Brake balance

Brake balance has been an ongoing issue with the Tamora since I bought it and this problem (along with a house move, a baby, and the fueling issues) is the reason the car has yet to do a trackday. Which is disappointing.

I first found there was a problem when we went to CAT Driver Training at Millbrook in February 2012. This was something I had planned to become familiar with my first powerful RWD car before doing trackdays.

The initial exercise was threshold braking on the mile straight. This revealed that the car will lock its rear wheels easily before the front wheels, resulting in plenty of smoke initially, and not great braking. In further tests I was having to judge my braking to the point where the rear wheels were on the edge of locking and the fronts not doing as well as much as they could. This is an inefficient and unstable way to brake.

We had to cut our driver training day short, but CAT were very good about it and offered to pick-up in the future where we had left off.

Some more information is here (blog) and here (Pistonheads).

Stopping at TVR Power on the way home from Millbrook they had a look at the brakes (and at a drip from the radiator). A quick visual inspection had shown the brakes were standard bur the rear pads were EBC Greenstuff. The front pad type was not visible without removing wheels / callipers. Jason at TVR Power suggested changing all front and back to the AP Racing provided pads which are Ferodo DS2500 front and DS25HP rear. AP originally designed the braking system for the T cars.

A quick bit of research suggested that EBC Greenstuff may have a much higher coefficient of friction than the AP Racing pads. The EBC site suggests a coefficient of 0.55, whereas the AP Racing site suggests more like 0.44 for their pads when warm, and less when cold. This would mean the rears could have been braking with 37.5% more force than designed if the fronts were cold, and that might well be enough to cause the problem.

I am not sure why the AP pads gave a lower coefficient of friction, but it would be interesting to know the temperature response characteristics of the EBC pads. They may work less well when very hot.
When the car was in with Power for a new radiator and rear pads (April 2012), the front brake pads were examined. The fronts were shown to be the correct AP pads (DS2500) so were left in place and the rears were changed to DS25HP, (a low-noise variant of the DS2500). The front pads were later changed for fresh DS2500 in November 2012).

New rear pads

Dom then took the car for a good test drive, and said the brake balance seemed fine. However, when I got round to testing it, the balance was still off and the rears locked first on dry tarmac. I think the driver’s side rear was locking, not sure about passenger side. If they both locked I think it would stall the car which I am sure I would notice.

Next up was a 3000 mile European road trip. This was fine – the brake problem only manifests itself right at the limit of braking, and I wasn’t pushing that hard.

Following the road trip the car was due its annual service at Track v Road, the garage that sold us the car.

They suggested fitting an AP Racing adjustable brake bias valve in the rear brake lines. They checked the front brakes and thought they were fine.

A write-up of the service, with some pictures of the valve installation, is here.

Brake bias valve with brass knob at back left of pedal box

I had hoped the valve would fix the problem. But it didn’t. What makes it worse is that the valve rattles when it gets towards the end of its adjustment, and the end when it should do the most to fix the bias issue. See this writeup for details of the testing and results. See this thread on Pistonheads for some discussion of the problems.

When the car was with Neil Garner for suspension work (which improved the setup), he drove it and found the brakes to be ok, but that may have been on a road surface with lower friction than the Millbrook straight, and hence the weight transfer was not as much. Also, it is hard to test such things on the road.

On a second unsuccessful visit to Millbrook (October 2013 – Report Here), the brakes were better, but even with the knob fully out the braking was rather more even front/back than I would like. I want a nice safe bit of front bias. That day went wrong for fueling reasons instead, but I made it to lunchtime…


What to try next?

I had some communication with Ford to find out the specification of the standard brake proportioning valve which is fitted – Ford part 7117274. If I knew how that valve responded, just some simple numbers, then I would know whether the AP valve was adjustable in the right range. However, the initial response from Ford was not helpful. After some further communications it looked like they were going to send me the specification, but they sent me some useless information instead. Ford is a no-go for that kind of detail.

I also spoke to AP Racing a few times. While they may have originally designed the system, in the 90s, the knowledge of the details is lost. But at least they were helpful and tried.

Pads and disks front and back are all as they should be, so this adjustable valve is the only non-standard part. After garages have looked at them I believe all brake callipers etc. are working fine. The Gaz Gold Pro suspension could have some effect on weight distribution. I think the rear damping needs to be increased since it can get through the travel too quickly on certain undulating roads. That may make a slight difference to the weight transfer, but can’t imagine it will be that significant.

Next stop is Topcats Racing – with their experience in car set-up for motorsport, and access to space to drive quickly and test the brakes properly, hopefully this will do the trick.