TVR 350i Brake Mods.... Not the normal question!
Discussion
Hi guys...
At BBWF last year I parked next to a white wedge which was obviously well sorted for sprinting...
During the day I spent some time talking to the owner and the subject of brakes came up (not unusual in converstaions between wedge owners!) sorry can't remember his name!
He suggested removing the brake bias valve/regulator from the rear circuit allowing more force to be applied to the rear brakes..
My car, an otherwise standard S2 350i, has Princess 4 pots and 300mm drilled/vented disks and stainless hoses all round. (Tower View kit of a couple of years ago)
The only issue I have with the current setup is that it is possible to lock up the front when in an "emergency" situation (try that with the standard setup!). If you "load" up the brakes in the correct manner the the stopping power is very impressive.
What are peoples opinion on this mod?
My concern is that the mod will produce to much rear bias which would produce a highly unstable car under heavy braking.
Has any one fitted one of the variable proportuning valves and would this be a better mod.
Regards,
Brian (and a Red 350i)
At BBWF last year I parked next to a white wedge which was obviously well sorted for sprinting...
During the day I spent some time talking to the owner and the subject of brakes came up (not unusual in converstaions between wedge owners!) sorry can't remember his name!
He suggested removing the brake bias valve/regulator from the rear circuit allowing more force to be applied to the rear brakes..
My car, an otherwise standard S2 350i, has Princess 4 pots and 300mm drilled/vented disks and stainless hoses all round. (Tower View kit of a couple of years ago)
The only issue I have with the current setup is that it is possible to lock up the front when in an "emergency" situation (try that with the standard setup!). If you "load" up the brakes in the correct manner the the stopping power is very impressive.
What are peoples opinion on this mod?
My concern is that the mod will produce to much rear bias which would produce a highly unstable car under heavy braking.
Has any one fitted one of the variable proportuning valves and would this be a better mod.
Regards,
Brian (and a Red 350i)
In my opinion you never want too much rearward brake bias, it doesn't tend to lead to a stable car. The best way of distributing braking force is via an adjustable brake bias, this acts from the pedal box, and proportionally distributes the braking force between the two master cylinders, and is not just a hydraulic limiter to the circuit to which it is fitted.
The adjustable hydraulic limiter is something you might want to try, just to see if the car feels better, either in braking performance/stability. I presume you are thinking of one to fit in line, and not adjustable from within the cockpit?
The adjustable hydraulic limiter is something you might want to try, just to see if the car feels better, either in braking performance/stability. I presume you are thinking of one to fit in line, and not adjustable from within the cockpit?
Don't change your brakes unless you understand what you're doing. With all due respect your friend in the wedge is giving you very bad advice.
It is important that your brake balance is correct under light braking, and under heavy braking. The necessary balance is different in these two situations because of the weight transfer under braking. The brake balance under light braking is determined by the static balance of the brakes i.e. the mechanical geometry of the pedal, master/slave cylinders and discs/wheels. This determines the balance of the car under braking in very slippery conditions and when cornering. The brake balance under heavy braking is determined by the static balance plus the effect of the bias valve. This determines the handling of the car under braking in a straight line in very grippy situations. If you change the brakes significantly then you need to establish the correct static balance and then change the bias valve if necessary to achieve the correct static balance. An adjustable bias valve can come in handy here but remember this will only affect the dynamic balance not the static balance.
I've seen several people now who've massively upgraded the front brakes and moved the static balance too far forward, and then removed the bias valve completely so that they get more braking to the rear to stop the fronts locking too early under heavy braking. This is the WRONG ANSWER and can result in a car which is very dangerous to drive.
It is important that your brake balance is correct under light braking, and under heavy braking. The necessary balance is different in these two situations because of the weight transfer under braking. The brake balance under light braking is determined by the static balance of the brakes i.e. the mechanical geometry of the pedal, master/slave cylinders and discs/wheels. This determines the balance of the car under braking in very slippery conditions and when cornering. The brake balance under heavy braking is determined by the static balance plus the effect of the bias valve. This determines the handling of the car under braking in a straight line in very grippy situations. If you change the brakes significantly then you need to establish the correct static balance and then change the bias valve if necessary to achieve the correct static balance. An adjustable bias valve can come in handy here but remember this will only affect the dynamic balance not the static balance.
I've seen several people now who've massively upgraded the front brakes and moved the static balance too far forward, and then removed the bias valve completely so that they get more braking to the rear to stop the fronts locking too early under heavy braking. This is the WRONG ANSWER and can result in a car which is very dangerous to drive.
Thanks for that Peter,
that explains in reasonably simple terms the dynamics of the situation..
