Author Topic: Alluminium disc rotors, your thoughts? (Read 8537 times)

John Orchard · « **on:** July 08, 2018, 05:34:12 PM »

Hey Guy's,

l've been thinking of making an alloy rear brake disc for either my dirtbike or roadracer. l don't use a rear brake overly hard; vertially never on my roadracer, plus l don't like a savage rear brake.

l was thinking that alloy might have a lower friction coefficient plus be much lighter (less unsprung weight and all that), yeh it would wear out quicker but eh?

Idea's on what grade alloy; less chance of shattering or warping?

pokey · « **Reply #1 on:** July 08, 2018, 07:33:56 PM »

ally disks? Id share my drugs if I thought of doing that.

You do know that once the oxide is removed you have silver butter to play with.
Cant say Ive heard of anything other than ceramic, CF or iron disks. Probably a good reason for that.

Rear disks dont have much braking effect due to the inertia and weights being on the front. drum rears are just as good as you probably know. If you dont want weight use a smaller dia disc from a pitbike.

David Lahey · « **Reply #2 on:** July 08, 2018, 08:24:21 PM »

The strength vs temperature relationship for aluminium and its alloys is such that it would be useless for motorbike brake discs

David Lahey · « **Reply #3 on:** July 08, 2018, 08:26:59 PM »

As for weight saving compared with steel, aluminium (at room temperature) is approx 1/3 of the strength of steel and approximately 1/3 of the density of steel so there would be no weight saving

GMC · « **Reply #4 on:** July 08, 2018, 09:04:44 PM »

I think it would be a fail, the alloy would be too soft and the surface would peel and tear away. Not just wear quickly but rip itself to shreds quickly

Any heat treatable alloy would lose its temper from the braking heat
5083 has a close grain structure and might be the best bet but I think if there was anything in it the Formula One guys would have been onto it already.

I think Titanium might be a better option that alloy but again if F1 don't use it there must be a reason

John Orchard · « **Reply #5 on:** July 08, 2018, 10:12:36 PM »

Thanks, l can put that idea to bed ;-) What planted the seed in my head, a couple of years ago l saw a GP125 roadrace bike with an alloy rear disc; it might have just been there for looks?

djr · « **Reply #6 on:** July 08, 2018, 10:51:03 PM »

I think the only aluminium rear disc's I can remember seeing is those combined rear sprocket / rear disc set up's, that a few people have tried and then abandoned .
{ seem to remember a Hagon grass track / long track bike with this set up }
I guess you could get the surface coated with something to improve wear - anodised / chrome / nikasil maybe ?
but a lot of expense for little or no gain

skypig · « **Reply #7 on:** July 08, 2018, 11:36:59 PM »

Some MTB discs are (aledgedly) alloy sandwiched with Stainless steel. Including “Shimano Ice Tech”.
I wonder if a whole MTB setup would suit for your rear? Floating discs up to 203mm are common. Tiny calipers to suit.

mboddy · « **Reply #8 on:** July 09, 2018, 08:17:57 AM »

Zanzani disk for your road racer: http://www.motobi.com/zanzani_2015/dischi.html

sleepy · « **Reply #9 on:** July 09, 2018, 09:42:10 AM »

Alloy discs are quite common in open wheel speedway cars. Very critical to use the correct compound pads with them. Also using Titanium rotors now as well.
https://www.speedwaymotors.com/Wilwood-160-14185-Super-Alloy-Slotted-Rotor-10-Inch-6-x-5-25,268419.html
Not cheap but very light.

Momus · « **Reply #10 on:** July 09, 2018, 02:45:31 PM »

Against my better judgement I made an aluminium disc for a 100cc sprint kart about 20 years ago. Was made from the garden variety 5083 H321 plate and electroless nickel plated. It held up for at least a couple of meetings despite my dire predictions and I had no report of it's failure.

Aluminium has about twice the heat capacity of iron, or stainless steel, so for a given energy input an Al. disc half the weight would absorb the same amount. They must have been running pretty cool though.

Gippslander · « **Reply #11 on:** July 09, 2018, 06:41:02 PM »

Check the rulebook b4 using titanium
(happy to be corrected but as I recall it's ok to make components out of any old aluminum but titanium of which even the crappiest grade is stronger than ally is a not on - go figure)

sleepy · « **Reply #12 on:** July 12, 2018, 10:17:22 AM »

Aluminum rotors for use on front of speedway cars. Could be a little to thick for some bike calipers though at 8mm.

https://www.speedwaymotors.com/Speedway-Front-Aluminum-Brake-Rotor-10-875-x-312-Inch,20054.html

OverTheHill · « **Reply #13 on:** July 12, 2018, 12:24:32 PM »

Reminds me of about 20 years ago when my son bought home his first roadrace bike--an NSR250 turned into an RS250 with go fast bits. I took it for the obligatory thrashing down our road going far too fast & trod on the rear brake. Think it had those sintered bronze type pads & instantly ripped the disc to bits & packed it all up against the pads & locked the wheel solid. Was bloody lucky to escape from going sideways down the road. Had to carry it onto the trailer for the short trip home. Of course he said what the %^&* are you doing using the rear brake lol.

