New S-Caliper Pistons are on the Way

[Neunelfer]

I started digging into 930 calipers and Brembo 951 calipers and I got the idea that we simply needed to update the pistons for S-Calipers.

Samples have been cut and approved and the hard coating has been completed. We're starting to make pistons out of aluminum for our aluminum calipers. The thought came when we kept replacing pistons with stainless steel versions due to dis-similar metal fusion and flaking original pistons. I thought; why dis-similar metal when we could use similar metal? 930 calipers have aluminum pistons. 951 calipers have aluminum pistons. Some 908 calipers have aluminum pistons... why not S-Calipers.

The coating process was key for a street car. As technology progressed, so did the coating process. These new aluminum pistons will be harder and more durable than their original or stainless counterparts. Anodic Oxide Coating makes them the perfect piston for the S-Caliper.

They are feather weight was well. These weigh 1/2 of what a stock S-Caliper piston weighs.

Here's how they're coming along. Piston on the left is a test blank. Piston on the right has the Anodic Oxide Coating.

[Flieger]

Man, Eric. You are killing me. I will need to find an excuse to rebuild my still pristine S calipers. I agree that this is a significant improvement. Less weight, less corrosion, less heating of brake fluid. Maybe I will get some of those leightweight Brembos for the rear, too.

Some day...

[OliveR]

Great Eric !!!
Please keep me updated as soon as they will be available...

[chris_seven]

. Less weight, less corrosion, less heating of brake fluid.

Max,

I agree that they will be lighter but there are other issues that need to be considered.

There is no dissimilar metal corrosion issue between an Austenitic Stainless Steel such as an EN303 which was a common piston material that used to be used by AP Racing. The surface of the stainless is sufficiently passive (see Pourbaix Diagrams) for this not to be an issue.

The thermal conductivity of aluminium is much higher than that of austenitic stainless steel and this will surely increase the heat transfer into the brake fluid - I can't see how it can be reduced.

Many calipers with aluminium pistons need alpha/beta Titanium or Mica Thermal Barrier plates fitted to avoid fluid overheating.

One manufacturer of aluminium pistons claims they are more effective because they transfer heat away from the pad - I agree with this but they will conduct it straight into the brake fluid.

Titanium Pistons, which are common in F1 are used to reduce heat transfer. Ti alloys have a relatively low coefficient of heat transfer and are beneficial. These pistons clearly need surrace treatments to avoid galling. Titanium also has virtually identical expansion to mild steel and wouldn't impact on seal compression.

Seal Clearance will also change as the expansion of an Aluminium Piston will be greater than the equivalent steel piston and this will influence seal compression and hence the manner in which the seal pulls back the piston. I can see an issue here.

Anodic Oxide Coating - commonly known as anodizing - and looking at the finish these pistons have been hard anodized and this may also be an issue.

I would agree hard anodising will give great wear resistance but it will also increase seal friction and again influence seal roll back. Hard anodising is commonly applied to Vee belt Pulleys as it confers excellent wear characteristics and also improves belt grip by a significant degree - It does give a significant increase in friction.

It is possible to PTFE impregnate hard anodised surfaces but i am not sure how this would react/interact with brake fluid, but may be worth investigating.

It is clear that aluminium alloy pistons are used in quite a number of aftermarket calipers and this issue of seal compression/friction can be dealt with but the depth of the seal groove and its profile may need to be changed to maintain the correct friction behaviour.

I think aluminium is attractive because it is cheaper to make this type of piston as 6082-T6 can be machined to a very high level of surface finish and you can avoid the cost of centreless grinding but for a serious race car I would stick with Stainless Steel or you may have to do some detailed development.

[Flieger]

Hi Chris,

By "less corrosion", I meant compared to the original, non-stainless steel pistons. I have the stainless steel pistons right now in my S calipers I got from Eric.

By "less heating of brake fluid" I meant that it appeared to me at first that the piston was of the "modern", "cup" design, where there is a significantly larger hollow area between the pad and piston surface, with only the raised ridge around the rim contacting the pad. Now that I look more closely, I see that the view on the right is a backside view, so my first impression was incorrect. I agree with you that Aluminum used in the original style pistons will increase heat transfer.

