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Correcting front / rear brake bias

Correcting front / rear brake bias

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Sevenman

Original Poster:

590 posts

115 months

Tuesday 4th September 2012
quotequote all
I have been trying to sort the brake bias on my Tamora, but I assume this is relevant to the other T-cars as well.

I had an earlier thread on my postponed CAT Driver Training day at Millbrook, but have had some work done on the car since then.

As bought, the braking was too rear biased. The rears lock before the fronts, destabilising the car, reducing braking performance and producing smoke from locked wheels.

The first bit of corrective work was by TVR Power who changed the rear pads from EBC Greenstuff to the same Ferodo compound as the fronts. I can't remember the exact compound, but it is less aggressive than DS2500.

However, this didn't fix it. The rears still locked first.

The next bit of work was by Track V Road (from whom we bought the car). They put an AP Racing brake proportioning valve in the rear brake line, replacing the standard non-adjustable valve, and mounted it to the pedal box.

This allows the fraction of braking force to the rear to vary from 0.95 (fully screwed in) to 0.35 (fully screwed out - 1 3/4 turns).







As fitted by Track V Road the valve made a noticable rattling noise when driven over any non-smooth surface (no shortage of those), so that is a lot of rattling. But some adjusting revealed that it rattles only when adjusted in the range 1 1/4 to 1 3/4 turns out.

I did some testing recently summarised as:

- 1 turn out: Fronts lock first - Success, so test some other settings
- 1/2 turn out: Rears lock first - Expected
- 3/4 turn out - Rears lock first - Ok
- 1 turn out: Rears lock first frown This was working earlier...
- 1 1/4 turn out: Sometimes rears lock first

- More than 1 1/4 turns - Rattling returns, ran out of testing time.

I need to do some more testing towards the extremes of the range to see if I can get the behaviour to be reliable and just live with the rattling. AP say the valve shouldn't rattle and they will look at it if I send it to them, but that isn't so easy...

This is being an annoying fault and not cheap to fix. It is difficult to test as it only manifests in extreme conditions, and it is hard to try different solutions. I bought this car as being more suitable than my last for trackdays, but until I fix this there is no point taking it on track (other than for testing faults) and I can't complete my driver training until I am happy with the handling.

On the plus side, this pic is after a recent sunny day roof-down drive. I know not everybody likes the Tamora, but I think it looks fine and is good fun smile




TVR_owner

3,349 posts

114 months

Tuesday 4th September 2012
quotequote all
Gavin,
A standard Tamora without red rose brakes has a fabulous set up and balance. It should almost never lock the rears under controlled operations on road or track.

Different compound pads or tyre changes will impact this balance.

Was everything as it should have been?

Sevenman

Original Poster:

590 posts

115 months

Tuesday 4th September 2012
quotequote all
TVR_owner said:
A standard Tamora without red rose brakes has a fabulous set up and balance. It should almost never lock the rears under controlled operations on road or track.
That would be nice frown

As far as I can tell, everything is as it should be.

The car did this on the old tyres (Toyo T1-S) and the new tyres (Toyo T1-Sport).

2 garages - TVR Power and Track V Road have been over the braking system and all is as it should be. The compound is matched front and back, although the front pads are older (age unknown). Track V Road did not want to change them as there was still a lot of material left.

The suspension geometry is all set correctly (and it handles well), it is a year-old Gaz Gold Pro setup.

It would be good to know what reduction the standard proportioning valve provided, maybe I need the extremes of the adjustable valve to meet that or maybe it does not go far enough.

If the extremes of the adjustable valve don't work then I am a bit stuck... Unless the original standard valve was faulty and a new standard valve would fix it. Or my new adjustable valve is faulty.

I can't just start replacing the entire braking system when everything seems to be ok. I would need to turn to a garage who have a lot of experience in setting up brake systems, have plenty of ways to test the car, and are familiar with TVRs. Even with my TVR running cost expectations, this first year has been expensive.

