1976 XS650 rotor dimensions

[flamesnm]

Hi

Haven't posted for a while, but was out for the first ride of the new year today and realised the list of jobs I wanted to do over the winter didn't get done. When I put the bike together I did the Brembo conversion and always intended to swap out the industrial rotors for something lighter and better looking. I bought the floating disc from Mike's XS but didn't pay attention that it only fits '77 onwards ( offset is much too small ).

So I've been looking around for options and came across ISR in Sweden who can be purchsed in the UK through bikehps.com. They provide a custom form with measurements required to put together a custom one off rotor. It don't expect it will be cheap, but without engineering spaces/brackets it might be my best route.

Before I haul the wheels off to break out the calipers, I wondered if anyone else has the measurements for the stock front and rear rotors ( mine earlier XS has a rear disc conversion ).

The form for the measurements is here a photo of my XS with the industrial rotors is below. Thanks in advance for any information.
https://www.bikehps.com/isr/ISR_Custom_Disc_Order_Form.pdf

 

[figure8]

Is this the rotor you bought?
https://www.mikesxs.net/yamaha-xs650-brake-rotor-drilled-floating-77-84-front-with-spacers.html
If so you need an offset spacer for the rotor, I had to do a such for a brembo with that rotor.
http://www.xs650.com/threads/brembo-caliper-w-motolanna-rotor-spacer.45576/

 

[flamesnm]

Hi figure8

That thread is gold. Thank you.

 

[gggGary]

There is a US company with an ebay presence making custom rotor spacers. I made one for that mikes XS rotor (about 7mm), it's offset is less than even the "77 on XS650 rotors". I think this is a much better "kludge" than caliper spacers. BUT, another issue: it doesn't take a very thick spacer to move the rotor beyond the centering ring on the wheel flange. Kind of scary not having that rotor positively centered.
Also: finding CORRECT rotor bolts with a full size shank that extends all the way to the wheel flange is NOT an easy task.
This stuff gets complex quickly.

 

[flamesnm]

Thanks gggGary. I'm going to get in touch with them and point them to them to figure8s thread to see if they can come up with the correct sized spacer. It would be good to have a standard option for others going down this line.
1. Get this spacer from these guys.
2. Get these rotor bolts.

I'll see if I can put that together from info in figure8s thread depending on the response from that ebay comp.

 

[MaxPete]

Hi All:

Gary is dead right - this stuff gets complex - rapidly and I have to say that some of what I am reading concerns me. Not to brag but I am a professional mechanical engineer who specialises in vehicles and so I can "do the math".

Looking at a brake disc that is held on by say...6 bolts: the actual mechanism by which brake torque is transferred from the tire & wheel to the disc is more complex than it may appear. The real mechanism is that the axial force in the bolts (i.e. along the length of the bolts) generates friction between the flat surface of the brake disc and the wheel hub.

Thus, the real function of the disc mounting bolts is not to resist brake torque - it is to hold the disc onto the wheel hub tight enough to generate that friction.

If the mounting surfaces of the disc and the hub are not in intimate contact - there will be no friction and the bolts will be fully loaded in shear (ie. across their axis) - and bolts are NOT intended to work that way. No good engineer designs bolts to be loaded in shear because in that loading mode, they are much less than half as strong as bolts loaded in tension.

Shear loaded bolts place you at risk of having the disc break away from the wheel when you nail that front brake hard.

So - if you are making any sort of disc-wheel spacer, be sure that it provides full surface contact between the brake disc and the wheel hub, Simply using a few little round pieces of tubing to act as spacers of the correct length to place the disc the right distance from the hub - would be unsafe.

Sorry to be your Mom - but I don't want to see anyone get hurt.

Pete

 

[gggGary]

Well a goodly amount of surface anyways, many years have hollows in the wheel brake flange area.

This early front hub has steel inserts for the bolt threads later hubs don't have inserts, the aluminum is threaded.
On most newer motor cycles with more powerful brakes the rotors just have tabs.
FJR front


In 72 the rotor sat on it's own bearing and was connected to the wheel through 2 small tabs with rubber boots around them! ALL the stopping torque went through those. Shrug

Very good point about correct torque on the bolts!

 

[5twins]

I've used stainless replacement bolts on a few wheels already and haven't had any problems ..... yet, lol. The bolts aren't shouldered either, they're fully threaded. I don't even have lock tabs on this one, just wave lock washers .....

Flamesnm, your pic shows you have the later disc installed.

 

[flamesnm]

Flamesnm, your pic shows you have the later disc installed.

Whaaaaaaaaaaaat ? Now I'll have to get out with the calipers in the cold and measure up.

 

[retiredgentleman]

Good explanation there Pete!
When I installed Brembo calipers front and rear, I was happy to find that I could use my stock discs, and no spacer was required. The offset is such that my discs bolt directly to the wheels, the same as the stock design. I do think spacers are fine as long as they are done correctly, and the correct torque is used.

 

[flamesnm]

Finally got out into the garage and it may be the later disc ( I don't have an earlier one for comparison ) but the offset is definitely still different from the floating rotor from MikesXS so there's no easy bolt on. Still need a spacer.