disc brake caliper rubbing against spokes

[Brucey]

Yes, good that thoughts agree on this.

Hylomar Universal Blue non setting jointing compound. Rated to 250C (PTFE tape rated to 260C).

don't get me wrong, I have used Blue Hylomar many times and it has been excellent (a Brucey top tip is that if you have an apparently dried up tube of hylomar it can be brought back to life by using acetone as a solvent and storing the concoction inside a properly airtight container afterwards). I confess I don't know exactly what happens to hylomar when it gets too hot, but the reason PTFE tape is 'only' rated to 260C is that is no longer a reliable long-term sealant at higher temperatures; it will in fact survive brief excursions to much higher temperatures than this and of course it never melts, so it is quite well suited to brake caliper use IMHO.

[AndyB1]

Is it possible to use PTFE tape on a 3mm screw? PTFE tape is really designed for larger diameter threads.

[Cyckelgalen]

The screw on the Spyres is around 6 mm I'd say, the hex key to adjust it is 3 mm. I see no problem in using Teflon tape on a 5-6 mm screw, though Teflon thread would be easier to apply.
I believe there are PTFE tapes rated well above 300ºC. PTFE seated ball valves can be soldered to copper pipes and survive peak temperatures of 400º. But unlike a brake calliper, they have to endure those temperatures very briefly and only once.

One of the ways around spoke rubbing, discussed before in this thread, is increasing the rotor size. I have tested that and gone from 160 to 180 mm at the rear, and there is now plenty of clearance to accommodate the Spyres or any other calliper. The braking has improved significantly and that is with the same pads and callipers and before upgrading to compressionless cable housing. Well worth the effort and the small investment. Apart from the rotors, you only need an inexpensive new adapter for the rear and moving the current 160 mm adapter to the front as it is identical to a front 180 mm adapter.

[Brucey]

IME PTFE will eventually turn into white powder (and noxious fume) at very high temps but it is certainly better than most other (if not the best of) polymers at high temps. There is no problem using thin PTFE tape (the cheap kind that isn't certified for gas use) even on very fine pitch threads.

[Cyckelgalen]

I wonder, what sort of temperatures can a calliper reach in extreme situations, like prolonged descents with heavy riders or carrying loads of touring gear?
Would PTFE be a serious contaminant if traces of it reach the rotors and pads, or would it just burn off?

[AndyB1]

Hi Cyckelgalen,
Agree with what you say about increasing disc size - makes me wonder why 160mm discs are the norm and not 180mm?

[Cyckelgalen]

160 may be the norm on gravel and road bikes, but not quite on MTBs.
One factor may be the lighter, flimsier chainstays and forks on road bikes, but most likely it is because paired with hydraulic callipers, 160 is plenty. My Trek 520 tourer came with 160 discs and cable callipers, and I felt I needed a bit more braking power while touring with full camping gear. The 180 rotor at the rear has been a great improvement, I can manage long descents even with the rear brake alone, totally unthinkable before.
I wonder if a 180 rotor at the front would have similar benefits. I trust the steel fork will manage the added torque, I was reassured by Brucey's comment that most bikes can handle an upgrade from160 to 180.

[Brucey]

.... I trust the steel fork will manage the added torque.....

note that there isn't really any 'added torque' in normal braking, only when you use the larger disc to it's full potential.

[Cyckelgalen]

Interesting. I expect to use the full potential of the brakes though. I often tour in mountainous terrain with a total weight around the maximum weight limit of my Trek 520, that is 125 Kg, combined weight of bicycle, rider and cargo.
I had tried to improve braking with different pads, cable housings and callipers, all with negligible results until I went for a 180 mm rotor (following your advice Brucey). The improvement has been very significant, with the added benefit of solving any eventual spoke rubbing.
Actually, having no more spoke rubbing issues, I went ahead to test a Spyre calliper on the bigger rotor, and it performed worse than my good old single piston Hayes. It may be, though because the Spyres require different brake levers than my current Tektro 340. I noticed that the Spyres' actuator arm is a lot longer and requires a lot more cable pull.

[Brucey]

it is a combination of the arm length and the ramp angle which determine the MA of a mechanical disc caliper. I think the Spyre, being a later design, is meant for use with the slightly longer NSSLR cable pull, which is not what the tektro levers deliver.

[Cyclothesist]

Another factor is ensuring the pads and the rotor are bedded in together. Swapping the caliper without adequate bedding in on the rotor will not give a fair reflection of performance. I found the Spyres and Hayes CX comps to be about equal with ST 4700s (which are NSSLR). The main difference being the Spyre pads last a lot longer (they're bigger) and are cheaper (they're a common Shimano format).