Brake proportioning front : rear
Discussion
Hi,
standard set up has been altered and front discs and calipers are slightly larger from a rwd xr4i.
I find that theres a slightly increased inclination to lock the fronts over the rear. Any recommendations about which adjustable brake balancing valve would be best in replacement of the OE fixed one?
standard set up has been altered and front discs and calipers are slightly larger from a rwd xr4i.
I find that theres a slightly increased inclination to lock the fronts over the rear. Any recommendations about which adjustable brake balancing valve would be best in replacement of the OE fixed one?
These bias valves only alter the dynamic balance i.e. the distribution under heavy braking. You need to get the static balance in the right ballpark first, otherwise you'll be forever trying to compromise between having the balance right under light braking (when you're tip-toeing round that wet roundabout) or heavy braking (emergency stops in the dry).
Morning, prophet of doom here again! You might not like this but it may save your life, livelyhood and/or an innocent S type.
Even TVR DESIGNED their brake system and it was tested. Provided it is working as it should the system is perfectly good for road use, it certainly is on my S1 anyway.
Mess with the system at your peril. If you do 'upgrade' (that is often a misnomer for change without knowing the implications) you must anticipate the effects on the rest of the system and design accordingly.
Fit bigger brakes at the front and you will increase front end braking, therefore they are more likely to lock. Simples!
They are most likely to lock in the wet. Loss of front end is much more difficult to control than loss of rear therefore likelyhood of greater consequences.
Following modern BMW at speed. Emergency in front. BMW has balanced brakes and ABS. You get front end lock and hit BMW at angle, spin round into oncoming traffic, injury, Police called, sportcar involved and apparently to blame, skidmarks from front wheels noted, car impounded and investigated, non standard front brakes identified. This is no longer 'just' an RTA, you are very seriously in the s... Ask your local experienced traffic cop if you don't believe me.
Even TVR DESIGNED their brake system and it was tested. Provided it is working as it should the system is perfectly good for road use, it certainly is on my S1 anyway.
Mess with the system at your peril. If you do 'upgrade' (that is often a misnomer for change without knowing the implications) you must anticipate the effects on the rest of the system and design accordingly.
Fit bigger brakes at the front and you will increase front end braking, therefore they are more likely to lock. Simples!
They are most likely to lock in the wet. Loss of front end is much more difficult to control than loss of rear therefore likelyhood of greater consequences.
Following modern BMW at speed. Emergency in front. BMW has balanced brakes and ABS. You get front end lock and hit BMW at angle, spin round into oncoming traffic, injury, Police called, sportcar involved and apparently to blame, skidmarks from front wheels noted, car impounded and investigated, non standard front brakes identified. This is no longer 'just' an RTA, you are very seriously in the s... Ask your local experienced traffic cop if you don't believe me.
greymrj said:
Even TVR DESIGNED their brake system and it was tested.
The Chimaera (maybe a bit faster but not particularly heavier) went from the titchy 240.. discs to the popular S Series upgrade, 260mm discs after a couple of years production.I thought the main reason for "bigger brakes" / rear disc conversion was better heat dissipation for prolonged use (wizzing up and down the Alps
) not for one off emergency stops ?ETA Informed my insurers of brake mods and they were ok with it, no inspection or whatever required.
Edited by phillpot on Monday 4th November 10:43
Well done Mike for letting your insurers know, you are clear therefore on the 'failed to notify' let out.
Apologies to those for whom the grandma and sucking eggs analogy applies!
Brakes essentially work by converting kinetic energy to heat energy. Apply pads at high pressure onto small discs and the heat transfer will be quick and the brakes perfectly effective, so small discs are not in themselves a problem.
If however the brakes are used again before they have cooled to the optimum then their ability to absorb more energy is reduced and the brakes are less effective, this essentially is brake 'fade'.
Increase the mass of the disc and it can absorb more heat, increase the cooling (usually by toroidal cooling) and it regains its ability to absorb more energy more quickly, usually a combination of both is used.
