CM rims...

[michaelmasseling]

Hey people,
I'm Michael and new here. Been reading the forum for a while though. Decided I wanted to be a member... Thanks for letting me join you all! I'm foreign (holland), so please forgive me my mistakes. My bike is an '82 fxs, being under constant change. Recently I could buy a nice 18" wheel. Coincidentally I heard that it had cm-rims as opposed to wm-rims. And that most tires, except specially made goodyears, don't fit the cm-rims . In fact, the normal tires may slip on the rim, causing to tear the tube... Has any of you heard of this to happen?

[76shuvlinoff]

I can't answer your question but welcome!

[CraigArizona85248]

Welcome aboard Michael.  Below is some information I found on this subject from another site I frequent.  Sounds like you are smart to be concerned about using a CM type rim.  I've was thinking about putting a 18" CM rim on the rear of my panhead until I read this information.  Still haven't given up on the idea, but I want to see if I can find a 18" WM rim that I like.

-Craig

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M.O.Ther        08-31-2007, 10:54 AM

I've always liked the look of 18 and 19 inch rear wheels better than 16s. But there's something different and possibly dangerous about the Harley 18 and 19 inch rims from the '70s that I'm not sure most people are aware of. These rims are CM profile rims.

What does that mean? CM is the engineering term for the size and shape of the rim and bead seat. WM is an earlier designation, and it means more than just a width, (WM3 = 2.15" etc.,) but a specific size and shape. Current rims are MT or TL profile, at least in those sizes. The problem with the CM profile is that it's .075" smaller in diameter than the others at the bead seat.

Here's what Dunlop says about it. "Fitment to Harley-Davidson 18-inch and 19-inch CM contour rims may result in slippage or air loss. Harley-Davidson 18-inch and 19-inch CM contour rims are not compatible with Dunlop tires" Harley's recommendation to owners of bikes with CM contour rims is to replace the rims or wheels when the tires wear out. My boss, a Harley dealer since the late '60s (in a family owned dealership open since '62) tells me I'm not to even sell tires to guys I know have these rims on their bike, even if they want to mount them themselves. Harley Service Bulletin M-731 dated 6/20/78 deals with this issue.

How to identify these rims: This actually applies to steel rims of the same era as well. The Aluminum rims have a square drop center, a 2.50" bead width, and one of the following numbers stamped in them.

Number Material Size Hub/Brake

43005-70 steel 18" steel spool rear drum
43008-70 aluminum 18" steel spool rear drum
43002-70 steel 19" '64-72 style front drum
43010-70 aluminum 19" '64-72 style front drum
42998-74 aluminum 19" aluminum hub front disc
43018-73 steel 19" aluminum hub front disc

I have also heard of some Lester cast wheels being made to the CM rim profile, however Harley cast wheels are not CM rims.

You can also measure. Measure the circumference of a known good rim (say a WM3 x 18) against one of these and there's almost 1/4" difference at ther bead seat. .075(difference in diameter) x pi = .236".

The problem: If you look at the cross section of a tire bead like a foot, normally the whole bottom of the foot and the back of the heel are in contact with the rim. In the case of a normal tire on a CM profile rim, only the toe and the back of the heel are in contact, and the bottom of the heel is about .037 away from the rim. Less contact means more chance for the tire to slip on the rim. Since these are all tube type tires, the tube moves with the tire and the valve stem rips out of the tube, instant flat tire.

The only tires ever made for CM profile rims were Goodyears, marked "for taper bead base only" and are all at least 25-30 years old by now, not to mention impossible to find.

So it's Harley's, Dunlop's, and probably all the other tire manufacturers recommendation that you don't run these rims with normal tires on your scoot. But some people do and will anyway, including me. In part 2, I'll give you some tips in case you want to ignore the experts' recommendations.

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M.O.Ther        08-31-2007, 10:56 AM

Part 2
If you’ve decided to knowingly go against the experts’ recommendations, here are a couple tips to keep in mind.
The bike itself: A seriously high torque motor or really good brakes, combined with a tire that gets good traction might be a combination worthy of second thoughts on these rims
Tire selection: All these rims are .250 at the bead seat. Recommended tires for that size run all the way from 3.25 or 90/90 at the narrowest to 130/90 or 140/80. I’d stay away from the extremes of that. The farther away from the ideal, the more “off” the angle of the tire to the bead seat.
Tire mounting: Use a tire mounting lube that will dry up, like a little dish soap in water. Do not use one that will stay slippery for a long time, like WD-40 or a silicone spray. Make sure the valve stem is perpendicular to the rim and check it often, if it starts to tilt, you’ve got a problem. Go toward the high end on the inflation pressure, remember the bead is being held against the rim with 32 psi, or whatever. It’s not the place to run low pressure for a smoother ride. The tire generally will center itself, but check to see that the tire is true after mounting. It’s a little easier in this case to get a fold of tube stuck under the bead. Consider the use of sheet metal screws through the rim and into the tire bead, but not deep enough to damage the bead cord. It’s an old racer’s trick, and could even give your scoot a little bit of a competition look.
These rims are cool looking, plentiful, and sometimes reasonably priced. A lot of guys run them, and have been getting away with it for years. I just think it’s better to make an informed choice.

[michaelmasseling]

Thanks Craig!
Usefull info. I think I'll be looking for wm-type rims as well.

[MMOCGuy]

shovelmass.

I just wanted to welcome you to the forum. You'll find answers to any of your questions here and all of our members are willing to help each other.

Welcome!!

Norm.

[michaelmasseling]

That's great!
Thank you Norm.