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MG MGB GT V8 Factory Originals Technical - Brake Upgrade

Front Only Brake Upgrades Increases Stopping Distance!

Double Dutch or not?

Paul
Paul Wiley

Paul
Is this information from experience?

What is the thinking behind it?

Mark
Mark

Surely braking needs to be balanced if there is not to be a risk of one end breaking away at the limits.

RMW

Mark,

A few numbers ball park

OE brakes 60% Front 40% Rear
Upgrade front now 70% Front 30% rear

Maximum front tyre grip at point just before lock up 1500lbs

So OE Front 1500 rear 1000 total 2500lb of stopping force

Upgrade Front 1500 Rear 650 total 2150
Nearly 15% reduction in stopping force!


Paul
Paul Wiley

Do you have adjustable brake balance? Surely with an upgrade more braking is only done on the front because the level of grip permits.

If tyre grip was the limiting factor even for stock brakes then all you will gain through an upgrade is reduced fade.

Are you saying that the OEM 60% bias is adequate to make full use of that 1500lbs of grip all the time? I disagree.

I don't buy into this concept.

Joe
Joseph Phillips

Joseph,

Maximum braking is just before point of lock up (hopefully front).

If the OE system can lock brakes then there is no more front grip available.

Heat is enemy and the reason for upgrading but increasing the fronk brakes stopping power reduces the rear tyre stopping force.

Paul
Paul Wiley

Thats my point. But surely the only way the balance would be effected is if there is more stopping power and grip available at the front.

unless you have adjustable brake balance, the only way to get 70% at the front is if you have better brakes and sufficient grip to make use of the better braking. The better brakes will only work if sufficient grip is available. If grip is the limiting factor then surely the extra braking efficiency will just lock the wheels? so you balance is the same. You will not get more braking on the front even with an upgrade if grip doesnt permit it. However, I don't feel this is the case with my MGB.

And anyway, 30% is still probably plenty for the rear. At Castle Combe I when I was trying to catch my mate I locked the rears twice on the way into quarry. The front didn't lock but the rear was locking all the time.

To be honest I don't really understand where you are coming from.


Joe
Joseph Phillips

God I need to write slower! sorry for all the errors.

Joe
Joseph Phillips

Assuming the fronts locked before (which they certainly should) and after, and assuming the same tyres, road surface and conditions, then all that change in ratio means is that the fronts have the same retardation as before but the rear has been reduced, hence less total retardation overall as your figures show. The only real point in brake upgrades should be to reduce fading if you are going to be putting a lot more braking effort into them i.e. going much faster than the vehicle was originally intended for and/or for circuit work. To get back to the original ratio and hence overall braking effort you will have to increase the size of the rear cylinders - not too difficult on a roadster as the GT items are bigger to cope with the greater weight and will only lock with greater braking effort, but the locating peg is in a different place which means drilling the backplate or replacing those too. A brake balance adjuster will not help on its own as the rear cylinders are already getting the full braking pressure, all an adjuster will do is vary it *down* from that, reducing rear braking effort even more. If the GT items lock on a roadster, or you fit even larger ones that lock, then a balance adjuster would be useful. But after all that you are still going to end up with exactly the same maxiumum braking effort overall, unless what you originally did to the fronts *was* to reduce fading.
Paul Hunt

I think thats wrong. AFAIK all the racers use rear wheel cylinders to take braking effort away from the back, to stop the rears locking.

Joe
Joseph Phillips

Joe,

Racers would normally use sticky tyres which increases grip and therefore weight transfer - hence the need to reduce pressure to rear. Against this the car will be lower reducing weight transfer but the sticky tyres would usually rule.

Paul
Paul Wiley

Up to you guys, but I think your heading down the wrong track.

Even Roger parkers guide to V8 conversions says to use roadster wheel cylinders on GT V8 conversions.

I will be putting MGB wheel cylinders on my MGB GTV8. As I have said, from experience on track the rears will lock.

Joe
Joseph Phillips

Ooops, my comment about having to increase the rear wheel cylinder size before you can use a brake balance adjuster *is* wrong, but for a different reason. I was thinking of a pressure *limiting* valve that goes in the rear line. The adjuster would more or less replace the 4-way branch on the RH inner wing and *would* divert pressure between front to rear as required.

Changing wheel cylinders can go either way. GTs on a roadster gives more braking effort on the rear, roadsters on a GT gives less. The factory V8 GT *does* use roadster backplates and cylinders, but that is because it has greater retardation at the front due to the wider tyres and hence more weight transfer, which is what sticky tyres do. Although if sticky tyres are on the rear too wouldn't the end result be the same? The comparisons I gave above were based on using the same tyres and surface as I said.
Paul Hunt

Paul,

Sticky tyres all round- you increase grip at both ends but now more weight transfer so more front load and have to reduce pressure to rear.

The RV8 with wider tyres reduced rear drum I think.

Paul
Paul Wiley

Paul, on my MGA, I have just installed mgb ft discs with Wilwood pedals and master cyls(3/4") bore. It has a ft/back balance bar between the two brake cyls. I also have very wide(205/65/15") tires.

I haven't set it up yet. Do you have any suggestions?

Thanks, fred
Fred H

Fred

With a balance bar you should be able to maxise system in real life rather than theory
Try this theory

http://www.miata.net/sport/Physics/07-Circle.html

Depends what you are balancing for, but weight transfer IMO (does not count for a lot)changes in wet conditions and fuel load decreasing etc or carrying loads, so preventiong rear wheels locking first for your typical driving conditions.

Paul
Paul Wiley

Fred,

If you wish to ball park,

Then look at weight transfer formulae prob 70F 30R about 1g!
Line pressure is required but without bias this is same both ends

Clamp Force is Line Pressure x caliper piston area

Brake Torque is effective radius x piston area (one side)x Line Pressure x Number of padsx COF of pads divided by 12

If tyres same all round then brake force at wheels is a multiple of same amount.

Paul

Paul

This thread was discussed between 19/01/2004 and 25/01/2004

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