Slowing Down An Airhead

One Way To Slow An Airhead

When you read the ride reports of the early 1980’s the Brembo brakes were as good as it got but there were a lot of drum braked cars still on the road then so the BMW brakes were excellent for their time. Fast forward thirty years and you are playing in traffic with distracted drivers who have ABS and stability control under their foot. No matter how quick you react you are at a severe disadvantage.
Everyone has seen the aftermarket big disc conversions aimed at early G/S BMWs. They typically go for around $500 plus freight from Europe. Here is a DIY option that cost me $75 to build and fit to my R80ST.
From 2002, the Kawasaki ZX9 was fitted with 320mm diameter discs and the bolt holes line up with the pitch circle used on airhead front hubs. It is necessary to make a spacer to adapt the disc to the right off-set and to make an adaptor bracket to mount the OEM Brembo calliper to suit the new larger diameter disc.
Attached are two sketches, one for the hub adaptor and one for the calliper bracket. I machined mine from billet but as an after-thought, it would have been easier to laminate two 12mm plates and rivet them together. The bracket is sketched as a laminated version. Bear in mind that the calliper needs to mount further aft so the bracket is in effect a Z shape to keep the same distance from the wheel centre line. There is little room to play here as you need to maintain clearance on the spokes. There is an image taken from above the bracket that shows the off-set.
Provided you maintain the hole centres by sizing it on a copier to get the holes in the right place the sketch should be good enough to trace as an outline. Although I haven’t tried it, by making the riveted version it should be possible to do a dual disc conversion by handing the bracket the other way at assembly to fit the other fork leg. By peening the rivets into countersunk holes then dressing the heads flush it should be possible to make the bracket appear as a solid piece.
I machined my hub adaptor on a small centre lathe so I didn’t measure the PCD of the holes. Instead, I clamped the adaptor to the hub and spotted the holes with a neat fitting drill then put the hub in the drill press and drilled the holes to 8.5mm diameter. Five new bolts M8 x 90mm are required to attach the disc. I used stainless steel hex bolts with the original nuts and washers.
While measuring the improvement in performance would be a pain in arse without a comparable stock bike to run against my seat of the pants meter came down from ‘frighteningly bad’ compared to my R1150GS to ‘adequate’. At a guess it is at least a 25% improvement in stopping distance.

Woz

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