Front tyre size and load rating

From your numbers you should be within the rated loading up to over twice the legal limit.  Any load rating will cater for uncertainty (I.e. marginal overload) because design codes contain safety margins which reflect the uncertainty both in loading and in material resistance.

My background is not tyre design.  So jargon may well differ.  Maybe someone else can offer a view?

Tony,

Ask Avon, I have a feeling the answer they give will not be that encouraging. With a full tank of fuel in the mix this adds another 20Kg to weight.

As you know exceeding the tyre rating or any safe working load limit and entering the safety margin is not an ideal place to be.

From memory the 3.5'' x 19 Avon Roadrider has a better rating than the 90/90-19'' but still not great.

And yes, I asked Avon about this regarding the MK3 a few years ago, I now use the 100/90-19. The bike handles well enough, but the Roadrider Mk2's are not as good as the Conti classic attacks I tested for a couple of years, but cost and wear rate and the fact they now no longer do rear fitment mean they are not a viable proposition to fit. 

There is also a 325x19 Roadrider which is the same width as the 90/90 [3.9"] but the circumference is 26.1 as apposed to 25.5" for the 90/90 load/ speed is 54V, specs on the 100/90x19 are 4.3"w 26.4" di & 57V

I have come up against speed ratings several times on my bikes . In theory if you have an accident and you are using an unsuitable tyre you could have an insurance problem.  I dont think Mr Brown was bothered when he did 200 mph  on Nero  using an Avon SM  rear !!.

Hi Tony,

Add to your calculation the weight of 5 1/2 gall tank full of petrol, plus the weight of two sets of riding gear and you probably end up not too far shy of 1000 lbs, which is starting to get on the limit everywhere.

Based on comments regarding quickening up the handling on the NOC website, I did a couple of Alpine trips two-up on my Mk 111 with the smaller 90/90-19 tyre and was very conscious I was on the limit but took the view the manufacturer would have added a built in safety margin. Not sure with hindsight that was wholly sensible ? Norman White however advised me the factory recommendation was specifically 100/90-19 for the heavier Mk 111 and I switched back to this size.

The reality however is that for two-up touring the larger tyre is more than capable of providing the handling most of us require and I would go for that one. In my experience, the smaller tyre provides little discernible additional benefit in this context.g

Hi Tony

I use Salter Ultimate Accuracy bathroom scales which I checked with our calibrated aerospace work scales. They were spot on with a 92kg reference weight (me) and also accurate with other weights we tried (other people) You simply weigh each end of the bike and add both to get the total. Takes careful balancing so may need to do it 3 or more times to get a repeatable value. These scales lock on to the final reading for a few seconds.

Use a wooden square to protect the glass. I have a convenient step in my garden path the perfect height so it's level with the scales and wood top.

My lightened Mk3 weighs 214.5kg/472lb with a full 23 litre/ 5gal Interstate tank ready to ride. I weighed all the various fluids a while back and the total including fuel comes to 21kg/46lb.

Dry weight is 193.5kg/426lbs but I use modern radials on the correct rims which have load ratings of 59W front and 70W rear. Your total weight with a standard Mk3 will be close to 1000lb as Alan says.

However I used TT100, Red Arrows, Pirell Phantoms and Dunlop Arrowmax tyres on my Atlas in the 70s and 80s. Never had a tyre problem at all with plenty of fully loaded, two up alpine touring. Weight would have been close to 1000lb. Modern versions of the old cross plies should be at least as good but not a patch on radials for load capacity, longevity or grip.