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350TSS · 15-11-2017, 09:03 PM

I have come to a conclusion concerning the rear brake hanger. The issue I have been struggling with is that the original S4 (fixed) hanger has a 12mm inward displacement from the inner-most edge of the casting at the wheel spindle to the two holes where the caliper is attached. With the 25mm spindle the OD of the wheel bearing is 52mm with a machined boss in the wheel casting that holds the bearing about another 8mm outside that on each side. The diameter of the boss in total is therefore 52mm + 8mm + 8mm = 68mm. The distance between the centre of the wheel spindle and the caliper bolt hole centres on the casting is 106mm.
My original intention was to machine a hub that had a bearing onto the wheel spindle out of 60mm bar and bolt a 10mm plate onto the inside edge of the hub. This will plainly not work as the size of the wheel boss and the 12mm inward displacement means that the hub would have to be machined from much larger bar. (clear the 68mm wheel boss say 72mm, allow 6mm either side for structural integrity then a 12mm either side to take 8mm bolts to attach the swivelling 10mm hanger plate so my hub would have to be machined from 72mm + 6mm + 6mm + 12mm + 12mm = 108mm bar. It would weigh a ton and all un-sprung weight as well. The hub would need to be 32mm widthways
The real issue/problem is the inward displacement of the hanger, my solution is to spacer the disc out by 12mm from the wheel, meaning that the hub can be as small as the bearing I can buy/make + say 6mm each side for structural integrity + 12mm each side for attachment of the plate by 8mm bolts.
I have located some single row sealed ball races with 25mm bore x 37mm OD x 7mm wide, I intend to use two inserted from either side of the hub with a 6mm division in the middle of the hub so the inner races bear against the swinging arm on one side and the wheel bearing on the other. The hub need only be 20 mm wide and 37mm + 6mm + 6mm +12mm +12mm = 73mm.
Upside is that the rear disc will be out of the lee of the tyre and so should be better cooled also the turning forces transmitted through the reaction rod to the frame will be through the centreline of the inner bearing (rather than 12 mm inboard of the centre-line of the bearing. The downside I can think of is that the bolts retaining the disc or rather the threads retaining those bolts will suffer a much higher shear load. The spacers obviously will have to be accurately made to prevent disc run out and exacerbation of that shear load. The reaction rod end at the crank case may also need a spacer/ bracket, subject to exhaust pipe and brake pedal clearance issues. Can anyone envisage any other difficulties with spacing the disc out from the wheel?

15-11-2017, 09:03 PM	#377
350TSS Too much time on my hands member Join Date: Jan 2013 Location: Shipbourne Bike: M900 Posts: 1,419	I have come to a conclusion concerning the rear brake hanger. The issue I have been struggling with is that the original S4 (fixed) hanger has a 12mm inward displacement from the inner-most edge of the casting at the wheel spindle to the two holes where the caliper is attached. With the 25mm spindle the OD of the wheel bearing is 52mm with a machined boss in the wheel casting that holds the bearing about another 8mm outside that on each side. The diameter of the boss in total is therefore 52mm + 8mm + 8mm = 68mm. The distance between the centre of the wheel spindle and the caliper bolt hole centres on the casting is 106mm. My original intention was to machine a hub that had a bearing onto the wheel spindle out of 60mm bar and bolt a 10mm plate onto the inside edge of the hub. This will plainly not work as the size of the wheel boss and the 12mm inward displacement means that the hub would have to be machined from much larger bar. (clear the 68mm wheel boss say 72mm, allow 6mm either side for structural integrity then a 12mm either side to take 8mm bolts to attach the swivelling 10mm hanger plate so my hub would have to be machined from 72mm + 6mm + 6mm + 12mm + 12mm = 108mm bar. It would weigh a ton and all un-sprung weight as well. The hub would need to be 32mm widthways The real issue/problem is the inward displacement of the hanger, my solution is to spacer the disc out by 12mm from the wheel, meaning that the hub can be as small as the bearing I can buy/make + say 6mm each side for structural integrity + 12mm each side for attachment of the plate by 8mm bolts. I have located some single row sealed ball races with 25mm bore x 37mm OD x 7mm wide, I intend to use two inserted from either side of the hub with a 6mm division in the middle of the hub so the inner races bear against the swinging arm on one side and the wheel bearing on the other. The hub need only be 20 mm wide and 37mm + 6mm + 6mm +12mm +12mm = 73mm. Upside is that the rear disc will be out of the lee of the tyre and so should be better cooled also the turning forces transmitted through the reaction rod to the frame will be through the centreline of the inner bearing (rather than 12 mm inboard of the centre-line of the bearing. The downside I can think of is that the bolts retaining the disc or rather the threads retaining those bolts will suffer a much higher shear load. The spacers obviously will have to be accurately made to prevent disc run out and exacerbation of that shear load. The reaction rod end at the crank case may also need a spacer/ bracket, subject to exhaust pipe and brake pedal clearance issues. Can anyone envisage any other difficulties with spacing the disc out from the wheel?