve? Sounds too high tech for a TVR, and against the general drivers car ethos methinks.
The reason why the rear disc are bigger than the fronts is supposed to be to pull the rear down and minimize rear lock-ups. Maybe thats what "the book" refers to.
Edited by philshort on Wednesday 28th November 17:52
Um no. The idea of braking the rear harder to stop them locking up must set the odd distant alarm bell ringing? The geometry of the brakes determines the proportion of front versus rear braking effort under gentle braking. There is a brake regulator in the rear brake line that adjusts this proportion as you brake harder and harder, to compensate for the weight transfer. It works by restricting the flow to the rear circuit. On recent TVRs this is an acceleration-based valve (ball bearing pushing against a spring sort of thing). On mine I've replaced this fixed regulator with an adjustable one so that I can tweak the characteristics to suit my uprated brakes. Ideally you want a curved brake bias, but these regulators give you a stepped bias which is close enough.
I think there are several reasons why griffiths and Chimaeras tend to lock the rears. The first is TVR generally seem to put too much braking on the rear wheels. The second is the rear suspension doesn't have any anti-lift geometry and the back lifts quite a long way under braking, when it reaches the top of its travel this temporarily unloads the rear wheels which makes it easier to lock them up. If you are smoother and more progressive as you roll on the brakes this isn't such a problem but if you are too abrupt it is easy to lock the rears. And if you just jump straight on them without waiting for the weight to settle on the fronts, you can lock the fronts too. The third is that TVRs have lots of engine braking and this aggravates all the other factors trying to lock the rears up.
When you're working the tyres that close to the limit of grip you need to think about the dynamic weight distribution, with springs, dampers, bump stops, roll bars and so on all involved. It gets quite complicated, but generally the smoother you are the harder you can brake without locking up.
Cheers,
Peter Humphries (and a green v8S)
Edited by Greenv8s on Wednesday 28th November 19:27
quote:
I think there are several reasons why griffiths and Chimaeras tend to lock the rears. The first is TVR generally seem to put too much braking on the rear wheels. The second is the rear suspension doesn't have any anti-lift geometry and the back lifts quite a long way under braking, when it reaches the top of its travel this temporarily unloads the rear wheels which makes it easier to lock them up. If you are smoother and more progressive as you roll on the brakes this isn't such a problem but if you are too abrupt it is easy to lock the rears. And if you just jump straight on them without waiting for the weight to settle on the fronts, you can lock the fronts too. The third is that TVRs have lots of engine braking and this aggravates all the other factors trying to lock the rears up.
Cheers,
Peter Humphries (and a green v8S)
Does it follow that increasing the size of the front disks is not such a bad thing? (I see the first step on the brake upgrade path is to simply replace the front disks with slightly larger, cross drilled ones - do people do this as a matter of course when it comes to replacing worn disks?) I suppose it is possible that greater braking at the front might unload the rears even more? I'm beginning to think there are too many variables! Smooth and progressive braking sounds like the answer but it would be nice to know that emergency stops in poor conditions are not going to put me into a spin!
Thanks for the replys.
quote:
The idea of braking the rear harder to stop them locking up must set the odd distant alarm bell ringing
Did I say that? Is that what I meant by "pulling the rear down"?
It wasn't really, but I suppose, thinking a bit further, what the hell exactly transforms that initial rear bias into balanced braking which doesn't lock the rears? There would have to be some sort of self adjusting bias valve in there somewhere!
I wish I was clever!
Phil
quote:
Does it follow that increasing the size of the front disks is not such a bad thing?
You've hit the nail on the head. On the V8S, the 'sweet spot' was with 285 mm on the front and standard calipers, rear left as standard (250mm). This gave very slightly too much braking at the front so the car tended to straighten up slightly under braking which is very reassuring if things are getting out of shape. Unlike the originals that made the back distinctly wandery (is that a word?) under braking. But I'd be cautious about going much beyond about 285mm without looking at the brake balance, any further and the fronts will be too eager to lock up under heavy braking. 285mm was a nice balance just on the stable side of neutral.
Cheers,
Peter Humphries (and a green V8S)
Always seemed odd to me why TVR puts such wopping great discs on the rear, Touring Cars don't. The rear brakes are really only needed to settle the car not to seriously stop it. The last thing you want is to lock the rears up. Hollywood stunt drivers fit an extra controls so that they can lock the rears on purpose (nice slides) and you can do something similar using the handbrake.
