Mill rotors thinner and cross drill them?

I stumbled across a gent on reddit who milled his rotors much thinner and then cross drilled them. I've seen either of the modifications done before but I've never seen anyone do both. Seems like the risks of either modification are compounded greatly.

imgur.com/a/T84Pf

Opinions?

Agree. I seems like the chance of a cracked rotor would increase. Ed

...and/or a warped rotor. Don't wanna be there when the wobble starts up on the freeway. Could make for a hairy/dangerous ride...

. It the photos, it looks like those rotors are way under the spec for thickness. If so they will warp in short order.

The front disks are the same discs i just had reconditioned. The fronts are in spec down to 4.5mm which is quite thin. The rear is in spec down to 6mm. That hole pattern is nearly identical to the stock Z1R rotor hole pattern, however the smaller holes on the inside ring are not part of the z1r hole pattern. You can see the thickness difference between front and rear in one photo. It looks ok to me, except for that inside ring of holes maybe.

This is done on old Honda's all the time. A cb750 rotor is just plain stupid thick and made of stainless steel. To my knowledge, absolutely no one has ever warped or cracked one whether it is drilled or thinned or both. It has been found that you can thin them to almost half their original thickness, drill them amply, and they will still be strong...thereby saving a TON of weight. KZ rotors are made a much more reasonable thickness and I am not sure what material they are, so agreed, it may not be the best idea for old Kwakas. I think a lot of other 1970's jap bikes had the stupid thick stainless rotors too.

My instinct is telling me you went to far. I think the thin rotors will heat up super fast and give chance for multiple problems to occur.

If they fail / fragment at speed, I guess we will never know because, you wont be around to tell us. :pinch:

I hope they work.

Good luck

i cant speak for the original image because the guy could have thinned them too much, but mine are in spec at 4.7mm and the holes are from the factory as these are Z1R disks. I don't think there will be a problem. I just finished bedding in the new pads. feels great.

I never really understood why the rear discs are thicker. I've noticed the rear discs are 6mm service limiit, the fronts are 4.5mm ; but the fronts take a much higher braking force. Previous single front disc models had a 6mm service limit, new thickness was 7mm for single discs and 5mm each for single discs. .

(Some early disc brake kz Z1R and others I think initially had too powerful rear brakes)

Modern sportsbikes run thin discs from new, 3.5mm service limit is typical. add a set of sintered pads and a few trackdays and wear and warp is a bigger problem now.

Keeping the disc weight to a minimum reduces to polar moment of inertia = the 'flywheel effect' also reduces unsprung weight. Reducing weight here is every bike designer's priority, although it comes at a cost of reduced service life. I suspect the rear and early single front rotors were deliberately made with extra thickness and the service limit might be rather conservative.

The 0.5 - 1.0 mm between new and the service limit doesn't give much meat for skimming on warped and/or scored discs and I wouldn't use them far below the service limit. The discs look nicely reconditioned but most used discs would be too thin after skimming.

The ventilation drilling could be a good idea, but it looks a bit overdone there.

I believe the difference is whether you are running single disc (thicker - service limit 6 mm.) or double discs (thinner - service limit 4.5 mm.). It may be a good idea to get two single discs that are warped/ruined and have them slimmed down and run them in double disc setup. There are plenty of thinner drilled discs on old Kawis.....my 79 has them and I have no problems.

bluezbike wrote: . It may be a good idea to get two single discs that are warped/ruined and have them slimmed down and run them in double disc setup.

Yes, one of my projects has Nissin multipot calipers on the front running on a pair of rear discs, and a front disc on the rear. Apart from this rotational and unsprung weight increase, plus the reduced service life at the back I can't see a problem.

Have you compared the offset and diameter between front/rear discs?