Disc Advice for Historic Open Wheeler

[El Rob]

My Old man is restoring/building a 1960s Renault Open Wheeler and we have made our own brake rotors and tophats. Can you have a look at what we've done and offer advice?

We are using Girling Single piston each side Callipers.

Currently nothing will be floating, and the hats are bolted to the Grey Iron disc, is this bad? Should we do some sort of floating bolt type setup between the Top hat and the disc.

Do we need to allow for expansion between the iron of the disc and the aluminium of the hat? Currently the tophat is spiggoted and bolted on.

We were considering machining out any excess iron out of disc once we were happy with the way it was mounted.

thanks for any advice you have

[Sucker]

Will it be run pretty hard or more of a cruisey parade lap type of thing?

If it is going to get used pretty decently I would look into a floating arrangement.

[El Rob]

Will be run pretty hard but we're not trying to be Michael Schumacher, just want something that is consistent and safe. For reference here are some pics of how the car is coming together and looks and the suspension. Has a Renault 16ts (1.5 Litre Hemi headed All aluminium Pushrod) engine and should weigh around 350 to 400 kg.

[10sec rx7]

all old formula fords are like that, i used to get any disk that had the right back spacing and machine them down to 6mm thick and use them..

[Uncle Arthur]

What you have done will work but as S said, floating is better. The flange with the wheel studs looks a bit thin, be better if it was thicker to match the studs.

[El Rob]

Okay, thanks for that, mabye we just leave them the way they are for the time being? Down the track we can slot the holes and mabye get a fancy floaty nut made up to fit in the slots. What do you mean by better - like always trued up or something like that with the disks floating? Better feel? less chance of shudders or something like that?

Also do we have to worry about any expansion/cracking issues between the Iron and Ally tophat?

The flange is the Original Renault Hubs and are really hard and seem to be very good, forged steel. Also we want to keep them and the 3 studs as it's a feature of the car.

[Momus]

Good job, nice car and I like the 3 bolt wheel mounting and the K nuts.
Everything is massively strong assuming the wheel stud flange is steel..
I fit GT-3 Porsche front brakes to things from time to time.
They use a 2 piece rotor and hat with the hat made from cast iron and the mount flange about 6 mm thick.
They are bolted up solidly but made in a way that the contact between the disc and hat is small to limit heat transfer and allow the disc to do it's own thermal thing to some extent.
The porker arrangement uses 10 M6 bolts on a 170 mm or so circle for a car with a front weight under heavy braking that
is probably 450 Kg a side.

The usual rule with this type of rotor mounting is to have the disc id as big as possible to reduce the hot/cool gradient. With aluminium hats the very hot cast iron grows more than the relatively cooler ally (which has double the per unit expansion rate) and things move around together.
This is a time honored marriage that works well. Floating mounting is nice- and not hard to do if you can drive a mill but probably unecessary for the little Regie.. Floating will give better feel and disc life. Part of the feel improvement can come from the disc being to some extent isolated from stub axle, spindle or caliper mout/body flex and being 'pad guided' and gripping and releasing cleaner.

When you help to make something like this you should always say to the driver that if there are any real problems he will be the first to know...