MKIII rim width questions for 19" & 18" rear.

I am about to buy some new Excel rims and some new tires for the wheels I'm getting built and have a question or two about rim width.

Tire size will be 120/90V/18" - rear and 100/90V19" - front.

How wide do my rims have to be? The stock rims on my 1975 MKIII measure about 2 3/4" accross the top, measured rim sidewall to sidewall with a tape measure. Not the best way to do it but the wheels are still on the bike. Should I go for a WM2 (1.85) for front and WM3 (2.5) on the rear?

I assume those measurements are in inches???? Wouldn't the WM 3 be better for front wheel use and a WM 4 (2.5) be better suited for rear or am I completely out to lunch here?

WM2 and WM3 are correct for 100 and 110 tyres which is what I use. You can up the sizes but I have never heard what if any advantage. It spreads the tyre a little more and puts the sidewall at a lesser angle. From an old article on Mick Hemmings PR Commando I read he uses WM3 & 4 with 100/120. He knows what he's doing so there is probably something in it.

Here is the tyre/rim width fitment chart from Hagons:

http://www.hagon-shocks.co.uk/WheelTab.htm

Thanks, you guys rock. I figure WM3 on front and WM4 on rear. My reasong is more cosmetic than functionality as I am looking for a meaty look with my new wheels, so wider might be the ticket. Thanks again for the quick replies fellas!

I am considering Avon Super Venom AM18 front and rear (AM20 front is not available in a 100/90/19) or Bridgestone Battlaxe BT45 V-rated tires. I have read comments the Avons wear rather quickly so maybe the Bridgestones might be a better choice.

I put BT45s on this year after years on original Roadrunners, AM20, 21 and 18's. Rear wear seems about the same as AM21. The AM18 wears pretty quick, I only got 4500 out of mine before it squared off badly. 21's would go 7000 with the same kind of use. BT45's don't tramline as bad and they give more confidence on poor surfaces in the wet. They're not immune to breaking away but there definitely a more positive feel in that respect.

I'll probably go with the Bridgestones as you echoed what I have read about the Venom's. Thanks for your opinion Keith.

Which tires will fit on the rear?

So, I am still wondering if a 120/90/18 will fit with no fitment issues or rub. Is a 120/100/18 a good bet as well or will it rub? I'm a bit lost here and have to special order the tires in so I don't want to be stuck with something that won't fit and I want to get this all sorted out so I can get my wheel built.

Coco,
A 120/90-18 Dunlop works on my MkIII cafe racer, but I notched the chainguard for extra clearance. Without notching it looked like about 1/16 inch clearance between the chainguard and the edge of the tire. That was too close for my liking. A different tire profile may give you a little more or a little less.

Ron L said:

Coco,
A 120/90-18 Dunlop works on my MkIII cafe racer, but I notched the chainguard for extra clearance. Without notching it looked like about 1/16 inch clearance between the chainguard and the edge of the tire. That was too close for my liking. A different tire profile may give you a little more or a little less.

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Ron, thank you. That is the answer Iw as looking for. I spoke to a few guys who have done it but they had rear drums so I am just making sure before I take the plunge.

One more question before I go ahead and order tires. Is the tire on the rear of my MkIII supposed to be exactly centred between the swing arm?

There is talk about a crazy offset to the wheels on Commandos but as I look at my MkIII, the tire is closer to the right (rear disc caliper side) than it is to the left. I would assume the tire should be completely centred between the two sides of the swingarm.

I can squeeze 1 finger between the right side of (disc brake side) the swingarm and tire, and I can fit almost two fingers between the swingarm and tire on the left side. Something tells me this is not right.

Coco
That is how it should be. For some reason and I've never asked why, the Commando cradle and front mount are offset appx 1/4" to the left. Some will tell you it's 3/16" but on mine it's 0.244" rear and 0.291" front which is why I'm having the front machined and spaced to match the rear.
You need to accurately mark the swingarm centre (on the cross brace), find the tyre centre and measure difference. Almost an extra finger on the left sounds a bit much! Less than half would be close but depends where the wheel is in the adjusters. The front wheel must be exactly centred in the forks too. When you take the measurements make sure the rear wheel axle is exactly the same distance both sides from the swingarm pivot centre so you don't have misalignment here screwing up the offset data. Not that easy to do on the bike. Good luck.

ludwig said:

the ONLY reference point for your rear wheel (offset) is your front wheel .

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Sorry if I sound a bit ignorant, but I am not understanding what you wrote.

I am used to building bicycle wheels regarding offset or *dish* and the rim was always dished properly or centred between the bearing cones and lock nuts if it had disc hubs or centred between the hub flanges if there was no disc brake on the front. I realize motorcycle wheels are not bicycle wheels but I can relate to bicycle wheel building as a reference point.

I am unsure if the rear swingarm is off set or ii is symetrical. If it is symetrical then the rear wheel should be centred within the swingarm and if it is offset to one side, then the wheel position should compensate.

