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lightening rear disc rotor

8K views 45 replies 19 participants last post by  corner carver 
#1 ·
I have found a few bikes which appear to have the standard rear rotor but have been drilled out and lightened.

Can anyone shed any light on this, I think it looks good and wouldn't mind doing it myself.



 
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#4 ·
The best way is to use a waterjet.

Google waterjet/motorcycle rotors and there are a few companies doing this.

I did it on my 08 Cbr1000, saved .5-.75 pounds I believe.

The guy who did mine had a screen name of LHintze on the cbr forums.
 
#8 ·
The vintage car racers used to try drilling their own rotors. The drilled rotors almost always cracked. Holes made by a common type drill bit are really rough, and the roughness acts as a stress riser. The cracks always start at the drilled holes. If you do it (bad idea), better check the rotors for cracks before each ride.
 
#10 ·
oddly enough, I just bought a rotor off of ebay to experiment with in this regard.

A word to the wise: when buying a user rotor, even if a thickness is specified, ask again. ask what the SWEPT area's thickness is. If they don't know what you're asking for, don't deal with them.

I made the mistake of taking the listing at face value and have a rotor that might last a season (and I don't use the rear much at all). look at the pic. notice that the caliper's reading isn't quite the same as what it written on the rotor itself (by the seller).
 

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#12 · (Edited)
here's another shot post-cleanup and the first round of mods.

- cleaning it (dislike getting dirty where not required).
- removed most of the outer lip (the part that WAS 3.98 mm). left a few thou since I don't have a Blanchard grinder to do it the correct way.
- put a cross-hatch on the friction surfaces. mounted in a lathe, angle die grinder with a Scotchbrite disc, spin in opposing directions, flip and do the other side.
- drilled all the holes to 8 mm (7.94 mm (from 7.34 mm) to be more exact).
- countersink a few thousandths to break the edge.

the f'ed up appearance is mostly an optical illusion.

weight after cleaning was 832.1 g. after round 1 mods, it's 824.8 g. not going to do any crazy giant holes or slots and not looking to get it as light as possible. just some experimentation.
 

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#14 ·
now with 12 holes @ 11 mm, re-countersunk, and 807.88 grams. about 15 minutes work in the drill press. might have to get help to get the rotary table up on the mill so I can speed things up with whatever else I come up with.
 

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#15 ·
24 holes @ 11 mm, all holes chamfered, and 791.17 grams. This is the end of making chips from it for now. I will install it on the Monster sometime soon and test it out and probably won't bother removing it again until after winter comes. This wasn't done out of need, only for cheap entertainment and because I could. And, as a demonstration (only because this thread popped up at the same time I got the rotor).
 

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#17 ·
being so thin to start, it might not make it to another round of changes. if it does, the next change might be to just make all the holes 11 mm. simple and easy. or, maybe something more artsy-fartsy. time will tell.
 
#22 ·
another small clue but, it should be pretty obvious to some. out to the garage to start the install......
 

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#23 · (Edited)
went for a short ride (20 miles or so) to test the rotor out. I used the same pads as before. the other rotor (wavy) and pad combo howled badly when applied lightly/normally unless used hard once regularly. they work well otherwise.

besides the lightening aspect, I also made the rotor float with the pieces shown above. each holes has been drilled oversize, a sleeve slips in each hole, a .5mm shim is on either side. the shims and sleeves are bolted tight to the wheel. the rotor is free to move laterally +/- .5mm and currently about +/- .1mm radially (starting out 'tight'). this was done to decouple the rotor from the wheel - to make the assembly less rigid as far as vibrations are concerned. can't say if it works yet. need more pad material deposited on the rotor first.

but, it works for stopping the bike. no warpage (didn't try it when it first arrived and you never know about used bits).

time will tell. maybe another ride this afternoon.
 

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#24 ·
Working on another rear rotor and remembered I'd posted to a thread before. Good grief, 2+ more years fly by.

Anyhow, more miles on the rotor, less/different/lower Hz howling but, howling none the less. Not sure how many miles I've gotten from it but, like I said, I don't use the rear much. I'll keep using it until it's toast.

Now working on another rotor with much the same modifications to it. This one has the same pattern but the larger holes are 12 mm (currently) instead of 11. I think that they will ALL end up at 12 mm before I mount it. I am not going to make this one a floater (yet). This one has been glass bead blasted now that I have a cabinet (60-120 mesh, 80 psi). Probably going to use Ferodo Platinum or some other organic pads.
 

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#25 ·
Pay attention to placing the holes too close. I had 2 sets of OEM Brembo drilled fronts. One w/ one pattern, another with different pattern. One of these developed hot-spots between the holes. The other pattern did not.

This was while bedding-in new pads, where the rotors and pads are forced to get very hot (and you need a subsequent cool-down run!).

I'll see if I can remember to dig them out and photo. It's pretty unbelievable.

And that "swiss-cheesed" rotor that's photo'd in this string...wow. That would overheat in no time. I would not want that for a front rotor!
 
#27 ·
And that "swiss-cheesed" rotor that's photo'd in this string...wow. That would overheat in no time. I would not want that for a front rotor!

I don't see where anyone suggested using a pattern like that on a front rotor. I have to believe it was either done for show (on a street bike) or was used on a trackday or race bike to follow the rule that it had to have a 'working' rear brake.
 
#28 ·
Some of us really do not use the rear brake so I have had customers
drill rear rotors
use titanium rear rotors
use aluminum rear rotors
use tiny corsa rotor and caliper

I had a friend that used to have a rear master cylinder that contained a bolt inside so it would always have a strong brake pedal to pass tech inspection but was not a working brake (he was a expert racer that won races in his class).

On the front I had a customer who ran ceramic front discs on the road and have another who is running lindal (sp) discs that while crazy light do not seem to have as much brake power as a single disc setup.

Application makes the difference, some are fine to use given how they are to be used (or not used) other times it is a case of do your homework to know what to expect.
 
#29 ·
Is it odd that I like using my rear brake? Enough that I'm liking the idea of that floating rear brake idea.

Not as a full stopping force, but for scrubbing a little speed off the top. I scrubbed down my rear rotor and did a really good bed in with some EBC sintered pads and I find it quite useful, though it's a little wooden. I've considered trying to find a master that's 1mm smaller but with longer travel.
 
#30 ·
I have always used both but then I got into the habit many years ago when I used to disconnect the front brake switch so that I could "scrub" off some speed in a sneaky way. 99% of the time in traffic I used both so that I knew a brake light was visible to others. Old habits die hard I guess.
 
#31 ·
Every rider likes different things so i hate to say using the back break is bad. If I ride in the dirt I use my rear brake but when on the track or street I do not touch it. I see more people get into trouble on the track with rear brakes as they tend to lock and slide when there is little to no weight on them. This is why most sport bikes have pretty weak rear brake systems, now do not take it that there is not a good reason to use the rear. Plenty of real fast racers use the rear , though it often has little to do with actually slowing the bike down.

I would say the older bikes like carby supersports,monsters.851's and ST had stronger rear brakes than any of the newer superbikes including the 916 series. Just note that we do what we are trained to do and when you have to throw the brakes on full power someday you will also be at increased chance of having the rear wheel locked up and sliding. If you practice this enough that you are trained not to lock the rear (or what to do when it does) then all is good, if not you might want to think about training for that emergency stop.
 
#32 ·
If you ride any dirt roads you should learn to not rely too much on front brakes, especially stopping or slowing on downhill sections. Same holds true of stop signs near gravel pits due to possible gravel or sand on the pavement.
 
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