Machining/turning rotors to 5mm

LuckyLeprechaun

LuckyLeprechaun

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I've been reading over the forum lately about weight reduction. Many people mention using a different, thinner rotor, usually 5mm, to reduce weight. My question is, since the stock rotors are 7mm thick, couldn't you just cut or machine them down to 5mm?

While it may not be the best weight savings, a rough estimate of .7 pounds per mm shaved. It would however eliminate the need for buying different rotors, making custom spacers, mounting brackets, etc. You could also pair this with cross-drilling to reduce weight even more.

This isn't necessarily something I'm interested in trying, but I was just curious as to if it were feasible.

For reference, I have 34mm forks with a 2 piece rotor.
 
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LuckyLeprechaun said:
I've been reading over the forum lately about weight reduction. Many people mention using a different, thinner rotor, usually 5mm, to reduce weight. My question is, since the stock rotors are 7mm thick, couldn't you just cut or machine them down to 5mm?

While it may not be the best weight savings, a rough estimate of .7 pounds per mm shaved. It would however eliminate the need for buying different rotors, making custom spacers, mounting brackets, etc. You could also pair this with cross-drilling to reduce weight even more.

This isn't necessarily something I'm interested in trying, but I was just curious as to if it were feasible.

For reference, I have 34mm forks with a 2 piece rotor.
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...... rather than make them thinner,,,, do a drilled pattern on them ,,,which will lighten them as well as ventilate them , theres some great drilling patterns you can get from the guys regards oldbiker
 
On the older two piece rotors, I think the center is aluminum. Turning the rotors down on those will be more weight saving, percentage wise, than on the later one piece rotors.
I think the main problem is that they are very hard and can't be turned on a regular brake lathe.
This was discussed on a thread about just resurfacing rotors. I think the process is done by grinding away the surface where on a regular car rotor they shave off the surface.
I don't recall all the particulars of the discussion, but you might find it with a bit of searching.
Leo
 
I crossed drilled some XS rotors last weekend. Using 6mm dia, 5 hole pattern - 90 holes in total. Weight reduction per rotor 258grams.
 
I did some careful measuring on the 5mm rotor I'm running (off an SR500) compared to the original 7mm item. It seems to make this 5mm, Yamaha removed 1mm from the back and 1mm from the front of the disc surface. This would keep the offset the same and keep the disc centered in the caliper. I would assume you'd need to do the same on the older disc.
 
XSLeo said:
On the older two piece rotors, I think the center is aluminum. Turning the rotors down on those will be more weight saving, percentage wise, than on the later one piece rotors.
I think the main problem is that they are very hard and can't be turned on a regular brake lathe.
This was discussed on a thread about just resurfacing rotors. I think the process is done by grinding away the surface where on a regular car rotor they shave off the surface.
I don't recall all the particulars of the discussion, but you might find it with a bit of searching.
Leo
Click to expand...

I have no experience with the later one piece rotor, but I have machined a couple of RD 350 rotors to get rid of runout. The RD 350 rotors look very much like the old two piece XS rotors. It is a while ago, but I believe I used a fairly big boring bar with a SECO or similar tool bit (Cannot remember the exact generic term, but was not HSS, but a harder tool) I used a regular lathe, a new tool bit, and some aerosol cutting oil, most likely CRC Supercut. There was no problems machining these discs, using the slowest feed on the lathe, and also a fairly low cutting speed and a shallow cut.
So if you have access to a decent lathe, and some time on your hands, it is definitely doable. As the material properties of these old Yamaha discs are unknown, I would hesitate to recommend both significant reduction of thickness, and extensive drilling.

Some modern discs are as thin as 4 mm, and still drilled, like the common Brembo discs with round holes in the gold colored steel carrier, others are 5 mm, and some Brembo race discs are thicker than 5 mm. The higher mass (weight) of these discs gives it capacity for storing more heat, so temperature will rise slower during braking. So there are a few things to consider when modifying brakes, but it all depends on the intended use of the bike.
 
oldbiker said:
...... rather than make them thinner,,,, do a drilled pattern on them ,,,which will lighten them as well as ventilate them , theres some great drilling patterns you can get from the guys regards oldbiker
Click to expand...
..........................................hi you could try xs1100 ventilated discs,,, they are not expensive from what i,ve seen,,, i think make sure they are 1980 2 piece,,, regards oldbiker
 
I did read some things about people trying to machine them. It seemed like one person(don't remember who), had some luck getting it done at an O'reillys, I don't know if they still cut or not though.

