ti disk rotors

Discuss light weight issues concerning mountain bikes & parts.

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Joined: Thu Mar 29, 2012 11:24 pm

by dave888

hi dave888 safety issues aside i love titanium, feel, the colour, lightness
given the right component/safety. out of interest how do ti disk rotors work?
im feeling a bit stupid, are disk brakes meant to hold some heat that helps with the pad material
to slow the bike yet ti doesnt conduct heat dos it?

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by socratease

I'm hoping a metallurgist can chime in, since I'm sure to be off the mark.

Titanium does conduct heat, just not as well as steel. A brake will be turning frictional forces into heat regardless of what the material is: wood/steel/aluminum/carbon. The reason carbon rims are notorious for being poor stoppers in the road (and subsequently having high failure rates) is that the heat builds up in the rim, causing the brake pads to toast, loosing their grip, then all of a sudden you have a very poor coefeccient of friction between pad and rim. A similar thing happens in mountain disks during overheating, but the problem is compounded by expansion of the brake fluid, and eventually boiling.

If you've ever overheated your discs enough, you can see coloration of the rotor, as well as heavy glazing on the brake pads. The effect, once the hydraulic fluid has cooled down, is very similar to contaminated rotors.

In a titanium brake this would happen faster, since the material wouldn't cool down enough. However, in *some* applications, it should work, probably with a specific coating to improve heat dissipation (and probably improve the coefficient of friction between pads and disk), as long as running sintered pads.

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by ricerocket

This is semi-unrelated, but there's someone local to me who makes titanium rotors for cars (racing, street, etc).

Heat transfer through titanium is pretty much garbage. The reason he's gotten titanium rotors to work is because the rotors are ceramic coated, and use fully metallic pads and in a full custom caliper situation, I believe he uses titanium pistons. The theory there is to thermally isolate the pad from the hydraulic fluid, and contain the heat to the rotor itself. The pads will have to have a wide operating temperature range.


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by dave888

dave888 thanks i feel i knew that in a way, we all like lightweight parts but i believe in not faffing with the bike to make it dangerous
ive got avid bb7 disk w cleansweep g3 rotors so what do i know on mechanics?
thanks tho, i know theres info on the ww site on ti disks should of read those first eh.

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by yourdaguy

I have Jucy 7 and can't run them with G3 rotors because the swept area of the rotor is not as tall as G2 rotors. I think bb7 take the same pads. The pads on Jucy7 and BB7 are taller than the pads on the Avid brakes that use G3 rotors such as Elixer Cr. You might check your rotors and see if the pads are not sweeping down on the arms.
For certain parts stiffer is more important than lighter.

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by TheRookie

I suspect the issue is nothing to do with heat transfer, stainless steel for example makes rubbish brake discs the coefficient of friction is poor, hence the reason they are made of other steel alloys (that rust), so I wonder if Ti just makes a rubbish friction surface, hence why in the quoted car example the brakes use a ceramic friction surface, much like you can get for ally discs for bikes.
Impoverished weight weenie wanna-be!
Budget 26" HT build viewtopic.php?f=10&t=110956

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by 02GF74

some data:
Thermal Conductivity - k - W/(m.K)
Aluminum 250
Copper 401
Iron 80
Iron, wrought 59
Iron, cast 55
Steel, Carbon 1% 43
Stainless Steel 16
Titanium 22

braking works by converting forward energy into heat that mostly goes into the disc and then passes to the surrounding air.

different materials for the discs pads mean that the braking system can work better at different temperature - one example being F1 cars stop better when the discs and pads are not cold so it does not necessarily follow that it is desirable to keep the disc as cool as possible.

for a Ti disc, that is a relatively low thermal conductor, the surface may get hotter than say a steel disc but then Netwon's law of cooling state that heat transfer is proportional to the temperature difference - more heat will move from a hotter disc to the air.... I am notr saying that is a good thing but there are a lot of factors involved...... now I've lost track - what was the question agai? :?

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by Varaxis

F1 cars work on different materials, taking advantage of adhesion instead of friction to slow the vehicle down. Since they're going so fast and need to stop so often without a chance to cool down, they simply choose materials that work best consistently at the temps they're most frequently at. Same design concept is taken for things like super advanced jets, where they leak like crazy on the ground, but once at high altitude, functions optimally, due to thermal expansion.

As for Ti rotors and brake systems, there's no reason for friction not to happen, clamping onto it with a brake pad. The heat tends to go to and build-up at the most conductive parts first, which tends to be the brake pads. They recommend not using metallic pads, partially for this reason, as the pads transfer the heat to the brake fluid and that can cause issues like boiling any traces of water in the line (lower boiling point than the brake fluid), turning it into gas and creating the issues related to having air in the line and needing a bleed. Thermal conductivity shouldn't be a huge deal as heat from friction is deposited right where the friction happens; conductivity just spreads the heat throughout the rotor better, so there's no uneven hot spots.

Sometimes thermal conductivity is bad. There are cases in motos using 2 piece rotors with alloy carriers, with the alloy carriers conducting so much of the heat that it affects the grease in the hub bearings.

There's far more to consider than basic material science stats on different materials and choosing the right one. With enough forethought, engineering, and design, you can make things work with many different materials. For general consumers, is it wise to pay more for less, and for companies, can you make money through it? Some materials have other issues, such as ease to work with, brittleness or fatigue life, stiffness, expense, etc. For those that already have tooling for titanium, it wouldn't be too much extra work to make Ti into other products, without carrying over too much of the cost of the machinery to the cost of the product. If you already have tooling and know-how to work with steel...

There are already some Ti rotors out there and judging by those riding them, they are apprehensive about using them full time, probably due to the cost, but the Ti rotor users have a more positive impression of them over Alu rotors, at least between Carver Ti and Scrub Al rotors.

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