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Fat tire

Posted: Mon May 22, 2017 12:31 am
by Zombie Master
84 R100 mit rear drum
So I'm looking at my rear wheel and I see there is a 3mm gap between the rear drive gear casing, and the wheel. The reason for the gap is because the idiot PO (me :cry: ) thought he needed a fat-er tire on the rear. Now that I'm more mature, and have learned that BMW knows more about building these bikes than mechanics that suggest opinions on modifications like this. I'm wondering if I should go back to the OEM size tire, which would require re-shimming. I also see that the rear break shoe is not worn all the way across the brake shoe and there is a small area that is above the rest of the at the far edge of the shoe. I would suppose that the spline is also not fully engaged. I realize that I am fairly writhing in incorrectness. I have run the 120/90/18 size tire for many years without problem but should I change this back to der fatherland's specification? Ich verdiene keine Gnade :|

Re: Fat tire

Posted: Mon May 22, 2017 3:22 am
by ME 109
Have a close look at the fd spline and see if the wear seen at the end of the spline is a single step, or is there a second step, caused by moving the wheel outwards.

If there is a second wear step, removing the longer top hat for stock, will make the wheel spline cup ride up over the step and have a not so nice contact pattern.

Re: Fat tire

Posted: Mon May 22, 2017 3:48 pm
by Zombie Master
ME 109 wrote:
Mon May 22, 2017 3:22 am
Have a close look at the fd spline and see if the wear seen at the end of the spline is a single step, or is there a second step, caused by moving the wheel outwards.

If there is a second wear step, removing the longer top hat for stock, will make the wheel spline cup ride up over the step and have a not so nice contact pattern.
I'll be pulling that wheel off and having a look. Do you think it's a problem to leave things the way they are in the long run?

Re: Fat tire

Posted: Tue May 23, 2017 4:53 am
by ME 109
You lose a couple of millimetres contact on the spline, which is a percentage loss of maximum contact. In your case not much of an issue.
I'd be more concerned about the wear steps. If there is a reasonable second step, leave it with the 3mm gap.
When you take the wheel off you'll see what I mean about the wear step/s.
At the fd end of the splines.

The second step

Posted: Thu May 25, 2017 1:08 am
by Zombie Master
Yes there is a second step. A great example of why modifications usually end up becoming a problem. So now I can't go back to the OEM tire size? I have less spline connection which will certainly lead to earlier failure. Also It is difficult to remove my rear wheel. Expert advise from a supposedly factory trained mechanic. Keep it stock!

Image

Re: Fat tire

Posted: Thu May 25, 2017 3:13 am
by ME 109
So you can see that by putting the original spacer back in, the wheel spline will ride up on that step.
Your second step is not toooooo bad.

You can put the correct top hat back in, and one or both splines will concede to the point where full contact is restored.
If the step was significant, then a twist would develop in the wheel spline cup under power.
The fd spline will weather the storm easily, the spline cup however, is hanging on er, by it's teeth.
I haven't heard of a wheel spline cup breaking.

Re: Fat tire

Posted: Thu May 25, 2017 4:41 am
by Zombie Master
Well I have lots of rubber left on this tire, I guess I could run it out then change back, unless the step gets deeper. The difference is actually only 2.91 mm.

Other than finding a new top hat, are there any changes I would need make ?

Re: Fat tire

Posted: Thu May 25, 2017 5:10 am
by ME 109
How far is the tyre from the swing arm? Therein lies the answer of what to do now.
Will the current tyre rub if you change back to stock?

Stock top hat is all you need. Get the rh side, they're different lengths.

my fd clearance is about 0.6mm to 0.7mm which might be tighter than stock.
I'm not sure what the spec is, but I'd imagine about 1mm.

I find the skinnier the tyre, the more pissed the sport bike riders get. :mrgreen:
Perhaps try a Pilot Activ next tyre. They're a 4:00, fit with maybe 3mm swing arm clearance. Very popular airhead tyre. Can be difficult to seat on a snowflake tho.

Re: Fat tire

Posted: Thu May 25, 2017 11:26 pm
by Zombie Master
So due to the relocation of the brake hub, the upper brake shoe has a small area on the drive side edge that is raised. I'm wondering if I should sand that off? Would there be a problem corresponding problem with the brake drum surface as well?

Re: Fat tire

Posted: Fri May 26, 2017 5:52 am
by ME 109
Drum brake? I thought you had a disc.

Sanding or grinding the brake shoes would be best practice, at least for ease of getting the wheel on.
Have a look at the drum for a step. If it's not a significant step I'd put it back together and let the drum wear the shoe to suit. The shoes would flex a small amount and still make good contact with the drum, unless the step was significant.