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#265702 - 07/19/09 6:28 pm Maximum rear tire size  
Joined: Dec 2007
Posts: 33
proverbs11one Offline
BritBike Forum member
proverbs11one  Offline
BritBike Forum member

Joined: Dec 2007
Posts: 33
Kansas City, MO USA
I need a new set of tires on my 72 Commando. I ride the flat freeways of the middle USA, so 70 MPH cruise is typical. I would like to install a larger diameter tire on the rear to reduce the RPM's. Entire bike is stock with WM-2-19 both front and rear. Current tire on rear is AVON Roadrunner 4.10 H 19., mounted with rotation arrow pointed in the wrong direction.

QUESTION 1: Although my goal is to increase the tire diameter, I realize that tire width will increase as well. Looks like there is enogh side clearances at the chainguard to accomodate a wider tire, perhaps up a size or two. What is the largest size tire that will work with a WM-2-19 rim and still clear the chainguard?

Question 2: Why did the previous owner install the AVON with the rotation arrow pointing in a reverse direction of normal rotation?


Don Lozano
1974 COMMANDO
1973 COMMANDO
1972 COMMANDO
1970 T120R
1970 TR6C
1967 T120R
1965 A65T
1977 R100/7
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#265708 - 07/19/09 7:06 pm Re: Maximum rear tire size [Re: proverbs11one]  
Joined: Sep 2008
Posts: 103
Rick in Seattle Offline
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Rick in Seattle  Offline
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Joined: Sep 2008
Posts: 103
Mercer Island, WA / Austin, TX
Proverbs,

With a 19" WM2 rim you have limited capacity for a larger tire. Going to a WM3 will allow for a 110/100 or even a 120/100 before having interference problems with the swingarm. However, the problem of finding longer legs on your Norton is usually addressed by reducing the countershaft sprocket size down to,say, 19 teeth. Replacement sprockets are sold by all of the usual suspects; be sure to match to the chain size, usually 530.

You can find Avon Roadriders in your size at 90 bucks or so online. If you choose to rebuild your rear wheel to a more maintainable WM3-18", you'll spend 200 bucks or so in parts and another 100 bucks to have them laced, trued, and balanced.

When mounted correctly, the Roadrunner tread pattern is inverted from rear to front tire (rear tire is for acceleration, front for breaking). The DPO may have valued a bizarre sense of symmetry over safety and performance. You should religiously obey the rotation arrows.


Rick in Seattle
1970 BSA 441 Victor Special
1972 Norton Commando 750 Combat
1975 Norton 850 Commando Mk3
1960 Triumph TR3A
#265720 - 07/19/09 8:50 pm Re: Maximum rear tire size [Re: Rick in Seattle]  
Joined: May 2006
Posts: 97
R Model Tony Offline
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R Model Tony  Offline
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Joined: May 2006
Posts: 97
White Rock, BC
In fact, to lower RPM's you should INCREASE the countershaft sprocket size. I run 21T on both my Commandos and like it, but I know folks that run up to a 23. Acceleration will suffer somewhat but RPM's will be reduced at crusing speed. Regards, Tony

#265779 - 07/20/09 4:27 am Re: Maximum rear tire size [Re: R Model Tony]  
Joined: Sep 2008
Posts: 103
Rick in Seattle Offline
BritBike Forum member
Rick in Seattle  Offline
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Joined: Sep 2008
Posts: 103
Mercer Island, WA / Austin, TX
I was hoping I could correct my error before another member caught me, but no such luck. You're quite right, Tony, the countershaft sprocket must be increased to increase top end speed at constant rpm. The part cost is small, but the effort is large, as the entire primary, including the inner cover, must be dissasembled to get to the sprocket. Figure a full day to do it. You may need a 102-link chain as well.


Rick in Seattle
1970 BSA 441 Victor Special
1972 Norton Commando 750 Combat
1975 Norton 850 Commando Mk3
1960 Triumph TR3A
#265809 - 07/20/09 12:11 pm Re: Maximum rear tire size [Re: Rick in Seattle]  
Joined: Dec 2007
Posts: 33
proverbs11one Offline
BritBike Forum member
proverbs11one  Offline
BritBike Forum member

Joined: Dec 2007
Posts: 33
Kansas City, MO USA
Thank you for the wisdom. I found some Brigstone S11 tires for about $70 each. The specs say the tread has 2 different compounds. The sides are soft to provide better grip in the curves. The center is harder to provide better wear. The specs for the 110/90-19 say the minimum rim width is 2.15 inches.

I attempted to measure the width of an old WM2 rim, but the sides are tapered and rounded such that there is not a definitive surface to get a measurement. Has anyone mounted a 110/90-19 onto a WM2 rim? This will be for rear only. The front will be 100/90. Thanks.


Don Lozano
1974 COMMANDO
1973 COMMANDO
1972 COMMANDO
1970 T120R
1970 TR6C
1967 T120R
1965 A65T
1977 R100/7
#265839 - 07/20/09 2:38 pm Re: Maximum rear tire size [Re: proverbs11one]  
Joined: Oct 2007
Posts: 234
Ron Leisner Offline
BritBike Forum member
Ron Leisner  Offline
BritBike Forum member

Joined: Oct 2007
Posts: 234
Ohio
WM-2 is 1.85 inches wide. Minimum size for a 110/90 tire is 2.15 or WM3 with 2.50 inch recommended.

Who manufactures a 110/90-19 REAR tire? The 110/90-19 S11 is a front fitment only.

I would stick with a 100/90-19 Avon AM26 or lace up an 18" rear rim for a wider selection of tires. Just my $0.02


Ron L
1973 MkV 750 Roadster
1974 850 Interstate
1975 850 Cafe Racer
1968 750 Fastback
1967 P11
1969 Ranger 750
#266016 - 07/21/09 11:42 am Re: Maximum rear tire size [Re: Ron Leisner]  
Joined: Dec 2007
Posts: 33
proverbs11one Offline
BritBike Forum member
proverbs11one  Offline
BritBike Forum member

Joined: Dec 2007
Posts: 33
Kansas City, MO USA
Thanks Ron. From your list of Nortons, you have probably been through this tire search a few times. Appreciate the good advice. Avons it is.


Don Lozano
1974 COMMANDO
1973 COMMANDO
1972 COMMANDO
1970 T120R
1970 TR6C
1967 T120R
1965 A65T
1977 R100/7

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