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      09-07-2022, 09:47 PM   #23
aerobod
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Quote:
Originally Posted by dradernh View Post
If I wanted to get the most out of a shim made from any material, I'd first have Swaintech's White Lightning applied to the pads' backing plate or to the backing plate side of the shims: https://swaintech.com/race-coatings/...aust-coatings/.
Yes, that would have significantly more effect than any metal shim (as long as it isn’t compressible to any degree, which it shouldn’t be, and is very even in thickness). It would be interesting to see if it would stay bonded to a shim without cracking under brake pressure.

That type of ceramic exhaust coating can have thermal conductivity as low as 1.0 W/mK, about 4% of that of titanium. The standard thickness is about 0.4mm, so new pads may have to be shaved a bit to fit, but a 0.5mm steel shim coated both sides for 1.3mm total thickness could work and reduce the thermal conductivity overall by about 30%.

On the other hand just coating the pad backing may prevent any cracking, with just the 0.4mm coating giving about 15% reduction in thermal conductivity.

Last edited by aerobod; 09-07-2022 at 09:52 PM..
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      09-07-2022, 10:34 PM   #24
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Originally Posted by aerobod View Post
Yes, that would have significantly more effect than any metal shim (as long as it isn’t compressible to any degree, which it shouldn’t be, and is very even in thickness). It would be interesting to see if it would stay bonded to a shim without cracking under brake pressure.
My experience with Swaintech is that they likely know and would certainly be willing to share the likelihood of cracking due to pressure under these circumstances. I took my first job to them in person, and the impression I developed was that they are a very professional operation.

I used their White Lightning product on my E36 M3 race car's S50B32 headers and the reduction in underhood temperatures was significant. I didn't collect any measured or comparative data, but the anecdotal lift-the-hood-and-feel-the-difference-in-the-amount-of-heat-coming-off-the-engine experience was telling.

I used Swaintech's product on the race car after having first used it to good effect on a tuned Volvo V70R and then after being told by my shop that the greatest general danger to the longevity of the S50B32 was heat buildup/retention in the head. The shop owner had an advanced degree in internal combustion engines, so I was inclined to listen to what he had to say.

The basic instructions after I bought the car were to get the hood up immediately after exiting the car in the pits. That got me used to how much heat was coming off the engine in ambient temperatures ranging from 35° F to 95° F. As I say, after having the Swaintech coating applied, it was evident under all circumstances that it was reducing underhood temperatures by what I perceived to be a significant amount. It didn't hurt that the ceramic coating, along with gold foil on the firewall and transmission tunnel, significantly reduced the temperature in the driver's chair. Before that, I was baking in that chair on days above 75° F.

I thought Swaintech's prices were reasonable for the value they provided. That's going back a dozen years now, so I can't comment on how that is today.
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      09-08-2022, 06:02 AM   #25
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Originally Posted by aerobod View Post
Sounds like a lot of theory there, that would require controlled testing with thermocouples at each interface. The most important time for thermal conductivity is when the brakes are applied and any gap will be minimized under brake pressure. The rotors cool very quickly after that in the air flow. If they are up to cherry red at 900C or so, as soon as brake pressure is removed they go back to dull, dropping 500C or so in a few seconds, so any gap between the pad, shim and piston isn’t particularly meaningful when the brakes aren’t being used, the thermal capacity and conduction under brake pressure is much more important.

Fully blind testing with thermocouples on brakes either side with different shims fitted either side would be needed. I would be surprised if there is more than 5C drop on the back of a titanium shim 0.5mm thick compared with a steel one.

I’m sure manufacturers of them will claim otherwise as they have an expensive product to sell (even though the material cost by weight is only a couple of $), but they would need to provided properly conducted experimental results to show any meaningful improvement. Nothing to lose if they were $5 a set, though.

The point about non-brake use time is that the Ti shim/plate is not subject to a heavy thermal input like it is during the transient brake application period, and being significantly cooler than the pad backing plate (and due to its very low mass has a much faster transient response) it functions very nicely as a radiation heat transfer shield for the dust covers. In this case theory has equated to years of successful application by many (including myself since 2004). Just evaluate the radiation view factor between the pad backing plate and the piston dust covers - it's essentially 1.0 without the Ti shim while it becomes a much different condition once the Ti shim is inserted. That very thin Ti shim runs at a much lower average temperature than the pad backing plate (i.e. simply consider the thermal mass of the pad material and backing plate). Note while rotor thermal condition is obviously important, the discussion here is centered on what the dust covers "see" from a thermal radiation viewpoint (i.e. their view factor) which is essentially near 1.0 to the pad backing plate without the Ti shim in place.

You can't measure thermal interface resistances between the brake piston(s), Ti shield and Ti shield to backing plate with thermocouples very easily. As always, there is a lot of error in tc location/adhesion in addition to its own thermal mass and response needing to be accounted for in results analysis. The easiest way would be to (in a lab) apply a tc to the brake piston at a location as close as possible to the interface, and then have a tc inserted into the backing plate to a small distance from the interface (by drilling the pad and backing plate). As you know thermal contact resistance depends on the surface roughness of each of the four surfaces, surface hardness, contact pressure and any interstitial material present. The additional thermal resistance of the two thermal interfaces is a significant addition to the total thermal resistance path between the pistons and the backing plate. There is a vast quantity of empirical data on contact resistance of all types of thermal interfaces one could peruse if desired. Designing an experiment to measure it is easy if you have access to the equipment albeit a bit costly.

