Disk wear limits and track performance.

[amrbose14]

Has anyone any opinions on how much wear a front disk can take before the performance on track is likely to be limited (assume there's no warp)? I've just measured the width of my disks (using modified vernier calipers). I get ~27.5mm. New is 28mm (taken from Clark's garage web site for an early turbo), and the wear limit is 26mm. So they seem relatively fresh. But perhaps under hard use one might expect some problems before you hit the wear limit?

Chris

87 220t.

 

[bmnelsc]

I'd trust Porsche published limits in this regard. They engineered the stuff so they know when serviceability and safety is no longer satisfactory (as they don't like any liability - these limits are probably conservative).

Having said that - your brake enemy on a track car is heat so the thinner the disc the more susceptible to warping from spirited use. IMHO I'd use them until they hit the wear limit or warped - whichever comes first.

Just a comment - the local indies here don't bother resurfacing (or even measuring) discs - they just replace them when you need new pads....the claim is that they don't want to deal with brake squeal issues, etc when reusing existing rotors and presumably the cost of resurfacing plus down time while the work is done exceeds the cost of new rotors. Being old school and against the replace rather than repair culture - this practice bothers me a lot but I can't argue with the logic....

 

[amrbose14]

A quick google suggests specific heat capacity is proportional to weight, so presumably this gets worse as the disk gets lighter but only linearly. .5mm must constitute a very small proportion of the weight. So presumably OK. Possibly even less significant than that since I'd imagine the heat sinks into the hub.

In the context of dealer servicing it is possibly defensible in terms of economy to just replace disks every time, if that is a small cost compared with the labour and other parts. However on my wear rate it does seem that a disk should last ~3 pad changes.

cheers,
Chris

 

[amrbose14]

Does anybody use/own a disk run out tool? They seem to consist of a measurement gauge and either locking pliers or a magnetic base. The former are more expensive, but it looks like just selling some pretty basic bits together inflates the price really. Also - I can't see why the base needs to be located on the car body. As long as it's bolted to something heavy on the ground it should be fine, I think? Plenty of the magnetic base kind to be had on ebay for not a lot.

Chris

 

[edh]

I don't think it's a big deal in my experience

Pads don't work as well when very low - but some track pads (PF 97's for example) will wear out a set of discs before wearing a set of pads.

 

[zcacogp]

ORIGINAL: amrbose14

Does anybody use/own a disk run out tool? They seem to consist of a measurement gauge and either locking pliers or a magnetic base. The former are more expensive, but it looks like just selling some pretty basic bits together inflates the price really. Also - I can't see why the base needs to be located on the car body. As long as it's bolted to something heavy on the ground it should be fine, I think? Plenty of the magnetic base kind to be had on ebay for not a lot.

Chris

If you mean a Dial Test Indicator (DTI) then yes. A basic (piston) kind, rather than the more expensive moving finger kind, but it cost me about £30 on eBay a few years back, with a magnetic base, and is very useful for measuring disk run-out.

If you are anywhere near London then you are welcome to borrow it. It's easy (if fiddly) to use.


Oli.

 

[amrbose14]

OK. Ta.
Chris

 

[amrbose14]

ORIGINAL: zcacogp

ORIGINAL: amrbose14

Does anybody use/own a disk run out tool? They seem to consist of a measurement gauge and either locking pliers or a magnetic base. The former are more expensive, but it looks like just selling some pretty basic bits together inflates the price really. Also - I can't see why the base needs to be located on the car body. As long as it's bolted to something heavy on the ground it should be fine, I think? Plenty of the magnetic base kind to be had on ebay for not a lot.

Chris

If you mean a Dial Test Indicator (DTI) then yes. A basic (piston) kind, rather than the more expensive moving finger kind, but it cost me about £30 on eBay a few years back, with a magnetic base, and is very useful for measuring disk run-out.

If you are anywhere near London then you are welcome to borrow it. It's easy (if fiddly) to use.


Oli.

Sadly, no where near London. But it sounds like a good tool to have to assess disk conditions, especially in the light of track use. Presumably also useful when fitting new disks. Against all the other costs... it's peanuts.

cheers,
Chris

 

[944Turbo]

Do you need a spacer of some kind to do the wheel nuts up onto - presumably it is only worth checking the run out with some torque on the wheel nuts?

 

[zcacogp]

ORIGINAL: 944Turbo

Do you need a spacer of some kind to do the wheel nuts up onto - presumably it is only worth checking the run out with some torque on the wheel nuts?

That is something I have wondered on several occasions. Broadly, no, I measure run-out without the wheel on, but with the two retaining screws per disk in place. Spacers and torqued-up wheel bolts would be better but I have never managed to arrange such a set-up.


Oli.

 

[944Turbo]

Interesting, thanks Oli

 

[Hilux]

Does anybody use/own a disk run out tool?

Clamp or tape a nail or small screwdriver or engineers scribe onto the caliper and adjust until it just starts to hit the disc as it turns near the perimeter outside edge. Turn the disc and measure with feeler gauges when its not hitting.

There's yer run out.

 

[amrbose14]

Tolerance on the run out is .1 mm. I guess your method would pick up the run out when it matters?

ORIGINAL: Hilux

Does anybody use/own a disk run out tool?

Clamp or tape a nail or small screwdriver or engineers scribe onto the caliper and adjust until it just starts to hit the disc as it turns near the perimeter outside edge. Turn the disc and measure with feeler gauges when its not hitting.

There's yer run out.