Rotors & Bells

[Error404]

Ok I thought it might be useful to have a thread based on two-piece brake discs, using rotors & alloy bells.

I'm designing some custom bells for myself right now for use with separate rotors, really to reduce unsprung weight Vs a large single piece disc, but also to give some flexibility with disc offset / caliper fitment across different rotor sizes, to enable me to run larger discs on track simply by using a radial spacer on my existing caliper bracket.

So in terms of bells, not being a professional engineer or designer I have selected 7075 as a possible material, at 12mm thickness - what are the experts thoughts on this.... Driftin Jo et al?

In terms of rotors, I was looking to use a common lug hole PCD if possible to give more flexibility, but struggling to find any information on anything other than AP discs - who else makes good quality rotors? i.e. not compbrake *****

AP
Alcon
Bembo?

Cheers
Alex

[Rilla]

no way you could just use/modify RS6 brake set up?

[Error404]

unnecessarily large and heavy IMHO.
I'm looking around 330 x 30 rotors which with the bell should weigh circa 7.5kg Vs the 16.5Kg of the RS6s or 12.5kg for RS4s

[Rilla]

ah fair comment then.

[willg54]

Id say 7075 is a fair choice. Might be worth anodising them.

Check thermal expansion of the bells, might cause some issues in the disc mounting.

I was thinking of doing this when it comes to changing mine. Wondering if I can get cnc time at work lol

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[driftin jo]

If it was me I would use 2024 over 7075 as it is stronger at higher
temps, but if you are going to make them thick, like 12mm, you could probably
just use 6082, it would be a lot cheaper.
I made some for a friend of mine and they were to his design, 2024
and 8mm thick with 5mm radii and hard anodized.
I think he worked out he had a safety factor of 5, with a minimum recommended
factor of 3, he could have made them 6mm thick.

[Error404]

Good info thanks guys. The AP discs I have based my design on have slotted lug holes to allow for thermal expansion.
To be fair 12mm was just a rough guess, so if I went with 2024 I could get away with with 7-8mm thickness, more like 9-10mm for 7075? Need to look at the cost / weight ratio as I'm not building a competition car so the last gram doesn't really matter, if 6082 is a good option in terms of mass/strength/cost balance I should consider that

[bilko0795]

Alex
try a firm called Godspeed in bridgend S.wales they have their own website. they make custom rotors.

[Error404]

Thanks for the link Ray, they look like they have good prices on AP rotors too

[twoqu]

To be honest, I think you are going to struggle to get much flexability on the one size fits all bell idea!
Most aftermarket discs tend to keep the annulus around 50mm so the larger the disc, the larger the pcd. You might get a marginal increase in disc diameter but certaintly not worth the extra expenditure.
Also 30mm is a odd fitment. Most are 28mm or 32mm!
Then there is the choice of floating or fixed. Having seen the results of the Movit fixed bell failure, I was toying with a floating conversion. Additionally the caliper/pad combination is also going to be a factor especially if you are planning to use OE calipers (Porsche?) as the pads are larger than used for aftermarket caliper!
Not sure if the Wilwood stuff is still around. The discs were affordable but mostly imperial sizes. There was aplace in Ireland selling Brembo discs. They had a 328mmX28mm that was under a £100 but that was a while back.
Think you need to finalise the disc size, diameter/width. Decide on whether to go floating or not as that will dictate the fixings, bolts/bobbins/anti rattle clips etc and design the bells accordingly!

[willg54]

What's the movit bell failure?!
Don't start me worrying...my bank balance wont be happy!

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[terrybullwon]

Pm me what you want Alex I have a few sets !

[Error404]

To be honest, I think you are going to struggle to get much flexability on the one size fits all bell idea!
Most aftermarket discs tend to keep the annulus around 50mm so the larger the disc, the larger the pcd. You might get a marginal increase in disc diameter but certaintly not worth the extra expenditure.
Also 30mm is a odd fitment. Most are 28mm or 32mm!
Then there is the choice of floating or fixed. Having seen the results of the Movit fixed bell failure, I was toying with a floating conversion. Additionally the caliper/pad combination is also going to be a factor especially if you are planning to use OE calipers (Porsche?) as the pads are larger than used for aftermarket caliper!
Not sure if the Wilwood stuff is still around. The discs were affordable but mostly imperial sizes. There was aplace in Ireland selling Brembo discs. They had a 328mmX28mm that was under a £100 but that was a while back.
Think you need to finalise the disc size, diameter/width. Decide on whether to go floating or not as that will dictate the fixings, bolts/bobbins/anti rattle clips etc and design the bells accordingly!

Good info
I probably wasn't clear in what I intended with this proposed set-up - I wanted to design a custom bell to run something like a 330mm rotor at the same disc offset as my road discs, thus using the same calipers (996s) with the same caliper bracket, using an 8mm radial spacer. That way having a road set and a track set which can be changed pretty quickly when required.
By the sounds of it I need to have a look at the pad sweep I hadn't really considered the difference in annulus to be honest. I really wanted floating to maximise the benefit of the two piece set-up

[twoqu]

What's the movit bell failure?!
Don't start me worrying...my bank balance wont be happy!

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Baically the bell fractured seperating it from the disc! It was at CC one year.
If you look at the bell design there is a lip that fits inside the disc whereas AP use a flat flange.
If I can find them I will illustrate the difference along with some relevant measurements if that is of any use to the OP!

[twoqu]

Good info
I probably wasn't clear in what I intended with this proposed set-up - I wanted to design a custom bell to run something like a 330mm rotor at the same disc offset as my road discs, thus using the same calipers (996s) with the same caliper bracket, using an 8mm radial spacer. That way having a road set and a track set which can be changed pretty quickly when required.
By the sounds of it I need to have a look at the pad sweep I hadn't really considered the difference in annulus to be honest. I really wanted floating to maximise the benefit of the two piece set-up

The problem is going to be the pad width! I take it you have 314mm discs at present.
I think you are going to be stuck with OE discs unless there is an odd aftermarket one which will undoutedly be expensive.
Unfortunately the 996 caliper does not work with the 345mm S8 disc. Think :corey: had to knock it down to something like 340mm. Unless there is something in the Merc fitments that is nearer the size. Altough you will still have the bell issue.
It may be possible to modify an OE disc to take a bell and effectively machine it to the desired diameter subject to the annulus being sufficent. Looks like an evening spent trawling the Brembo catalogue!
That why people stick with the OE discs really. Easily obtainable and cheap but heavy. Otherwise its a complete change to disc/bells and calipers.