Ludwig is correct, the front wheel is the start point and must be centered in the forks, the rear is aligned off the front.
On the offset question, like I said, the cradle, front mount and therefore swingarm & rear wheel is 1/4" nominally offset to the left of the frame centreline and therefore the front forks centreline. So, Norton built the rear wheel offset 1/4" to the right to get it nominally aligned with the front. I don't know the reason for cradle offset, maybe to get clearance for the chaincase from the frame, that's a guess. Someone knows. I always felt it was dumb question and I should be able to figure out the answer!
The swingarm is symmetrical on 750's and I assume Mk3 850 also so a centred rear wheel would be misaligned with the front. That's why you have right offset, but too much maybe.
Sort the front out first and move to the back. If you get the front centered and rear right offset in the 1/4" ballpark you will be close. To do it highly accurately you need to strap the bike vertical, make sure the front wheel is straight ahead (using plumb line on each side dropped from the top of the rim) and then you can go ahead with the string trick allowing for difference in tyre diameter by spacing the string either side of the front tyre.

Thanks keith, I get it now. I think putting an 18" rear wheel and 120 tire is going to be more of a tricky situation than I originally expected. With that much offset to the right side a 120 raer tire will definitely not fit so I am unsure how others like Mick Hemmings have done it (maybe it was not on a MkIII). Might be safe to go with a 110 but I wanted to try a 120 on the back.

Ron L said:

Coco,
A 120/90-18 Dunlop works on my MkIII cafe racer, but I notched the chainguard for extra clearance. Without notching it looked like about 1/16 inch clearance between the chainguard and the edge of the tire. That was too close for my liking. A different tire profile may give you a little more or a little less.

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Ron, did you run into any problems with the offset of the rear wheel when putting the 120/19-18 on? besides nothing out the chainguard, my problems seem to be with clearance on the right side and not the drive side.

My problem is I have to order the wheels and work will not be done locally so this is a one shot deal for me and I have to get it right the first time.

I encountered no problems with the brake side of the wheel. I laced the wheels myself retaining the stock offset. My wheels are aligned, but I am also using a Dunstall double disc front end. I'm not sure I agree with all this, I'll have to do some measuring on my other bikes when I get back out in the shop before I comment.

Ron L said:

I encountered no problems with the brake side of the wheel. I laced the wheels myself retaining the stock offset. My wheels are aligned, but I am also using a Dunstall double disc front end. I'm not sure I agree with all this, I'll have to do some measuring on my other bikes when I get back out in the shop before I comment.

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Thanks Ron. I'm using or will use a Brembo front disc kit from CNW, mounted on the right side opposed to the stock disc being mounted on the left.

Buchanan will be doing my wheels for me, which is why I need to sort all of this out before I get them to make my set. I still want to do an RM3 on the front and RM4 rim on the rear so I need to know if those widths will work, which I assume they will.

Coco
If you use a BT45 the widest dimension across the tyre (tread edge) is amazingly 110mm +/- 0.5 on mine but that's on a WM3 rim (2.5"). It might follow that the 120 is 120mm but not sure if the WM4 rim will increase this a touch and make the sidewall the widest point? Looks like Ron has the answers but I'd think the chainguard is the closest point even with the wheel offset to the right. On non Mk3's one issue is getting the tyre past the drum fixing lugs, maybe you don't have that problem.

Keith1069 said:

Coco
If you use a BT45 the widest dimension across the tyre (tread edge) is amazingly 110mm +/- 0.5 on mine but that's on a WM3 rim (2.5"). It might follow that the 120 is 120mm but not sure if the WM4 rim will increase this a touch and make the sidewall the widest point? Looks like Ron has the answers but I'd think the chainguard is the closest point even with the wheel offset to the right. On non Mk3's one issue is getting the tyre past the drum fixing lugs, maybe you don't have that problem.

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This is turning into a small nightmare so I want to get everything sorted before I jump in and order all these parts.

I was told by RGM motors that a WM4 rim would be too wide for the rear so going by that I might do WM3 front and back.

I measured the stock commando rim with a tape measure accross the rim and it is 2.5" wide. I still think a WM3 would be narrower than a 120 tire so I don't think the sidewall would stick out farther than the tread surface, but maybe I'm on glue and am missing something.

Tire choice is down to Bridgestone BT45 and Avon super Venom 100/90v - 19 on front and 120/90v - 18 on rear, both of which I can obtain locally by special order.

Coco, My Combat has a Mk III swing arm with a 2.5 Excel with a 120/90/18 Metzler ME55A with a custom chain guard and a 530 chain.
No problems, but it's close. For various reasons I can't fit a stock chain guard, but it looks like it would be problem.

My Dreer has a 110/90/18 with the stock guard. I'll have to check the wheel size, but I think it is a 2.15. The front is a 90/90/19 with I think a 1.85. Again, I'll have to check.

The 100/90/19 barely clears the stock front fender. I don't know anyone who has put a 2.15/19 on the front. It may spread the tire enough to cause an interference. I'm not sure. Just be prepared for this.

Oh the joy of customization.