As to the seal clearances, would not the Aluminum piston seal better since it more closely matches the rate of thermal expansion of the caliper body? The seal clearance with the stainless steel piston should get larger but the Aluminum would stay closer to the "static" clearance?

How much experience do you have with the Titanium sheet metal backing plates? How well do they work for thermal isolation? I would be interested in either temperature data or in qualitative analysis, e.g. "The only change I made was the Titanium backing plates between track sessions and the brake fluid stopped boiling".

Thanks for sharing some of your wealth of knowledge, as always.

~Max

[Neunelfer]

Stay tuned on the rears Max. That's all you'll get out of me!

These are Hard anodized. They are going in my calipers... probably not Chris's though... LOL.

[chris_seven]

Max,

I agree that you would think that the expansion would be 'better' but the seal groove will have been designed to suit the piston material that is being used , hence my comments about changes of radial compression due to the greater expansion. Using an alloy such as 4032-T6 may help as it has lower thermal expansion but I am not sure how well it will hard anodise due to the increased silicon but it would machine well in a T6 condition.

In the limit too much seal compression and a high seal friction could cause the pistons to stick when they are hot. I would imagine that this is an unlikely scenario but some testing is needed to get the piston diameter correct annd keep the seal characteristics. I don't think they would just be a 'like for like' swap. It should be fairly easy to calculate the correct cold clearance.

I have used Ti plates (6AL4V Grade 5 - 1mm thick) on a Mk1 Lotus Cortina Rally Car that used to overheat Castrol SRF !! They seemed to work reasonably well and improved the brake pedal, which was becoming a problem after about 10-12 miles. The longest stage we used them on was 20 miles and we never had any temp measuring equipment available.

We have now modified the rear brakes and improved the general braking of the car but we have left them in place as thet don't do any harm.

If you make some be sure to use an Alpha-Beta Titanium Alloy as pure Ti has a much higher heat transfer coefficient and won't be as effective. We water jet cut them as the edge finish is better than with a laser.

I am tempted to try a Mica plate in future as SWB 911s with original calipers and solid discs can develop a long pedal during a 2-3 hour race. Mica may be a bit fragile and may need regular replacement but should be an even better barrier.

The thermal coefficient of 6Al4V is about 7 W/m^2K. Aluminium is around 220 W/m^2K and EN303 Stainless Steel is around 16W/m^2K. Mild steel is typically 45 W/m^2K

Eric - I probably will stick with EN303 as we have used it for years with good results.

[Flieger]

Thanks for the tips about the Titanium alloy and the Mica backing plates.

I am not sure if the seal friction is a bad thing, since the deformation of the square seal is part of the piston retraction mechanism, like a diaphragm spring in a clutch pressure plate.

[chris_seven]

Max,

I do agree that some friction is needed to pull back the piston and it is the control of what goes on that is important.

The diagram you have shown is correct in principle but I am not sure the detail is quite right.

The seal groove shown is wider than the square section seal which isn't quite correct - the groove is normally parallel sided athough very modern calipers do have grooves that are not flat bottomed. I have seen than with about a 10 degree taper. This is done to cotrol the raial compression and modify the way the seal retracts the piston.

I think that the seal deforms in the radial clearance gap between the piston and cylinder and clearly the stored energy pulls back the piston.

Too big a gap and a high seal friction can cause the pad to retract too far, hence residual valves or anti knock back springs. (Just as a point of interest the pins and bushes fitted in the S calipers are anti knock back devices)

If the clearance is too small there will be insufficient deformation and the piston may not retract. If there is a high seal friction and no seal deformation I think this problem could be worse but I would agree friction in this case may not be a major issue.

[Flieger]

Ah, yes. The "knock-back" mechanisms. http://www.early911sregistry.org/for...n-Step-by-Step

Thanks for your insights. I think I will stick with the stainless steel. Unless Eric can make some inverted/cup pistons by removing the knock back mechanism.