As you may have realised I wasn't entirely convinced by the advice hence me not implementing it, but asking the question here first.
I have driven a car which locked up the rears under heavy braking, an experience I would not like to repeat (took ages to clean the drivers seat!). The car involved was one of the very first Nova GTE's. I believe that the spec of the valve was changed on later versions.
Would you suggest I leave well alone, other than replacing the disks and pads and trying to keep the rear brakes in tip-top condition?
Have you used one of the variable hydraulic valves? or anyone else for that matter. I assume all you can adjust is the amount of pressure sent to the rear pistons. You are never going to get from 100% front through to 100% rear. If I replaced the standard valve with an adjustable one then I would have to spend some time on the track while I got the bias setting near optimal.
I assume that the standard valve came from the TVR parts bin, has anyone any idea what it was originally fitted to?
Regards,
Brian (and a Red350i)
PS may well bend your ear further at back Home if you're going Peter
that explains in reasonably simple terms the dynamics of the situation..
As you may have realised I wasn't entirely convinced by the advice hence me not implementing it, but asking the question here first.
I have driven a car which locked up the rears under heavy braking, an experience I would not like to repeat (took ages to clean the drivers seat!). The car involved was one of the very first Nova GTE's. I believe that the spec of the valve was changed on later versions.
Would you suggest I leave well alone, other than replacing the disks and pads and trying to keep the rear brakes in tip-top condition?
Have you used one of the variable hydraulic valves? or anyone else for that matter. I assume all you can adjust is the amount of pressure sent to the rear pistons. You are never going to get from 100% front through to 100% rear. If I replaced the standard valve with an adjustable one then I would have to spend some time on the track while I got the bias setting near optimal.
I assume that the standard valve came from the TVR parts bin, has anyone any idea what it was originally fitted to?
Regards,
Brian (and a Red350i)
PS may well bend your ear further at back Home if you're going Peter
briantvr350 said:
Have you used one of the variable hydraulic valves? or anyone else for that matter. I assume all you can adjust is the amount of pressure sent to the rear pistons. You are never going to get from 100% front through to 100% rear. If I replaced the standard valve with an adjustable one then I would have to spend some time on the track while I got the bias setting near optimal.
It only alters the pressure applied to the rear brake circuit, it doesnt alter the front, you can get a 100% front bias from this device, but dont recommend it.
If it's a cast iron valve on the 350, it's probably off an early Ford Sierra. They are often called "G" vlaves. Inside, there's a ball bearing that rolls up a slope as you brake. The harder you brake, the further up the slope it rolls. Eventually, it blocks off a port in the end and reduces the pressure to the rear brakes. They are very crude devices and it is possible to "beat" them by applying the brakes very sharply so that the rear brakes get full pressure before the ball has had a chance to roll up the slope. You can change the level of deceleration at which this happens by altering the slope.
As has been said though, woe betide anyone who has a go at this and gets it wrong! It could be ages until you get the right combination of circumstances to lock the rear wheels (road surface, car load distribution, deceleration, corner, fuel load etc...)
As has been said though, woe betide anyone who has a go at this and gets it wrong! It could be ages until you get the right combination of circumstances to lock the rear wheels (road surface, car load distribution, deceleration, corner, fuel load etc...)
I'm going to have to get under the wedge again and have another look!
From memory, back when I changed over to braided hoses, I didn't see any bias valve / regulator / limiter / proportioner / do-hickey / thingamy-jig.
Is this likely, am I not looking in the right place, or has a previous misguided owner removed it?
The rest of the brake circuit appears standard as prooved by the inability to lock wheels under almost all conditions! I guess it's a kind of poor mans ABS LOL It certainly promotes a more laid back driving style.
Interesting thread, please post up your final findings.
One day I hope to upgrade the brake system to change it from the worst I have ever encountered to the best, so all information is welcome.
From memory, back when I changed over to braided hoses, I didn't see any bias valve / regulator / limiter / proportioner / do-hickey / thingamy-jig.
Is this likely, am I not looking in the right place, or has a previous misguided owner removed it?
The rest of the brake circuit appears standard as prooved by the inability to lock wheels under almost all conditions! I guess it's a kind of poor mans ABS LOL It certainly promotes a more laid back driving style.
Interesting thread, please post up your final findings.
One day I hope to upgrade the brake system to change it from the worst I have ever encountered to the best, so all information is welcome.
Don't remove the brake bias adjuster. You will have a lot of issues as the system is based around that adjuster. If the balance is moved to the back the car will be very unstable under braking. If the rear brakes lock up first, the chances are you will spin. Its the same as the problem with deliberately using the engine as a brake.