Sorry for going off on detailed tangents here...I'm an old (emphasis on old) engineer who spent years doing thermal design, analysis, testing, etc, in the old days (not on brakes but on military computer design which depended heavily on conduction (with numerous thermal contact resistances) and convection...hence I can easily fall back into that role...and bore people like no end.
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Last edited by CSBM5; 09-08-2022 at 06:23 AM..
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      09-08-2022, 09:01 AM   #26
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Quote:
Originally Posted by CSBM5 View Post
The point about non-brake use time is that the Ti shim/plate is not subject to a heavy thermal input like it is during the transient brake application period, and being significantly cooler than the pad backing plate (and due to its very low mass has a much faster transient response) it functions very nicely as a radiation heat transfer shield for the dust covers. In this case theory has equated to years of successful application by many (including myself since 2004). Just evaluate the radiation view factor between the pad backing plate and the piston dust covers - it's essentially 1.0 without the Ti shim while it becomes a much different condition once the Ti shim is inserted. That very thin Ti shim runs at a much lower average temperature than the pad backing plate (i.e. simply consider the thermal mass of the pad material and backing plate). Note while rotor thermal condition is obviously important, the discussion here is centered on what the dust covers "see" from a thermal radiation viewpoint (i.e. their view factor) which is essentially near 1.0 to the pad backing plate without the Ti shim in place.

You can't measure thermal interface resistances between the brake piston(s), Ti shield and Ti shield to backing plate with thermocouples very easily. As always, there is a lot of error in tc location/adhesion in addition to its own thermal mass and response needing to be accounted for in results analysis. The easiest way would be to (in a lab) apply a tc to the brake piston at a location as close as possible to the interface, and then have a tc inserted into the backing plate to a small distance from the interface (by drilling the pad and backing plate). As you know thermal contact resistance depends on the surface roughness of each of the four surfaces, surface hardness, contact pressure and any interstitial material present. The additional thermal resistance of the two thermal interfaces is a significant addition to the total thermal resistance path between the pistons and the backing plate. There is a vast quantity of empirical data on contact resistance of all types of thermal interfaces one could peruse if desired. Designing an experiment to measure it is easy if you have access to the equipment albeit a bit costly.

Sorry for going off on detailed tangents here...I'm an old (emphasis on old) engineer who spent years doing thermal design, analysis, testing, etc, in the old days (not on brakes but on military computer design which depended heavily on conduction (with numerous thermal contact resistances) and convection...hence I can easily fall back into that role...and bore people like no end.
I can appreciate a shim will make a difference, but the initial premise that titanium will make a significant difference over any other material is not borne out by the difference in thermal conductivity. Also from my aerospace engineering background, if we needed to use a high strength alloy where thermal conductivity was an issue, inconel alloys are a better solution than titanium alloys in almost all scenarios.

The surface roughness comes in to play in terms of surface area to allow conduction, but if only say 50% of the surface is in contact when brake pressure is applied, that would be the same as using a shim twice the thickness and would be an effect independent of material used. If the reflectivity of the metal has an appreciable effect when pressure is not applied to the system due to surface finish, then that is an effect of the type of surface finish as opposed to the material. The same goes for perforated, rippled or “sprung” shims - all effects that can be applied to most metals, not just titanium.

I appreciate the difficulty in taking the measurements, but without the evidence of thermal conductivity benefits beyond what the science indicates, I don’t think it is worthwhile to use titanium shims over any other type of metal shim.
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      09-08-2022, 09:45 AM   #27
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Originally Posted by aerobod View Post
I appreciate the difficulty in taking the measurements, but without the evidence of thermal conductivity benefits beyond what the science indicates, I don’t think it is worthwhile to use titanium shims over any other type of metal shim.
Yep, that's the point I alluded to in my first post above by pointing out that the thin shim is not conducive to providing much thermal conduction resistance in the first place (i.e. since the "L" is so small in basic conduction equation) and why the two contact resistances introduced in the conduction path are significant relative to the conduction resistance of the metal. Ti likely became a marketing gimmick to some extent early on about 20 years back now. I don't think it's worth the expense of Ti versus a cheaper stainless steel with conductivity in the same range depending on the alloying.
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      09-08-2022, 10:09 AM   #28
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Quote:
Originally Posted by CSBM5 View Post
Yep, that's the point I alluded to in my first post above by pointing out that the thin shim is not conducive to providing much thermal conduction resistance in the first place (i.e. since the "L" is so small in basic conduction equation) and why the two contact resistances introduced in the conduction path are significant relative to the conduction resistance of the metal. Ti likely became a marketing gimmick to some extent early on about 20 years back now. I don't think it's worth the expense of Ti versus a cheaper stainless steel with conductivity in the same range depending on the alloying.
Yes, we have been saying the same thing in some respects, the titanium seems to be a marketing gimmick, especially as some stainless steels have similar thermal conductivity. It is about the shim and it's design, as opposed to the material it is made from.

For the amount of material in 0.5mm shims, a set would weigh about 30 to 50 grams. With titanium sheet available in quantity for USD$25 per kg, that would be $1 for material cost, combined with water jet cutting costs of about $0.10 per set if 20 sheets are stacked and say 1000 sets are cut at a time, add in packaging and handling costs and $2 is the likely production cost in quantity. The $100 per set that seems to be a typical selling price does give a nice margin.
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