Hi all,

The GBS Zero I'm helping a mate build will have a Saab 2.0 turbo engine with around 200bhp and will rarely see any track action but at the moment he wants to stick with the Sierra donor 240mm vented discs, standard calipers, and rear drums. They'll be unservo'd as per most kits as well.

The Zero with that engine will be no lightweight special, probably quite hefty once its all trimmed out nicely with weather gear too.

Is the general opinion that this setup is suitable for a kit car? I kind of think that although there shouldn't be prolonged heavy breaking, the 1.6 sierra stuff is a bit feeble really, especially without assitance as it would have been in the donor. Any opinions one way or the other?!

[Edited on 16/3/10 by GRRR]

Should be fine - there are lots of track cars being driven very aggressively using unservo'd standard Sierra brake components and they don't struggle to stop. Even if the car is lardy by se7en standards, it's still an awful lot lighter than the Sierra....

Tom

im building a 200hp (i hope) dax rush with standard sierra gear.

wilwoods "upgrades" etc are mainly aimed at reducing weight rather than giving significantly greater braking effort.

It'll still be a lot lighter than a Sierra, and the servo doesn't actually improve the brakes, just makes the pedal lighter.

Fortunately it's one part that pretty easy to upgrade with bigger disks and calipers if you feel the need, so why not suck it and see?

how much does a sierra weigh? about 1250kg?

not something i've given much thought, but brakes are there to stop the momentum, not power. i guess the weight it has to stop is the main concern. his car will be a fair bit lighter than the sierra the brakes came from

A suitable master cylinder is the main change you could make in order to get the pedal effort down to a sensible level. A Sierra MC is 22mm which is a bit big. Several people [me included] have changed to a 19mm MC and are very happy with the results. Its a fairly cheap modificaiton as well - either a Fiesta non servo MC or something similar. Mine came from a Fiat 124. and cost £20 new.

Otherwise the brakes will be fine.

Stu

[Edited on 16/3/10 by whitestu]

Brakes absorb energy to stop a car. The amount of energy to stop (simplistically ignoring wind resistance etc) is 0.5*m*v^2

So for the same velocity (i.e. speed) half the weight requires dissipating half the energy - this is why light cars can struggle to warm up big brakes.

Your car will weigh a lot less than half the mass the sierra brakes are designed to stop (1250 kg car + 4x100kg adults + luggage with a safety margin for towing)

The squared velocity is a killer for tuned tintops which will (on the track) go a lot quicker and, even if lightened, require better brakes - but a 7 will struggle with top speed simply due to the aerodynamics until you fit really big engines.

In summary you'll be fine so long as the brakes are in good condition, and as MikeRJ says it's easy to upgrade later.

[Edited on 16/3/10 by iank]

Mainly problems are usually high pedal pressures caused lack of pedal leverage --- because of space restrictions pedal mechanical advantage is less than half that of Sierra or similar combined with lack of servo the pedal pressure is going to be a lot higher than modern drivers are used to despite the much lower weight of the car.

Way round as already mentioned is a smaller bore master cylinder --- any suitable 17mm to 19mm bore tandem master cylinder works fine.



[Edited on 16/3/10 by britishtrident]

Cool thanks for the replies! This forum's great, you get a Physics lesson at the same time

Will have to investigate the alternative master cylinders.