GMC · « **Reply #14 on:** July 12, 2018, 06:26:19 PM »

According to the moms Titanium is illegal for axles, not sure about other items

GMC · « **Reply #15 on:** July 12, 2018, 06:41:01 PM »

Keep in mind that the word "Alloy" is a term that means a mixture of elements, we all commonly refer to Aluminium alloy as 'alloy' so dont confuse it with titanium alloy which is also mixed with other elements for best effect.

Coating the alluminium would make sense

Wasp · « **Reply #16 on:** July 12, 2018, 08:10:51 PM »

I have made some Tit discs for go kart dudes . Was going to make some for my own chrome trolleys , but never got around to that anymore . It does save a lot of weight , especially on a big disc , performance is OK . However you can not use sintred pads .

Gippslander · « **Reply #17 on:** July 14, 2018, 05:48:50 AM »

GMC - you are correct about Titanium - I should have checked the rule says:

12.13.0.1 The use of titanium in the construction
of the frame, the front forks, the
handlebars, the swinging arm, the
swinging arm spindles and the wheel
spindles, is forbidden unless OEM. The
use of light alloys for wheel spindles is
also forbidden unless OEM. The use of
titanium alloy nuts and bolts is allowed.

So brake parts look ok

John Orchard · « **Reply #18 on:** July 14, 2018, 10:21:19 AM »

l wonder if the rules about titanium are for cost reasons or safety? l remember there was a safety issue with axles.

sleepy · « **Reply #19 on:** July 14, 2018, 10:53:41 AM »

Quote from: John Orchard on July 14, 2018, 10:21:19 AM

l wonder if the rules about titanium are for cost reasons or safety? l remember there was a safety issue with axles.

Not to sure on the reason but I know if it isn't machined correctly with sharp corners and the like it is very prone to failure.

80-85 husky · « **Reply #20 on:** July 14, 2018, 11:23:43 AM »

dont forget the F1 and GP teams have been playing with all this stuff for yonks so they will have discovered all the reasons why things arnt done

micks · « **Reply #21 on:** July 15, 2018, 12:42:04 PM »

it was for safety
http://www.ma.org.au/licences-rules/rules/bulletins/?hilite=%27titanium%27
heard a story about a bloke Proddie racing (rgv days) who pulled the gusts out of his rear brakes because he didn`t use it plus would save weight etc. all good until he went thru scrutineering

Gippslander · « **Reply #22 on:** July 15, 2018, 05:05:34 PM »

Thanks Micks, that is part of MA I did not know about - a good resource

Some Googling:

Strength (and hardness) is inversely proportional to ductility; this means the greater the tensile strength (and hardness) the less ductile and more brittle is your material. Since it is already established that Ti alloys (like, Ti-Al-V) have high values of strength, it follows that they would be hard but brittle, which means while it would require greater stresses to fracture a Ti alloy than say Aluminum, once that stress is reached, a Ti alloy will instantaneously fracture and fail, unlike an Al alloy that will plastically deform significantly before failing.

What is peculiar about titanium and its alloys is that even very limited amounts of other elements, in quantities that in steel would be acceptable as impurities or would be beneficial, particularly oxygen and nitrogen are detrimental for toughness. Titanium is a sensible fella. He embrittles very rapidly if you allow him to oxidise or pick up other elements (he tends to pick them all up and by the way that’s why he's so difficult to machine, he has a tendency to allow other elements to diffuse into itself very easily).

Lozza · « **Reply #23 on:** July 18, 2018, 12:24:45 PM »

Quote from: John Orchard on July 08, 2018, 05:34:12 PM

Hey Guy's,

l've been thinking of making an alloy rear brake disc for either my dirtbike or roadracer. l don't use a rear brake overly hard; vertially never on my roadracer, plus l don't like a savage rear brake.

l was thinking that alloy might have a lower friction coefficient plus be much lighter (less unsprung weight and all that), yeh it would wear out quicker but eh?

Idea's on what grade alloy; less chance of shattering or warping?

Aluminium discs are plasma sprayed I *think* Zanzani were the first to do that on the Kobas bikes early 80's ........................but local hot shot Aaron Morris who picks up a few rides with the French R2CL team in France (Le Mans24h and Suzuka) told me after 2 or 3 hours the rear brake goes metal to metal, they don't change it because of the time it would take at a pit stop and it feels just like you put a valve spring between the lever and m/c