10% rule or 10% front, 20% rear rule

[Jerry_75_Guy]

Could some repost the info or post a link to info on the 10% rule regarding tire inflation?

 

I recently chanced upon a site claiming that 10% front and 20% rear was the way to go (which I can't seem to find again); which is better, and does it even make a difference?

[ShovelStrokeEd]

Neither is better nor worse than the other, both are based on an old racing rule of thumb where racers started with much lower tire pressures than you would tend to run on the street. The rule dates from way before pyrometers were cheaply and readily available. It was an attempt to correlate internal pressure to tread temperature and that doesn't always relate in either a linear or predictable fashion.

 

Here's why. Almost all of the temperature variation and hence pressure change in a tire comes from flex of the carcass causing hysterisis in the rubber and belts to make heat. That heat is transferred to the air within the tire. Tread temperature is slightly related due to the same flex causing heat in the tread as well as transfer of pavement temperature to the tread and the scrubbing action of minor slip during cornering or acceleration (positive or negative). Rubber is a pretty good thermal insulator and not much of tread temperature will actually transfer into the air within the tire. Go ride through the desert in a wet suit if you don't believe me.

 

OK, so now you are left without a rule of thumb. What's a poor boy to do? Didn't think I would leave ya like that, did you? Naw, not me, I'll make you work to develop your own rule of thumb. All you need to know is the weight distibution of your bike under a couple of conditions, you really need only one and you can do a little more 'rithmatic to figure out where you should be. You are going to have to define a couple of goals as well. Do you want maximum tire life? Mazimum traction? Something in between? A comfortable ride? Quick, responsive steering? Hmmm, lotsa stuff to think about.

 

OK, here is what I did for my bike. I took it to a truck scale and got a report on the actual weight on each axle. Then I compared it with the load rating of each tire at that tire's max, cold inflation pressure. Then I calculated the % of maximum load my tire was forced to endure and reduced my cold inflation pressure by an equal percentage. It is easier if you divide the actual load by the max load and apply that number to the pressure. On my Blackbird, fully laden with my normal assortment of luggage, it worked 38/40. I get pretty good tire wear this way, nice stick in corners and pretty nice response out of the steering. Yeah, it was a little more work and a couple of minutes with a calculator but so far, it works and I don't have things like tire construction adding any more variables.

 

Tire construction? Sure, a stiff carcass tire like a bias belted ME880 is going to flex a lot less than something like a Pilot Power or Pilot Race, if you used the 10% rule on those two tires, the 880 would wind up under inflated and the Pilot Race would probably be over inflated and not yield the response it should. I would also think the 20% rear would be major BS. To get a 20% rise in pressure, the internal temperature of the tire would have to increase by 20% of its Kelvin temperature. Let's say you started at 70 degrees Fahrenheit, that's 294 degrees Kelvin. 20% over that is 353 degrees Kelvin or 175 degress Fahrenheit. Way to warm.

[Jerry_75_Guy]

Very illuminating! Thanks very much Ed.

 

I had a gut feeling that the 20% rear rule would be a bit warm, but no experience/knowledge to support it.

 

Now to find the nearest scales.....

 

P.S. Not to be an 'apple polisher', but have you considered writing a book?. Seriously, I think that you could probably put together a pretty interesting, and useful volume.

[ShovelStrokeEd]

No writing of books for me. I do write technical manuals as part of my job and that is tough enough. 'Sides, the list of attributions and credits would wind up a bunch longer than the text. Just about everything I know came from a book somewhere. Luckily, I like to read.

[ltljohn]

Ouch, my head hurts now