Sevenman's TVR (was SAAB) Blog

September 2, 2012

Brake bias tests

Filed under: TVR — Administrator @ 9:03 pm

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.
An update on the ongoing brake bias struggle…

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 (see previous post), 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).

Valve

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. 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.

There is some discussion of the problem on this Pistonhead Thread

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.

Roof down

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