The Tower View brake kit was developed on the 520SE and used the standard brake balance. I do have an adjustable one fitted now (even bigger brakes etc and different master cylinder) and when setting up, it still needed quite a bit of brake bias to get a good balance.
It is true that the Cerbera doesn't have a brake balance as AP designed the system without it and use the calliper ratios to compensate.
The Tower View brake kit was developed on the 520SE and used the standard brake balance. I do have an adjustable one fitted now (even bigger brakes etc and different master cylinder) and when setting up, it still needed quite a bit of brake bias to get a good balance.
It is true that the Cerbera doesn't have a brake balance as AP designed the system without it and use the calliper ratios to compensate.
Edited by shpub on Tuesday 27th June 10:50
Based on very limited experience...
It was quite possible to lock the front wheels on our 350i with the standard twin pot solid disk setup by pressing the brake hard and rapidly. We now have a 280mm vented disk Hi-Spec four pot setup on the front and have found that under hard progressive straight line braking on a track the rear can lock before the front. The rear tyres (Continental sport) are old and possibly not as grippy as the newer fronts (Bridgestone RE720). The bias valve is in place and standard AFAIK we have certainly not touched it.
I suspect that suspension setup, tyre choice/inflation and possibly even fuel load could all affect your results. I cannot really give advice but I thought a comparison might help.
It was quite possible to lock the front wheels on our 350i with the standard twin pot solid disk setup by pressing the brake hard and rapidly. We now have a 280mm vented disk Hi-Spec four pot setup on the front and have found that under hard progressive straight line braking on a track the rear can lock before the front. The rear tyres (Continental sport) are old and possibly not as grippy as the newer fronts (Bridgestone RE720). The bias valve is in place and standard AFAIK we have certainly not touched it.
I suspect that suspension setup, tyre choice/inflation and possibly even fuel load could all affect your results. I cannot really give advice but I thought a comparison might help.
FrenchTVR said:
It is on mine too, fixed to the horizontal just below the master cylinder.
You're quite right! Just below the servo! I feel a little stupid now! 
Thanks
And on that bombshell I can justifiably go back to bed having learnt my "new thing" for the day.
BTW at the risk of dragging this off topic. Is it a dual or single circuit system? Looking at the servo I'd guess single but I have been wrong in the past!
Leo
toltec said:
Based on very limited experience...
It was quite possible to lock the front wheels on our 350i with the standard twin pot solid disk setup by pressing the brake hard and rapidly. We now have a 280mm vented disk Hi-Spec four pot setup on the front and have found that under hard progressive straight line braking on a track the rear can lock before the front. The rear tyres (Continental sport) are old and possibly not as grippy as the newer fronts (Bridgestone RE720). The bias valve is in place and standard AFAIK we have certainly not touched it.
I suspect that suspension setup, tyre choice/inflation and possibly even fuel load could all affect your results. I cannot really give advice but I thought a comparison might help.
It was quite possible to lock the front wheels on our 350i with the standard twin pot solid disk setup by pressing the brake hard and rapidly. We now have a 280mm vented disk Hi-Spec four pot setup on the front and have found that under hard progressive straight line braking on a track the rear can lock before the front. The rear tyres (Continental sport) are old and possibly not as grippy as the newer fronts (Bridgestone RE720). The bias valve is in place and standard AFAIK we have certainly not touched it.
I suspect that suspension setup, tyre choice/inflation and possibly even fuel load could all affect your results. I cannot really give advice but I thought a comparison might help.
Have a similar setup (although 300mm hi-spec front), and running bridgestone re720s allround. Never had the rears lock, but can lock the front if I want (=not). So it could be old tires at the back, or the system needs flushing to asure the bias valve is clean inside.
I replace the brake fluid like every 6 months to a year, and using dot 5.1.
Other possibility, and hoping it is not the case, is that you have better brake pads at the back and standard at the front. This should never be done, as more grip at the back can cause also rear locking before the fronts lock. You should be able to check this, or find out what you have used as a replacement (front/rear) last time.
Rob
toltec said:
Based on very limited experience...
It was quite possible to lock the front wheels on our 350i with the standard twin pot solid disk setup by pressing the brake hard and rapidly. We now have a 280mm vented disk Hi-Spec four pot setup on the front and have found that under hard progressive straight line braking on a track the rear can lock before the front. The rear tyres (Continental sport) are old and possibly not as grippy as the newer fronts (Bridgestone RE720). The bias valve is in place and standard AFAIK we have certainly not touched it.
I suspect that suspension setup, tyre choice/inflation and possibly even fuel load could all affect your results. I cannot really give advice but I thought a comparison might help.