BUT if you just add bigger discs and/or pads at the same pressure not only will you improve the brakes ability to cool, but also their absolute ability to change kinetic to heat energy, i.e. the transfer can be faster and therefore the disc can be stopped quicker.
Put simply, if you ram on you will stop the wheel turning more easily....you lock up easier.
That is essentially why ABS became essential; better brakes were causing more lock up accidents.
If you do not balance all the components of the brake system, it ain't going to work as designed.
Apologies to those for whom the grandma and sucking eggs analogy applies!
Brakes essentially work by converting kinetic energy to heat energy. Apply pads at high pressure onto small discs and the heat transfer will be quick and the brakes perfectly effective, so small discs are not in themselves a problem.
If however the brakes are used again before they have cooled to the optimum then their ability to absorb more energy is reduced and the brakes are less effective, this essentially is brake 'fade'.
Increase the mass of the disc and it can absorb more heat, increase the cooling (usually by toroidal cooling) and it regains its ability to absorb more energy more quickly, usually a combination of both is used.
BUT if you just add bigger discs and/or pads at the same pressure not only will you improve the brakes ability to cool, but also their absolute ability to change kinetic to heat energy, i.e. the transfer can be faster and therefore the disc can be stopped quicker.
Put simply, if you ram on you will stop the wheel turning more easily....you lock up easier.
That is essentially why ABS became essential; better brakes were causing more lock up accidents.
If you do not balance all the components of the brake system, it ain't going to work as designed.
greymrj said:
Morning, prophet of doom here again! You might not like this but it may save your life, livelyhood and/or an innocent S type.
Even TVR DESIGNED their brake system and it was tested. Provided it is working as it should the system is perfectly good for road use, it certainly is on my S1 anyway.
Mess with the system at your peril. If you do 'upgrade' (that is often a misnomer for change without knowing the implications) you must anticipate the effects on the rest of the system and design accordingly.
Fit bigger brakes at the front and you will increase front end braking, therefore they are more likely to lock. Simples!
They are most likely to lock in the wet. Loss of front end is much more difficult to control than loss of rear therefore likelyhood of greater consequences.
Following modern BMW at speed. Emergency in front. BMW has balanced brakes and ABS. You get front end lock and hit BMW at angle, spin round into oncoming traffic, injury, Police called, sportcar involved and apparently to blame, skidmarks from front wheels noted, car impounded and investigated, non standard front brakes identified. This is no longer 'just' an RTA, you are very seriously in the s... Ask your local experienced traffic cop if you don't believe me.
I always thought the biggest issues with the S1 were the alloy wheels and engines, but it appears there may be other issues.Even TVR DESIGNED their brake system and it was tested. Provided it is working as it should the system is perfectly good for road use, it certainly is on my S1 anyway.
Mess with the system at your peril. If you do 'upgrade' (that is often a misnomer for change without knowing the implications) you must anticipate the effects on the rest of the system and design accordingly.
Fit bigger brakes at the front and you will increase front end braking, therefore they are more likely to lock. Simples!
They are most likely to lock in the wet. Loss of front end is much more difficult to control than loss of rear therefore likelyhood of greater consequences.
Following modern BMW at speed. Emergency in front. BMW has balanced brakes and ABS. You get front end lock and hit BMW at angle, spin round into oncoming traffic, injury, Police called, sportcar involved and apparently to blame, skidmarks from front wheels noted, car impounded and investigated, non standard front brakes identified. This is no longer 'just' an RTA, you are very seriously in the s... Ask your local experienced traffic cop if you don't believe me.
From what I remember of my S2 the brake balance was pretty goo, although the brakes were pretty soggy. (Probably due to the el cheapo 1144 pads I was using.)
On the V8S the balance of the standard brakes was way out and fitting 20% bigger diameter rotors on the front gave me a static balance that was only slightly to the front. It worked very well and meant the car tended to straighten up slightly under braking rather than the original very tail happy setup. But if you take that too far the fronts will be prone to lock too soon. This static balance is determined by the mechanical leverages of the whole path between the pedal and the tyre, so if it's wrong you need to change the mechanical setup to correct it. A balance bar is one way to do that; changing the disc sizes is another way.
