Clubsport Rear Brake Upgrade
Discussion
My aged (but lovely) Clubsport was originally fitted with the tiny Willwood twin-pot Calipers, with correspondingly tiny pads.
Having come by some of the later spec rear uprights and some of the Willwood 4-pot small bore Dynalite callipers (thanks Bert
), I’m now in a position to upgrade the rear brakes to the later spec.
Unlike the Willwood 4-pot large bore Dynalite callipers on the front, the rear ones have thin pressed steel rubbing strips at each end of the brake pad aperture. If I remove these, then the pads from the front callipers will fit the rear callipers.
The question is, what pad pattern should I be using? Same as the front but back plate filed to suit (would need to remove less than 1mm of plate length)? Some other dedicated pad? Or perhaps I could just discard the rubbing strips, which would allow the pad back plate to bear on the (relatively soft) aluminium calliper, as in the front callipers.
Thoughts please
Having come by some of the later spec rear uprights and some of the Willwood 4-pot small bore Dynalite callipers (thanks Bert
), I’m now in a position to upgrade the rear brakes to the later spec.Unlike the Willwood 4-pot large bore Dynalite callipers on the front, the rear ones have thin pressed steel rubbing strips at each end of the brake pad aperture. If I remove these, then the pads from the front callipers will fit the rear callipers.
The question is, what pad pattern should I be using? Same as the front but back plate filed to suit (would need to remove less than 1mm of plate length)? Some other dedicated pad? Or perhaps I could just discard the rubbing strips, which would allow the pad back plate to bear on the (relatively soft) aluminium calliper, as in the front callipers.
Thoughts please
Martin B said:
My Clubsport has 4 pot calipers front and back and all the pistons are the same size (fronts are vented discs, rears are solid), are you sure they are the normal ones fitted to the Clubsports?
It sounds like Radical explored different rear brake options on the Clubsport/Propsort. With your setup Martin (same piston area at front and rear callipers), I would assume that basic brake balance is achieved by the use of master cylinders with different bore sizes (smaller bore on the front to achieve greater pressure in the front brake lines and hence greater effort at the front pads). Smaller calliper piston area on the rears than on the fronts (as in my case) will have much the same effect, ie, forward-biased brake effort.Mark, thanks for the offer - I'll be in touch. Andy, ditto.
Cheers L
It is down to the leverage to the tyre contact patch and the speed of rotation of the disc in relation to the tyre contact patch. The trend of going to massive discs on cars (330mm dia etc) with very low profile tyres makes the disc perifery speed closer to the speed of the tyre contact patch, the smaller the disc is in relation to the tyre OD then the disc perifery speed reduces greatly and hence the effectiveness of the brake.
I will have a think about how I can explain it a bit better!
I will have a think about how I can explain it a bit better!
Hi Martin - I've given some thought to your point about the effect of wheel rotation speed on brake effectiveness 
I think you’re right about the rear brakes being less effective due to the larger rolling diameter of the rear tyres, compared to the fronts, but I would explain it simply in terms of the relative “leverage ratios”.
By this, I’m referring to the ratio of the disc pad radius to the tyre contact patch radius (both measured from the centre of rotation, i.e., the axle centre line).
Taking the example of a 240mm disc. Let’s assume the centre-of-pressure of the disc pad acts at a radius of 100mm (i.e., 20mm from the free edge of the disc). On the front, we have tyres with a 500mm rolling diameter (250mm rolling radius), say. On the rear, we have tyres with a 550mm rolling diameter (275mm rolling radius).
So,
Leverage ratio for front brakes = 100:250 = 0.400
Leverage ratio for rear brakes = 100:275 = 0.364
Relationship between front and rear brakes = 0.400:0.364 = 1.09
In other words, for this case, the front brakes will be 9% more effective, all other things being equal.
Going back to your point about the front and rear wheel speeds of rotation being different, you can use this fact to construct an argument around the idea of the energy absorbed by the disc. If the pad contact force is same front and rear, but the fronts are rotating more quickly (1.09 times as quick), then the fronts absorb more energy. In other words, the front brakes are more effective at converting the kinetic energy of the car into heat energy in the discs (hence the need for the front discs to have more mass and or more cooling).
So it’s just a case of whether you want to think in terms of Forces Applied (per my argument) or Energy Absorbed (per your argument).
Has it stopped raining yet so I can go out and play?
I think you’re right about the rear brakes being less effective due to the larger rolling diameter of the rear tyres, compared to the fronts, but I would explain it simply in terms of the relative “leverage ratios”.
By this, I’m referring to the ratio of the disc pad radius to the tyre contact patch radius (both measured from the centre of rotation, i.e., the axle centre line).
Taking the example of a 240mm disc. Let’s assume the centre-of-pressure of the disc pad acts at a radius of 100mm (i.e., 20mm from the free edge of the disc). On the front, we have tyres with a 500mm rolling diameter (250mm rolling radius), say. On the rear, we have tyres with a 550mm rolling diameter (275mm rolling radius).
So,
Leverage ratio for front brakes = 100:250 = 0.400
Leverage ratio for rear brakes = 100:275 = 0.364
Relationship between front and rear brakes = 0.400:0.364 = 1.09
In other words, for this case, the front brakes will be 9% more effective, all other things being equal.
Going back to your point about the front and rear wheel speeds of rotation being different, you can use this fact to construct an argument around the idea of the energy absorbed by the disc. If the pad contact force is same front and rear, but the fronts are rotating more quickly (1.09 times as quick), then the fronts absorb more energy. In other words, the front brakes are more effective at converting the kinetic energy of the car into heat energy in the discs (hence the need for the front discs to have more mass and or more cooling).
So it’s just a case of whether you want to think in terms of Forces Applied (per my argument) or Energy Absorbed (per your argument).
Has it stopped raining yet so I can go out and play?
I have experienced the difference in braking due to tyre size on my old Morris Minor hillclimb/sprint car (450 bhp).
It has 13" wheels with 280mm discs on the front, with the 20" dia Avon slicks the brakes were imense, then with the 13" road wheels (yes street legal!) and 185/60 13 tyres the brakes were no way near as good! The tyres only had probably 2" difference in OD but the difference in braking was/is huge.
It has 13" wheels with 280mm discs on the front, with the 20" dia Avon slicks the brakes were imense, then with the 13" road wheels (yes street legal!) and 185/60 13 tyres the brakes were no way near as good! The tyres only had probably 2" difference in OD but the difference in braking was/is huge.
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