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

I agree with Ashley that consultation with Avon (in writing) would be a wise precaution, but as general principle surely a tyre rated for x kg @ y mph will be well within limits at x kg @ 0.5y mph?

I've had a front end wobble on my 750 Interstate now for a few years. It happen when slowing down from 60 to 30mph. It's not really bad if I hold the bars but if I let go with both hands it starts to get quite bad. I was running 100/90/19 front and rear.

I had a new pair of Roadriders fitted for a rally 90/90/19 front and 100/90/19 rear and also changed the cush drive rubbers in the rear hub. Wow, what a difference. No more wobbling at any speed with or without hands on the bars. I've got so much more confidence in the bike now that I've been riding like an 18 year old at times. The tyres cost me £290 including new tubes and rim tapes plus balancing fitted to loose wheels. Tyres pressures don't seem to make much difference to handling so I'm running about 32 f and 36 r. The bike just falls into corners now and feel really stable.

My experience of running the correct tyre for long distance with heavy load. Two adult men, Honda 250, Yorkshire to Corsica driving one hour on and one hour off, in 2 days. Plus luggage. The Avon sidewall in the rear developed a large blister, spotted on arrival, and I got a new Conti in Ajaccio. Obviously overheating and delaminating. No problem with the Conti of the same size.

Ordered rear tyre for Commander on internet. Smaller section than ordered turned up. Hassle to return so fitted. Loaded and heading Yorkshire to Portsmouth with wife and luggage, the tyre definitely felt warm to the touch at break stops. Kept my speed down in hot conditions. Rain good. Changed for correct section when suitably worn.

I guess based on this, my advice would be to watch the tyre temperature build-up, due to hysteresis (flexing) at high loads and road temperature. Too high for too long and some tyres delaminate.

If in doubt stop and compare temp of rear with front by touch. 

Peter 

Peters technique of working to temperature is probably how  bike manufacturers arrived at  pressure recomendations back in the old days.It has worked for me . I check tyre temps by hand at stops on long journeys and it has given me warnings when pressures need adjusting .I have done countless 1000 mile journeys accross Europe in temps from freezing  to 100f .  Vans needed pressures of between 45  and  80 psi. Sometimes  running empty pressures would be up to  10 psi below what the handbook said  to improve  comfort and grip. Tyres sometimes  lasted  40 000 miles .Often tyres were working at Max allowable pressure and loading and could get  nearly too hot to touch . Cheap tyres never lasted. Some expensive French made Michelin tyres just refused to wear out and died a ripe old age after being transfered from van to van. Its an interesting subject.