Once you have the static balance right you can use a bias valve to make the front/rear balance 'knee over' at the right pressure to approximate the weight distribution curve versus braking, but the bias valve doesn't affect the static balance and you need to get that right first.
Although bigger brakes tends to make more braking force available so it's easier to lock the wheels if you brake hard enough, it also paradoxically makes the brakes more progressive and less inclined to grab (due to lower pressure and higher speed at the pad/rotor interface) so they are much easier to drive hard without locking the wheels.
On the V8S the balance of the standard brakes was way out and fitting 20% bigger diameter rotors on the front gave me a static balance that was only slightly to the front. It worked very well and meant the car tended to straighten up slightly under braking rather than the original very tail happy setup. But if you take that too far the fronts will be prone to lock too soon. This static balance is determined by the mechanical leverages of the whole path between the pedal and the tyre, so if it's wrong you need to change the mechanical setup to correct it. A balance bar is one way to do that; changing the disc sizes is another way.
Once you have the static balance right you can use a bias valve to make the front/rear balance 'knee over' at the right pressure to approximate the weight distribution curve versus braking, but the bias valve doesn't affect the static balance and you need to get that right first.
Although bigger brakes tends to make more braking force available so it's easier to lock the wheels if you brake hard enough, it also paradoxically makes the brakes more progressive and less inclined to grab (due to lower pressure and higher speed at the pad/rotor interface) so they are much easier to drive hard without locking the wheels.
Useful. The issue I think for most non rear disc S drivers has been fast road / track use can induce early fade at the front. This leads to the desire to improve heat dissipation and new larger discs and calipers and a moderate change in balance towards the front .
Under normal use mine is just fine as is any Chim with the same set up but when really really pressing on on the track into a bend I find there to be too much bias to the front.
Maybe the V8s is the answer?
Under normal use mine is just fine as is any Chim with the same set up but when really really pressing on on the track into a bend I find there to be too much bias to the front.
Maybe the V8s is the answer?
Totally agree with Peter (greenV8S), it looks like he has thought it through and not just changed one component without recognising the consequences.
He is also quite correct in that SLIGHTLY bigger brakes can be made to lock easier but if you know your car well they should enable you to get better and more progressive braking. That is if you do not just ram on! But you do need that subtlety in your driving, otherwise in an emergency......
He is also quite correct in that SLIGHTLY bigger brakes can be made to lock easier but if you know your car well they should enable you to get better and more progressive braking. That is if you do not just ram on! But you do need that subtlety in your driving, otherwise in an emergency......
your problem is that all brake limiting devices are to fit in the rear brake line, so limiting the possibility of the rears locking before the front. A proper twin master cylinder with adjustable balance (as favoured by rally cars) is what you may need. Not sure if you have the room for one of those.
When I put rear disks on mine the balance was restored somewhat. Just a thought they look cool too and are much easier to work on IMHO
Fronts are 278 mm vented rear 273mm solid iirc Granada cosworth. Wouldn't have thought 260mm disks would have been a problem have the pads bedded in or are the new?
Good luck would be interested in how you get on with adjusting the bias.
Damian
Fronts are 278 mm vented rear 273mm solid iirc Granada cosworth. Wouldn't have thought 260mm disks would have been a problem have the pads bedded in or are the new?
Good luck would be interested in how you get on with adjusting the bias.
Damian
magpies said:
A proper twin master cylinder with adjustable balance (as favoured by rally cars) is what you may need.
A twin master cylinder with a balance bar is one way to adjust the static balance - the other ways involve changing piston dimensions or position. But once you have got the static balance right, you still need a bias valve to vary the brake balance as the amount of braking increases, to compensate for weight transfer. It's not an either/or thing - the static and dynamic balance both have to be correct.Gassing Station | S Series | Top of Page | What's New | My Stuff