I was thinking about either having it cut on a lathe, which it seems is iffy, or possibly taken down on a mill. Lay it flat on the table, machine until flat, resurface.

I was thinking about doing what 5twins said it looks like Yamaha did on his rotor. Take X amount off the front, and an equal X amount off the back, thereby keeping it centered and at the correct offset.

oldbiker - I realize that you could just drill them, I was just thinking that if so many people are going to thinner rotors, why couldn't you just make the stock one thinner. Also, I believe 5twins rotor is both 5mm thick, and drilled. I have not seen a lot of info about the xs1100 rotor swap, but from what I have seen, I thought it was for 35mm forks only. I also don't think I've ever seen a side by side comparison of weights.

XSLeo - I just checked, and it does appear that the center of my rotor is aluminum.

arcticxs - Those are all very good points and things to consider.
 
I had my stock rotors turned down at oreilly. I powdercoated both rotors completely and had them resurface them to take the powder off both brake surfaces. They had a hard time, took a couple hours, but did get it done. They charged me double because they said the material was so hard it took twice as many passes to get them flat.
 
Haven't done it but you could take the two piece rotors apart and do them on a surface grinder... I think the flea bay disk resurfacer uses a surface grinder.
 
Soliddrummer - So what did it end up costing if you don't mind me asking

gggGary - A surface grinder is a really good idea

I just did some quick, rough math, and it looks like if you drilled 90 5mm holes in the 5mm rotor, you would save about 70 grams.
 
I just drilled mine (86 holes IIRC) and unloaded close to .4 pounds. I worked the surface with 80 and 120 grit in an orbital sander to take out minor grooves and de-burr my drill job and keep it all nice and square at the edge of the holes. I would think grinding would be the best option for thinning, but I'm happy with just the drill jov myself.
 
I was very careful,and did mine on a Blanchard grinder (lots of blocking,and set up) but that was just to get the grooves out of them. I did not go below the min. thickness. i then drilled them. I was always under the impression that if you went below min. thick,they are subject to warp from heat??
 
Going through my manual, it looks like the minimum recommended rotor thickness is 6.5mm. So I suppose it is a bad idea to machine the rotors any thinner. Why that's the minimum recommended thickness I don't know though. I think some people are running stock calipers with 5mm rotors with no problems, so that shouldn't be the reason. Maybe it's the unknown alloy that the rotors are made out of, Yamaha thought it couldn't take the stresses or heat being any thinner. It could just be Yamaha erring on the side of caution, but since there's no way to prove that, it's much better to be safe than sorry.

Based on that, it looks like drilling is the way to go. You both lose some weight and have some gains in braking, while staying within manufacturers specs.
 
I've been meaning to weigh these discs for some time and finally got around to it. I was quite surprised. The later stock 7mm disc is pretty porky at about 6.6 lbs. The early two part unit with the aluminum carrier isn't much better at about 6.4 lbs. It surprised me how much lighter the 5mm SR disc was. At 5.25 lbs, it's like 1 1/4 lbs lighter. These were all undrilled examples. I will weigh them again after drilling. I use a pattern consisting of 15 "swirls" of eight 3/16" holes, 120 holes total.
 
5.25 lbs, that is 2381 gram for the SR disc. Still ridiculously heavy. A Braketech cast iron, fully floating 320 mm disc is 13-1400 g, 1000 g or more than 2lbs lighter. And surely a much better brake.
As they do not make any XS specific discs, the Ducati ones could be used, together with a 12 or 13 mm disc spacer, to get sufficient spoke clearance for a Brembo 4-pot.
My made to order, 5 mm thick ISR disc with 22 mm offset for the XS is around 1600 g or slightly more than 3.5 lbs, but needs no spacer. And a single ISR/ Brembo 4-pot is at least twice as good as the stock (Euro model) dual disc brake setup, at one third the weight.
On my 1995 Ducati Monster, I replaced the stock Brembo disc with an ISR disc, using EBC HH pads, some 50 000 km ago, with good results.
 
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