Volvo caliper upgrade.

[abowie]

Girling front brake callipers as fitted to the Volvo 240 are sometimes fitted as upgrades for series 1 E Types. I have been looking at this and am confused as to the benefits. The standard 2 1/8" front callipers have a total piston area of 45.76cm2. The Volvo callipers have 4 pistons, but only 36mm ones, giving a total of 40.72cm2. The Volvo callipers can be used with 1/2" thick discs as opposed to the 3/8" discs for the standard series 1 which would presumably offer some improvement in heat dissipation. Otherwise, on paper they have less piston area. So are they actually an improvement?

[PeterCrespin]

Larger pad area, lower unsprung mass?

[Zei220]

The smaller piston will apply a higher pressure to the pads.

[Dave K]

Larger pad area, lower unsprung mass?

As Pete says larger pad area, its a good cheap upgrade I ran them for years and have a write up with photos floating around the internet somewhere.

Dave

[abowie]

As I understand it the critical thing for stopping a car quickly is braking torque. This is the product of the disc radius, the clamping force of the caliper and the coeffiecent of friction of the pads.
The clamping force is the product of the brake line pressure and the caliper piston area. This is what changing the front calipers will affect, and the thing that confuses me about the Volvo swap is that it should be lessened because the piston area is decreased. Pad area doesn't affect clamping force; it only affects pad longevity and fade resistance by improving heat dissipation so bigger pads should offer some improvement in fade if you were using the brakes on a track for example. Brake line pressure is a solely a function of the master cylinder and pedal design and will not change if the calipers are altered.
The Volvo calipers are heavier than the stock ones so they don't improve unsprung weight. The increased mass of the caliper would however help heat dissipation and therefore improve fade.
Being able to change to a thicker or even a ventilated disc would also inprove heat dissipation and therefore fade. Both of the accounts of installing the Volvo calipers I've seen have involved using the stock series 1 discs though.
So I remain confused. In essence using the Volvo caliper should offer improvements with fade in a track situation but if anything the applied torque would be less because of the decreased clamping force and possibly because of a decrease in the disc radius.
Dave, what benefits did you find? The reason I'm interested is that I've bought a set of calipers (non vented disc ones) that I might fit to my 3.8 OTS.

[christopher storey]

Pad area most certainly does affect the force which can be applied to the disc. In the first place, it affects the mean lever arm which is exerted by the pads, and in the second place it affects the total force applied by the hydraulic pistons. This is most easily illustrated by considering the case where the pads were only , say, circular and 1/8 inch diameter . In such circumstances, however much force was applied to the pads by the pistons, you would get next to no retardation because the swept area of the disc would be minimal

[Dave K]

I forgot to mention you can use S2 discs which are 1/2" thick against the 3/8" standard S1 discs.
Another good idea is to use 'Greenstuff' pads which work really well with the upgrade.
What benefits did I find, better overall braking in general.

Dave

[abowie]

Pad area most certainly does affect the force which can be applied to the disc.

I don't think that it does. All that it can do is to spread the pressure applied by the master cylinder to the caliper piston over a greater area.

In your example the force applied to the disc is the same as with a larger pad, just over a tiny area. This causes the brake to fail because of overheating and material failure of the pad. Presumably, as long as pad area is greater than that required to prevent acute failure of the pad on braking it will still work. This of course applies to one stop only; repeated stopping relies on better heat dissipation, which the bigger pad should give.
The mean lever arm issue depends on where on the radius of the disc the pad contacts, not on it's size per se. Again in your example if the tiny pad's size means that it's mean lever position is moved inwards, thus decreasing the radius, then it would decrease braking torque. The Volvo pads are longer but not taller than the stock ones so this probably doesn't have much effect, although I haven't actually measured it.

Using the 1/2 inch discs would be a good idea, and I routinely use the Greenstuff pads with their better coefficient of friction.

Dave did you get better pedal feel, subjectively better stopping on hard stop, less brake fade or generally easier brake feel on normal driving?
My concern using the stock brakes in the 3.8 is that they won't pull up as quickly in an emergency stop. This is why I'm looking at potentially replacing the fronts if they don't come up to scratch. I have put Coopercraft 4 pot calipers on one of my 4.2's and the other came with Volvo's, but with 24mm ventilated front discs as well.