Rear disc conversion

[amalgamated users from prior forum]

Message originally posted by: Lee
Just completed a Rear brake disc conversion for my Roadster using Dolomite sprint axle, Vw Golf rear brake calipers and Citroen Xantia 225mm Discs. been using it a couple of weeks now and am very impressed with the extra braking power. Look out for full write up in pitstop soon.

[amalgamated users from prior forum]

Re: Rear disc conversion

Message originally posted by: Lee
Couple of more pics

[b_caswell]

Re: Rear disc conversion

Be careful as if you have too much rear brakes the back end can become very lively. If a car on SVA locks its rear wheels first it will fail!!

[danny_nelson]

Re: Rear disc conversion

I have recently fitted disc brakes to the back of my Berlinetta, and I have fitted a Wilwood proportioning valve to facilitate the balancing of front to rear brake bias.

[amalgamated users from prior forum]

Re: Rear disc conversion

Message originally posted by: Lee
While it was on the fancy steel wheels I had it on the local MOT brake testing machine and the brakes are more bias towards the front.But watching a bias valve on ebay and may fit it anyway..........

[jon_cox]

Re: Rear disc conversion

Brake bias is fairly simple. As a good rule for a rear wheel drive car the bias should be 60-70% front. For a front wheel drive car add another 10%. In theory, under normal driving conditions your rear brakes do very little work. When braking the cars centre of mass moves forward increasing the loading on the front suspension / tyres therefore giving the front tyres more grip. This in turn reduces the weight at the back of the car making it much easier to lock the rear wheels under heavy braking. However, if the rear wheels are still being driven by the engine then this reduces the chance of locking up the rear wheels which is why your driving instructor taught you to depress the clutch pedal right at the last moment while doing the emergency stop. Then they went and invented ABS which means this is no longer true.
Lesson over, class dismissed.

[amalgamated users from prior forum]

Re: Rear disc conversion

Message originally posted by: Sue Roedel (MOC Editor)
Blimey, this is WAY over my head!

[tymill]

Re: Rear disc conversion

Hi Jon, I agree with your points about acceleration and the changing forces on the various wheel contact patches, but I think it could be a bit misleading to say that the centre of mass is moving around. The centre of mass of a car doesn't move around under acceleration in any direction - The centre of a car's mass just is where it is...

The key to understanding why there is additional force pushing down on the front of car when braking is how the centre of mass (and thus the car's inertia going forward) is significantly above the ground level. At ground level, however, the friction of the road against the tyre is pushing straight back parallel to the ground.

The inertia going forward and the push going backwards aren't quite lined up "head-on" with each other - thus a moment is created that causes the car to pitch forwards. This puts more total force pushing the front wheels into the ground, and less total force pushing the back wheels into the ground.

And since friction is mostly a function of how much force there is pushing the wheel straight into the ground - the front wheels get that much more traction when a car slows down.

The closer the centre of mass is to the ground, of course, the less of an effect the acceleration (in any direction) of the car will have on changing the loading on the various wheels.

This is why many types of race cars have as low a centre of mass as possible. And why old-style dragsters would have their engines mounted up as high and far back as possible - to maximise force on the driving wheels when accelerating forwards...

Luckily, Marlins have pretty low centres of mass and quite an even weight distribution in the first place - so they are less affected by these effects than most other cars.

I don't mean to step on anyone's toes, but I love how the physics of these things can actually be quite elegant when you get some models. But hey - I'll get off my science-teacher soap box!

Cheers,
Ty

[tymill]

Re: Rear disc conversion

a nice diagram.
From Brian Beckman's work reproduced on http://www.virtualracersedge.com/physics_of_racing.htm

[jon_cox]

Re: Rear disc conversion

Ty,
I was trying to keep it simple. I agree the moments and inertia play a huge part. But the result of the turning moment around the front axle is that the CofG moves forwards and higher. This is why they designed anti dive front suspension.

[b_caswell]

Re: Rear disc conversion

Sorry Sue this is not over your head its under your right foot.

Whats the advantage of uprating the rear brakes if we then put a proportion valve in the system to reduce the effort going to the rear. I can under stand a load compesator valve being on a saloon or van or lorry?

[amalgamated users from prior forum]

Re: Rear disc conversion

Message originally posted by: Sue Roedel (MOC Editor)
Brake pedals are over my head when I'm stuffed upside down in the footwell trying to connect a stupid little dashboard bulb that refuses to locate....Stuff it, leave it out and have dim lights.