TVR_owner

3,349 posts

114 months

Tuesday 4th September 2012
quotequote all
Curious...

Ride height looks high? What is front to rear differential height??

Sevenman

Original Poster:

590 posts

115 months

Tuesday 4th September 2012
quotequote all
How do you measure front to rear differential height?

Ride height again as set by Track V Road with geometry checked. Requested towards the higher end of ride heights rather than lower end as this is a car for varied driving and I try to avoid grounding it. It managed a 3000 mile European road trip in May / June with 2 of us and a boot full of luggage very well.

Damping set slightly firmer than some have it - In Gaz Gold Pro specific units, 12 from soft on the front and 10 from soft on the back.

The brake problem has remained though a range of damper settings and some tweaks to ride height.
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YRRunner

1,639 posts

139 months

Tuesday 4th September 2012
quotequote all
Sevenman said:
... Unless the original standard valve was faulty and a new standard valve would fix it.
That's where I would start.

TVR_owner

3,349 posts

114 months

Tuesday 4th September 2012
quotequote all
Sevenman said:
How do you measure front to rear differential height?

Ride height again as set by Track V Road with geometry checked. Requested towards the higher end of ride heights rather than lower end as this is a car for varied driving and I try to avoid grounding it. It managed a 3000 mile European road trip in May / June with 2 of us and a boot full of luggage very well.

Damping set slightly firmer than some have it - In Gaz Gold Pro specific units, 12 from soft on the front and 10 from soft on the back.

The brake problem has remained though a range of damper settings and some tweaks to ride height.
I would measure chassis at back of front wheels and chassis at front of back wheels. I suspect you know what difference to expect?

Sevenman

Original Poster:

590 posts

115 months

Tuesday 4th September 2012
quotequote all
I assumed the adjustable valve would cover the possibility that the standard valve was faulty, as well as giving me a more accurate setup and increased flexibility should anything change in future (e.g. larger front brakes).

I haven't yet proven that the adjustable valve won't work as I had to finish my testing before I had been through all the settings.

A big problem with fixing this is the testing - after doing any work to try to fix the fault the car then needs to undergo threshold braking tests from decent speeds, coming off the brakes when a wheel starts to lock. The mile straight at Millbrook Proving Ground is good for this, but a regular garage will struggle to do the testing.

Does anybody know the specifications of the standard valve in the rear brake line?

Sevenman

Original Poster:

590 posts

115 months

Tuesday 4th September 2012
quotequote all
TVR_owner said:
I would measure chassis at back of front wheels and chassis at front of back wheels. I suspect you know what difference to expect?
My understanding is that 0.75 to 1 inch of rake is expected, so the front should be a little lower than the back.

I have taken a few ride height measurements in the past, but mainly from low points around the edges of the car (e.g. front splitter, rear splitter) to check the ground clearance.

Measuring involved using an assortment of DVD / CD cases to come up with a stack which just fitted under the relevant part and the height of that stack was then measured.

TVR_owner

3,349 posts

114 months

Tuesday 4th September 2012
quotequote all
Sevenman said:
My understanding is that 0.75 to 1 inch of rake is expected, so the front should be a little lower than the back.

I have taken a few ride height measurements in the past, but mainly from low points around the edges of the car (e.g. front splitter, rear splitter) to check the ground clearance.

Measuring involved using an assortment of DVD / CD cases to come up with a stack which just fitted under the relevant part and the height of that stack was then measured.
Using the body as a reference point is almost a pointless excercise - due to inconsitent moulds and bodies not being on chassis square.

What you suggest sounds like a huge amount of rake for a front mid engined car with the engine so far back and I suspect that is the problem.

I'll check ours at the weekend, but would expect to see the car eiher flat, or maybe an 1/8" high at the back.

Have you experienced any oversteer issues?

