Replacement front disc rotor thoughts - Offsets, dimensions, compatibility

[GeorgeOC]

Hello all, I'd like to share some ideas about replacement rotors. First off, thanks to member TimeMachine for his help with providing a head to head comparison. Initially our conversation began when looking into the offset dimensions for <77' rotors, and those 77' onward. I am attempting to take that one step further, with info on sourcing a correct replacement, in my case for a 76' 34mm fork and early style caliper.

In the photos are 2 rotors. The older version is on the left, newer on the right. Here are, what I can tell are the specs for each rotor.

pre 77'
298mm
6x80 BCD
64mm bore
~17mm offset
7mm thickness

77'-84'
298mm
6x80 BCD
64mm bore
~12mm offset
7mm thickness

Admittedly, I was totally wrong up to this point before TimeMachine provided the image. I was under the impression that the older rotors had the smaller offset, thereby complicating my search. In my scouting for a replacement, I came across many near fit options, MikesXS, 650Central, ebay, Internet sellers etc. But the exact dimensions of all these options really seemed to be elusive, not to mention the subtle compatibility difference from 34mm to 35mm forks and calipers, and lack of good/cool options for 34mm forks. I should also state, the threads within the forum that provided the lists of compatible replacements were also very helpful.

to make a long story short... If you're looking for a front rotor upgrade and If you have an older 34mm fork, it looks like combining a 298mm 6x80mm PCD 64mm bore rotor, with the requisite rotor adapter may be the only option.

 

[GeorgeOC]

And as a sub question... Does anyone have expereicne with the height of the rotor braking surface fluctuate (decrease) to a point that more recent rotors like this example of an FZR replacement may not be usable?

Stock OEM braking surface height on either side of 77' appears to measure 40mm.

 

[gggGary]

I'll let the pics do the talkiing.

 

[TimeMachine]

 

[GeorgeOC]

View attachment 96534

Thanks TimeMachine, much appreciated.

gggGary, I wonder what the pad contact area height is on that newer style rotor?

 

[MaxPete]

This is really great data - thanks everyone.

Pete

 

[gggGary]

The swept band looks like it will in a bit further than the stock FZR pads, but I don't think the XS pads will hit the rivets. My round tuit plan is to mount an (early) fzr fork to an XS, use later model 4 piston calipers, space the fzr rotors out from the spoke wheel hub.

 

[TimeMachine]

Sounds very sporty gggGary , Probably on a wide front rim you lace up ? Very Sporty !

 

[gggGary]

 

[5twins]

Measure the up and down width of your pads, that will tell you the swept area on the disc. On the early models the pad is round so, duh, the diameter is the swept area. The later models have a curved rectangle pad, I don't know how else to describe it. My measurements (not necessarily correct) from a few years back show about 38mm of swept area.

This is an interesting topic but my recent thoughts are actually in the opposite direction - adapting an early disc to the later forks and caliper. Why? well first of all I think they look much cooler with that extruded aluminum center carrier. Second, that aluminum center saves nearly a half pound in weight. That can't hurt but honestly, you probably wouldn't even notice. But, it does give you more "bench racing" ammo, lol.

 

[gggGary]

scale is set to pound, ounce, 10ths

 

[gggGary]

some caliper weights, all with pads, the last a is a late model 4 pot Yammie.

 

[Muckroot]

Here are, what I can tell are the specs for each rotor.

I have the older 34mm forks, with the corresponding caliper.
From your numbers, couldn't I just get a fancy drilled rotor from mikes that fits the 35mm forks and have a shop machine a 5mm spacer to correctly space the rotor from the wheel? Every other spec seems to be the same as far as I can tell.

EDIT: I think that's what you're saying here. Question answered.

with the requisite rotor adapter may be the only option.

 

[suntrakr]

Hi GeorgeOC. Did you work out a rotor upgrade solution for 35mm forks? I'm running brembo and pandemonium adaptor but can't find a rotor to work with it. HAve the MikesXS floating rotor but it needs a fair amount of spacer to make work and will push the rotor off the lip of the mags (unproven but I think it will). Looking for closer upgrade alternative that has similar offset/dimensions to the stock rotor.

 

[arcticXS]

Hi GeorgeOC. Did you work out a rotor upgrade solution for 35mm forks? I'm running brembo and pandemonium adaptor but can't find a rotor to work with it. HAve the MikesXS floating rotor but it needs a fair amount of spacer to make work and will push the rotor off the lip of the mags (unproven but I think it will). Looking for closer upgrade alternative that has similar offset/dimensions to the stock rotor.

See my reply in your thread

 

[arcticXS]

I have the older 34mm forks, with the corresponding caliper.
From your numbers, couldn't I just get a fancy drilled rotor from mikes that fits the 35mm forks and have a shop machine a 5mm spacer to correctly space the rotor from the wheel? Every other spec seems to be the same as far as I can tell.

EDIT: I think that's what you're saying here. Question answered.

As keeping the stock caliper dictates staying with the stock 298 mm diameter, the possible advantages are limited to improved friction properties, weight savings, and pure cosmetics. One way to do it, is fitting a better,, thinner and lighter disc to the oem alloy hub. I actually did this, for my RD350, which has the same caliper as the 73-76 XS, and a 267 mm disc that has the same design as the early XS disc. I got an XS disc hub from a forum member here, and ordered a custom made 298 mm disc for it from ISR in Sweden. 5 mm thick, drilled and lightened, like a TZ disc, just like the one in the attaced pic. Looks great, but is still on the shelf due to caliper mounting issues. ISR use a steel with much better friction properties than oem. So it ended up lighter, better and nicer looking. Also lightened my wallet around 200 $.......

Alternatively, these MAY fit:
https://www.jl-exhausts.com/300mm-floating-disc-2321-p.asp
Or these:
https://www.ettsuan.com/yamaha-tz350-pattern-front-floating-race-disc-new-p-879156.html

There are also lighter reproduction calipers available, I have one on myRD350. Very happy with it after several years:

https://www.economycycle.com/shop/y...er-front-rd250all350all40076-78-reproduction/