It was quite possible to lock the front wheels on our 350i with the standard twin pot solid disk setup by pressing the brake hard and rapidly. We now have a 280mm vented disk Hi-Spec four pot setup on the front and have found that under hard progressive straight line braking on a track the rear can lock before the front. The rear tyres (Continental sport) are old and possibly not as grippy as the newer fronts (Bridgestone RE720). The bias valve is in place and standard AFAIK we have certainly not touched it.
I suspect that suspension setup, tyre choice/inflation and possibly even fuel load could all affect your results. I cannot really give advice but I thought a comparison might help.
There are plenty of things you could have changed that would give you this result. The most likely thing is that the high performance pads in the front are a "harder" friction material than the TVR choice. This might help reduce the onset of fade but they will probably have a lower coefficient of friction against the disc. This will make the rears (at least initially) disproportionately more powerful. Similarly, you could have changed the position of the effective pad centre at the front and / or the piston diameter in the calipers. The TVR setup would have been calibrated initially and tested during its type approval tests with their setup but you will have altered it now. You could try altering the angle of the valve to alter the point at which the presure is reduced to te rear wheels. I'd say you might have to have it a bit flatter than it currently is but honestly, this isn't a DIY job and I'd be VERY wary about trying it on my car!
leorest said:
FrenchTVR said:
It is on mine too, fixed to the horizontal just below the master cylinder.
You're quite right! Just below the servo! I feel a little stupid now! Thanks
And on that bombshell I can justifiably go back to bed having learnt my "new thing" for the day.
BTW at the risk of dragging this off topic. Is it a dual or single circuit system? Looking at the servo I'd guess single but I have been wrong in the past!
Leo
Dual circuit split front to rear.....well mine is
wedge girl said:
leorest said:
FrenchTVR said:
It is on mine too, fixed to the horizontal just below the master cylinder.
You're quite right! Just below the servo! I feel a little stupid now! Thanks
And on that bombshell I can justifiably go back to bed having learnt my "new thing" for the day.
BTW at the risk of dragging this off topic. Is it a dual or single circuit system? Looking at the servo I'd guess single but I have been wrong in the past!
Leo
Dual circuit split front to rear.....well mine is
Thanks boys and girls for all your useful advice...
When I asked the question, I maybe had the idea that taking the valve out of the rear brake circuit wasn't the best advice I'd ever been given..
I didn't perhaps realise the complexity of the dynamics of the situation. I'm sure the excellent explanation given by GreenV8S is a simplification on the real situation where all sorts of other factors need to be considered such as the grip of individual tyres to the road, weight on each wheel at any particular instant, the efficiency of the piston, pad, brake disk on each wheel etc etc etc..
The original brake upgrade was only fitted after much research on here and talking to people; we can all remember the weekly "why I fell off this week" posts from SHPUB while the 520SE was being developed. Having driven with the upgrade for some while now possibly the only thing I would like to change is the weight of the Princess 4 pots. That said I would be very wary of changing to certain makes of light alloy calliper having read various horror stories on here and elsewhere..
So what am I likely to do..
Replace the rear disks and pads, change the brake fluid, leave everything else well alone and go out and enjoy driving the thing..
If anyone wants a chat in person, the car will be parked on the prom at Back Home and will be making a pilgrimage to Bristol Avenue for the 1st factory tour. Don't think there are many red wedges about with 3 plus 3 registrations.
Regards,
Brian (and a Red350i)
When I asked the question, I maybe had the idea that taking the valve out of the rear brake circuit wasn't the best advice I'd ever been given..
I didn't perhaps realise the complexity of the dynamics of the situation. I'm sure the excellent explanation given by GreenV8S is a simplification on the real situation where all sorts of other factors need to be considered such as the grip of individual tyres to the road, weight on each wheel at any particular instant, the efficiency of the piston, pad, brake disk on each wheel etc etc etc..
The original brake upgrade was only fitted after much research on here and talking to people; we can all remember the weekly "why I fell off this week" posts from SHPUB while the 520SE was being developed. Having driven with the upgrade for some while now possibly the only thing I would like to change is the weight of the Princess 4 pots. That said I would be very wary of changing to certain makes of light alloy calliper having read various horror stories on here and elsewhere..
So what am I likely to do..
Replace the rear disks and pads, change the brake fluid, leave everything else well alone and go out and enjoy driving the thing..
If anyone wants a chat in person, the car will be parked on the prom at Back Home and will be making a pilgrimage to Bristol Avenue for the 1st factory tour. Don't think there are many red wedges about with 3 plus 3 registrations.
Regards,
Brian (and a Red350i)
Edited by briantvr350 on Friday 30th June 12:24
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