Sevenman

Original Poster:

590 posts

115 months

Tuesday 4th September 2012
quotequote all
That might be too much rake, but I won't know how much there is until I measure it. It would be useful to have some more information on the geometry / setup of T-series cars as used by the factory / specialists that could be shared online.

The body-height measurements were taken because, at the time, I was most interested in grounding and clearance, which they were useful for.

I don't think there are oversteer issues:
  • Item 1 Testing the cornering balance of the car on a handling pan at Millbrook, when following a circle of constant radius and building up speed, the tendency is for the car to start to understeer slightly, which can turn into lift-off oversteer or could probably be balanced with more power.
  • Item 2 The instructor then drove the car and said he liked the way it handled at the limit and was how he liked to have his cars set.
It seems unlikely to me that the above would be the source of brake problems, but I don't know what is, and it will be a while before I can finish playing with the adjustability of the new bias valve.

Sevenman

Original Poster:

590 posts

115 months

Tuesday 4th September 2012
quotequote all
I just noticed the following on the information page for the AP valve:

[i]Installation
To obtain the best performance using this valves, the brake balance should be biased towards the rear so that with the valve piped in to the rear line and screw cap right in (clockwise) where virtually no reductions occurs, the balance is as much to the rear as will ever be needed. screwing the cap outwards to a lower setting will progressively reduce the rear line pressure giving more bias to the front, for use when more grip is available.[/i]

I haven't tested it in poor conditions where the greatest rear bias could be tolerated - e.g. cold wet day (I will ignore snow / ice), just in dry and good conditions.

I worked on the basis of setting the bias for very good conditions, then the car will simply become more front biased as conditions deteriorate which is a safe handling characteristic.

Hopefully some more tests at the far end of the valve's adjustment will tell me if it provides sufficient reduction. It would be useful to know the specification of the standard valve - Does anybody have that information?

ChrisPap

390 posts

77 months

Tuesday 4th September 2012
quotequote all
Hi,

It's an obvious thing, but is the valve definitely plumbed in the right way? The line closest to the adjuster should be pressure in from the master cylinder, and the lower one to the rear brake circuit.

It definitely shouldn't click! It's possible the internal valve has been broken by over tightening?

Also, I hope it's currently only installed as a test of theory and that when successful it will be piped up with hardlines or at the very least braided lines.

Cheers,
Chris

K4TRV

1,813 posts

175 months

Tuesday 4th September 2012
quotequote all
ChrisPap said:
Hi,

It's an obvious thing, but is the valve definitely plumbed in the right way? The line closest to the adjuster should be pressure in from the master cylinder, and the lower one to the rear brake circuit.

It definitely shouldn't click! It's possible the internal valve has been broken by over tightening?

Also, I hope it's currently only installed as a test of theory and that when successful it will be piped up with hardlines or at the very least braided lines.

Cheers,
Chris
Obvious thing number 2 - why in the foot well - the rear pressure limiter is on the offside area of the rear wheel? Shirely it's that limited (fixed) limiter which is replaced by the adjuster? So how does it get fitted near the brake pedal area? Has the fixed limiter been removed? Maybe I'm just ill informed?
Rgds
Trev

Sevenman

Original Poster:

590 posts

115 months

Tuesday 4th September 2012
quotequote all
Thanks for the posts.

ChrisPap said:
Hi,

It's an obvious thing, but is the valve definitely plumbed in the right way? The line closest to the adjuster should be pressure in from the master cylinder, and the lower one to the rear brake circuit.

It definitely shouldn't click! It's possible the internal valve has been broken by over tightening?

Also, I hope it's currently only installed as a test of theory and that when successful it will be piped up with hardlines or at the very least braided lines.

Cheers,
Chris
Regards to plumbing direction, I can't tell without a bit of work (removing the aluminium plate on the pedal box). For now I will assume the garage have fitted it the correct way...

The valve doesn't click. When approaching the limits of adjustment in the unscrewed direction there can be a rattle when the valve is knocked (hit valve with hand, or drive over a rough road).

