Boxerworks.com

Hi I just bought a 1974 R90/6 with only 17,000 miles on her. Absolutely mint condition, two owner, all original. it almost looks like it was kept as a show piece. I want to replace the tires with Avon Speedmaster 3.50 * 19 upfront and 4.00 * 4.00 safety milage in the rear. my question is if a r90/6 cant fit a 100/90/19, is it because of the width or height of the tire? the speedmaster is narrower but taller then the 100/90/19.

It's because of the width. The original stock tire was the same height as it was tall. A 90 series tire is only 90% as tall as it is wide, so it is actually shorter than an "even aspect" tire of the same size.

In addition, I'm sure some 100/90's could fit, while some may not. Every manufacturer, and every tire model, is actually a little different in that same size.

Those avons were great tyres back in 1968 fitted to my A10, there are some modern tyres made in the correct imperial sizes that will work very much better.....

Just a suggestion

Hullo JG, and welcome to the forum.
That's quite a find - congratulations. Some pics would be nice.
There isn't much room between the sides of the fork brace so that is the limiting factor. My R90 came with a metric sized front tyre and it rubbed on the brace (sorry, can't remember the brand). Stick with the 3.50s and you should be fine. I'm currently using Michelin Pilot Activs - 3.50x19 and 4.00x18 - and loving them.

I've found 90/90-19 work well on my /6's

richard t wrote:I've found 90/90-19 work well on my /6's

Ah, there you go. The one that rubbed must have been a 100/90x19. The rear tyre was over sized too and rubbed on one side of the swingarm. Had to deflate it and use force to get the wheel out of the frame. New standard sized tyres improved the handling dramatically, but that could have been something to do with the age of the ones replaced.

I would dare to say that 90/90/19 and 4:00/18 on any 70's > twin shock will not give issues.

100/90, 120 or 130/18 etc weren't standard fit on these bikes.

Buy a mono or a jap bike iffen yer want fatties.

Just a caveat on the 4.00 Bridgestone BT45, At the widest point it is 118mm wide and can be a bit do a bear to fit on pre 81 bikes, the Michelin pilot actives in 3.25 and 4.00 inch sizes are in contrast perfectly sized.

i decided on 3.25 - 19 avon speed master in the front and 4.00 - 18 safety master in the rear. basically sock size maybe a little bit smaller.So far my bike is in frustratingly good condition, i like fixing things.

jgleadle wrote:i decided on 3.25 - 19 avon speed master in the front and 4.00 - 18 safety master in the rear. basically sock size maybe a little bit smaller.So far my bike is in frustratingly good condition, i like fixing things.

Be careful what you wish for ...