Simply put, when braking there is a weight transfer forward which puts more weight on the front wheels which will allow more braking and so on until either the car slows to the desired speed or the the front wheels lock up (the amount of braking force to do this will depend on conditions, e.g., wet road/ice/tyre condition/speed/etc.). Normally the most effective braking is just under this break-away limit.
On a car without ABS the driver is in control of this locking-up and when the fronts lock-up just lift off the brake pedal slightly to allow the tyres to stop sliding. pressure can then be reapplied to the brakes to continue to slow the car. Do this fast enough (on/off/on/off) and you are effectively duplicating ABS, it is called cadence braking. This takes practice. It also takes deliberate effort of will as in normal 'blind-panic' braking, lifting your foot off the brake seems contrary to what is needed.
TVRs are capable of high speeds and need brakes to slow them down (rapidly at times). The throttle needs respect and so do the brakes. Just like smooth acceleration, smooth braking - just under the pressure needed to lock-up - is the way to go. This is driver experience nothing else. The braking limits of your car should be explored on a safe piece of private road (runway, track days etc.) and experience/confidence built up.
Now the plug:
Our brake conversion uses the Brembo World Rally Car gravel spec. 300mm fully floating discs with motorsport 4-pot calipers on the front with DS3000 pads, but we only change the pads on the rear as this is all that is needed there. We found that there was no need to adjust the front/rear balance. PetrolTed's review of our brakes can be read at: www.pistonheads.com/tvr/index.asp?storyId=2792
Oh yes, you can also lock the rears by not matching engine revs to the right gearbox/road speed when braking....
Clive Reed
Zertec Limited
info@zertec.co.uk
www.zertec.co.uk
Simply put, when braking there is a weight transfer forward which puts more weight on the front wheels which will allow more braking and so on until either the car slows to the desired speed or the the front wheels lock up (the amount of braking force to do this will depend on conditions, e.g., wet road/ice/tyre condition/speed/etc.). Normally the most effective braking is just under this break-away limit.
On a car without ABS the driver is in control of this locking-up and when the fronts lock-up just lift off the brake pedal slightly to allow the tyres to stop sliding. pressure can then be reapplied to the brakes to continue to slow the car. Do this fast enough (on/off/on/off) and you are effectively duplicating ABS, it is called cadence braking. This takes practice. It also takes deliberate effort of will as in normal 'blind-panic' braking, lifting your foot off the brake seems contrary to what is needed.
TVRs are capable of high speeds and need brakes to slow them down (rapidly at times). The throttle needs respect and so do the brakes. Just like smooth acceleration, smooth braking - just under the pressure needed to lock-up - is the way to go. This is driver experience nothing else. The braking limits of your car should be explored on a safe piece of private road (runway, track days etc.) and experience/confidence built up.
Now the plug:
Our brake conversion uses the Brembo World Rally Car gravel spec. 300mm fully floating discs with motorsport 4-pot calipers on the front with DS3000 pads, but we only change the pads on the rear as this is all that is needed there. We found that there was no need to adjust the front/rear balance. PetrolTed's review of our brakes can be read at: www.pistonheads.com/tvr/index.asp?storyId=2792
Oh yes, you can also lock the rears by not matching engine revs to the right gearbox/road speed when braking....
Clive Reed
Zertec Limited
info@zertec.co.uk
www.zertec.co.uk
As Pete Humphries & Clive Reed mention, the downshift, combined with breaking can easily lock the rears.
A friend was videoing my at the first corner (Lodge?) at a TVRCC Track Day at Outlon, and nearly every lap, the rears 'chirped' as I changed down.
I couldn't hear/feel anything particularly in the car, but having seen the video, was a bit pi55ed off that I hadn't made a better job of the heel & toeing.
However (correct me if I'm wrong Pete) some of the footage of Pete at Curborough on Roop's site, shows a similar thing. Is this just ineviable with the way the rear suspension works on TVR's?
Marcus
& Ocean Haze Griff 500
A friend was videoing my at the first corner (Lodge?) at a TVRCC Track Day at Outlon, and nearly every lap, the rears 'chirped' as I changed down.
I couldn't hear/feel anything particularly in the car, but having seen the video, was a bit pi55ed off that I hadn't made a better job of the heel & toeing.
However (correct me if I'm wrong Pete) some of the footage of Pete at Curborough on Roop's site, shows a similar thing. Is this just ineviable with the way the rear suspension works on TVR's?
Marcus
& Ocean Haze Griff 500
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