It is installed with solid metal lines at the moment - Other than braided lines I am not sure what else it would be installed with.

K4TRV said:
Obvious thing number 2 - why in the foot well - the rear pressure limiter is on the offside area of the rear wheel? Shirely it's that limited (fixed) limiter which is replaced by the adjuster? So how does it get fitted near the brake pedal area? Has the fixed limiter been removed? Maybe I'm just ill informed?
Rgds
Trev
I don't know if there were some differences between TVR models, but apparently there was a pressure reduction valve for the rear brake lines in the pedal box, which this valve replaces. It also means the valve can be adjusted when stopped.

Are you suggesting there are individual pressure limiting valves fitted in the lines close to the rear brake callipers?

ChrisPap

390 posts

77 months

Tuesday 4th September 2012
quotequote all
Oh, from the piture I thought that it was a rubber hose going in to the bias valve, but I see now it's just adjacent, and there is a hardline, though it's wrapped in yellow tape?

K4TRV

1,813 posts

175 months

Tuesday 4th September 2012
quotequote all
Sevenman said:
I don't know if there were some differences between TVR models, but apparently there was a pressure reduction valve for the rear brake lines in the pedal box, which this valve replaces. It also means the valve can be adjusted when stopped.

Are you suggesting there are individual pressure limiting valves fitted in the lines close to the rear brake callipers?
Hi, mine is an early 02 Tam and the limiter is towards the rear, offside before it feeds left & right rear callipers? Same place as my 93 Chim. Hence my comment that I've only experienced limiters near the rear split - in saloon cars/trucks the limiter would have a linkage to the rear suspension to modify the front/rear split dependant on load.

I'm still surprised the rear brakes have their limiter near the master cylinder and not at the rear before they split? There would only be one 'rear" limiter for both callipers.

Just my experience?

Trev

Sevenman

Original Poster:

590 posts

115 months

Tuesday 4th September 2012
quotequote all
ChrisPap said:
Oh, from the picture I thought that it was a rubber hose going in to the bias valve, but I see now it's just adjacent, and there is a hardline, though it's wrapped in yellow tape?
It does look that way - the yellow tape is from when I was trying to track down the rattle - I thought a pipe was hitting the alloy plate so taped it up - but then found it was the valve.

K4TRV said:
Hi, mine is an early 02 Tam and the limiter is towards the rear, offside before it feeds left & right rear callipers? Same place as my 93 Chim. Hence my comment that I've only experienced limiters near the rear split.

I'm still surprised the rear brakes have their limiter near the master cylinder and not at the rear before they split? There would only be one 'rear" limiter for both callipers.
I will have to do some more looking at the car and get it up on a ramp. It should be fine to have the limiter at any point in the line, be it the pedal box end or just before the split. It could be something that changed over time with TVRs.

I also need to work out what happens with brake servos etc. I will see if there is some detail in the Varley manual, although the Sag could be a bit different.

black11s

241 posts

163 months

Wednesday 5th September 2012
quotequote all
Sevenman said:
How do you measure front to rear differential height?

Ride height again as set by Track V Road with geometry checked. Requested towards the higher end of ride heights rather than lower end as this is a car for varied driving and I try to avoid grounding it. It managed a 3000 mile European road trip in May / June with 2 of us and a boot full of luggage very well.

Damping set slightly firmer than some have it - In Gaz Gold Pro specific units, 12 from soft on the front and 10 from soft on the back.

The brake problem has remained though a range of damper settings and some tweaks to ride height.
My tuppence worth:

This may or may not make a difference - the previous owner of my car had GAZ Pro Gold's fitted by Track V Road and they made a complete mess of it. I'm about to write up a full explanation elsewhere, but when I took it to the GAZ factory in Basildon yesterday (out of frustration) there were eyes rolling to the heavens at just what a mess they had made.
Ride height was wrong, spring rates were wrong, dampers then had to be overly stiffened to compensate etc.
After a few hours there (and a few quid admittedly) I have a completely different, and vastly better, car.
If your ride heights are as far out as mine were, this would affect the weight on each corner...

On locking brakes specifically, I took my car to Brands recently (although wasn't able to really push it due to the aforementioned suspension issues) and over heated the front pads. Once glazed over I found I have MUCH less braking performance. In fact, I've just been out and bought a new set of Red Stuff pads (I doubt the brand/version makes a lot of difference - just make sure they're all the same) which I'm about to have fitted. The amount of material left on the pad will make no difference - they can still be knackered with 80% left to go. Fortunately you can check this simply by looking at the surface of the pad and make a decision whether or not this is the cause. You won't have to discard the pads without knowing.

I race in the Fun Cup series, where pad material and choice of tyre is fixed. We recently had the same issue with the car - pads glazed over and the difference is staggering. At least double the pedal force was necessary to bring the car to a halt. As soon as the pads were changed we were back in business.
It is possible to bring knackered pads back to life by sanding the top surface off, but personally, for the sake of £90 or whatever I'd suggest fitting new ones.

Tyres are again an issue. I don't know about the use your car gets, but if its a garage queen the tyres may have been on there a long time. Even if they have plenty of tread, after a few years the rubber goes off and they'll be useless. This is even more noticeable in the wet.
Again, I changed my "good" tyres on mine, and immediately found a huge difference in traction from the back, and lateral grip. If you have an older set of rear tyres this could be adding to your problems.

Another racing reference to old tyres: Although we race with standard tyres the dry sets are ground down to remove excess tread depth, and wet sets are left with the full groove as you would expect.
Last year we didn't have a single wet race, so our wet tyres had been sitting in the truck unused.
This year when we did need them, we were 10 seconds a lap off the pace (around Croft). We double checked everything, tracking, dampers, camber etc etc to no effect. Eventually threw a new and, we thought, identical set on the car and immediately found our 10 seconds.

Sevenman

Original Poster:

590 posts

115 months

Wednesday 5th September 2012
quotequote all
black11s said:
My tuppence worth:
That was at least thruppence worth - Thanks for all the info.

I would be interested to see your suspension write-up. Track V Road have a good reputation and seem like very nice blokes. The Tamora setup seems good, but I have not driven, or been a passenger, in other TVRs. The car has been driven by Dom at TVR Power who had no comments on the ride, and the instructor at CAT driver training liked the on-limit handling (in a short test).

black11s said:
Ride height was wrong, spring rates were wrong, dampers then had to be overly stiffened to compensate etc.
After a few hours there (and a few quid admittedly) I have a completely different, and vastly better, car.
If your ride heights are as far out as mine were, this would affect the weight on each corner...
If I had that sort of thing done I would probably go to Chris at Centre Gravity, who has set up performance cars for friends and has a very good reputation.

black11s said:
Fortunately you can check this simply by looking at the surface of the pad and make a decision whether or not this is the cause. You won't have to discard the pads without knowing.
The garage had checked the pads - whether that included pad surface as well I don't know.

Changing the fronts, which are of unknown age, could be an option.

black11s said:
Tyres are again an issue. I don't know about the use your car gets, but if its a garage queen the tyres may have been on there a long time. Even if they have plenty of tread
When I bought the car it had old Toyo T1-S tyres on, which hadn't been made since about 2004 and I think the sidewall dates were 02 (front) and 03 (back) (See pic at this link if interested). The Tamora had only done 13'500 miles from new. So, despite having plenty of tread, I changed them for the new Toyo T1-Sports back in April, and have already put 3500 miles on them.

I am going to have to assess all the information, try to do a bit of looking at the car on a ramp, and come up with another plan to get it sorted. It might end up back at TVR Power as they are a lot closer to me than Track V Road, but are still 60 miles away. Having a specialist car is a bit awkward. I knew I should have gone for a Peugeot 206CC smile