The 10/20% rule for Tyre pressure - Ducati.ms - The Ultimate Ducati Forum
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post #1 of 17 (permalink) Old Feb 12th, 2019, 9:51 pm Thread Starter
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The 10/20% rule for Tyre pressure

I saw a small article on this subject and I've heard it mentioned several times before so I was wondering if there's anything official to confirm or deny what this guy is saying?
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A technique for those wanting to get the most out of their tires on the street is to use the 10/20% rule.

First check the tire pressure when the tire is cold. Then take a ride on your favorite twisty piece of road. Then, measure the tire pressure immediately after stopping. If the pressure has risen less than 10% on the
front or 20% on the rear, the rider should remove air from the tire. So for example, starting at a front tire pressure of 32.5 psi should bring you up to 36 psi hot. Once you obtain this pressure increase for a given rider, bike, tire, road and road temperature combination, check the tire pressure again while cold and record it for future reference.
https://www.mad-ducati.com/Articles/TirePressure.html

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post #2 of 17 (permalink) Old Feb 12th, 2019, 10:43 pm
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The answer is, for a tire to perform at its best it has to reach, but not exceed, the temperature at which the rubber compound gives the maximum grip.

The difficulty is, predicting the temperature that the tires will actually reach for each combination of bike model, suspension set-up, rider weight, cornering and throttle technique, air temperature, road temperature, road surface and curvature ... just to name a few variables. This is why tire testing is so important, and also why so much time is devoted to it by race teams.

For street bikes, Ducati plays the averages. When they do development work on each bike model, Ducati test riders and engineers work with their OEM suppliers, Michelin and Pirelli, to come up with a tire that gives a good overall performance (wet, dry, good milage, etc.) Along the way, the tire pressure that works best for these OEM tires for AVERAGE ROAD USE is determined, and thatís the number that ends up in the Owners Manual. As power increased and tire construction and rubber formulations changed over the years, so have Ducatís recommendation for tire pressures. Early 916ís for example, had a recommended pressures of 32F/35R, but 30.5F/32R is now recommended for 99Xís.

So when you move to a tire that isnít OEM, then you have to do your own tire testing to determine the best pressures FOR YOU, and you have to hope that the tire manufacturer has done the necessary development work to give you a good starting point.

The non-Ducati OEM tire manufacturers play the average-game as well, so when they recommend tire pressure starting points, they do it for an average bike, whatever that means. The trouble is the tire manufacturers donít ever tell you whatís the optimum tire temperature for the rubber formulation for each kind of tire. Some reps will tell you the optimal increase (%F/%R) in pressure from cold to running condition, which is more useful information than cold tire pressures.

So itís really all up to you. You'll get a lot of opinions on what tire pressure to run, but the correct tire pressure for you is not a matter of polling other rider's opinion or asking the manufacturerís rep at the track.

Here are the basics you'll need to decide for yourself.

For the street, start with the bike manufacturer's recommendation in the owners manual or under-seat sticker for OEM tires and similar models from other manufacturers. This is the number that they (Ducati and the OEM tire makers) consider to be the best balance between handling, grip and tire wear. Just as you would for a car, increase the pressure 2 psi or so for sustained high speed operation (or two-up riding) to reduce rolling friction and casing flexing.

In order to get optimum handling a tire has to get to its optimum temperature which is different for each brand of tire. Most of us don't have the equipment needed to measure tire temperature directly so we measure it indirectly by checking tire pressure since tire pressure increases with tire temperature. Tire temperature is important to know because too much flexing of the casing of an under-inflated tire for a given riding style and road will result in overheating resulting in less than optimum grip. Over-pressurizing a tire will reduce casing flexing and prevent the tire from getting up to the optimum operating temperature and performance again suffers. Sliding and spinning the tires also increase tire temperatures from friction heating.

S0, the technique for those wanting to get the most out of their tires on the street is to use the 10/20% rule.
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post #3 of 17 (permalink) Old Feb 12th, 2019, 11:58 pm Thread Starter
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Well thanks for that er Larry? At least you have confirmed the rule but is there something scientific to quote that demonstrates the rule? You know, to impress the skeptics.
I know it's just physics and there's a direct relationship between heat and pressure but there's another thing about tyres heating up or staying stable on a long trip for instance. So on that basis there's going to be a happy pressure given the weight loading and the speeds travelled that will be good for a given tyre. If the tyre is heating too much it is because it is underinflated and if it isn't hot enough it won't handle or grip as well.
So as far as I know some riders work out that optimum pressure by using the pressure increase to gauge the temp.
And that should be able to be calculated independent of the original recommendations because it is about maintaining a stable heat.
Can you shed any light there ?

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post #4 of 17 (permalink) Old Feb 18th, 2019, 8:08 pm
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Too much work for me. I know what cold temps I like ,set them and go.
post #5 of 17 (permalink) Old Feb 19th, 2019, 8:31 am
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I wish I could ride hard enough and stay on the edge of adhesion long enough to test tire grip. With racing compounds its a little easier because they release ( loose adhesion) very gradually, but street tires - even the best of them, tend to let go all at once and staying on the edge for any length of time without going down takes a lot of talent. More than most of us have.

On the street when a rider goes down because of grip its usually not by a little its usually a hard and fast hit, a ham-fisted mistake that isnt going to be saved by a pound or two of air pressure or a few degrees of temperature in the tire. We just aren't skilled enough to be riding that close to the edge.

I'm not sure if the OP is wondering about track riding or street riding but they are two completely different worlds. All I can say is you better stay WAY below adhesion limits while on public roads and with that in mind temperature and air pressure isn't going to be critical.
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post #6 of 17 (permalink) Old Feb 19th, 2019, 8:51 am
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The 10/20% rule for Tyre pressure

Totally agree. A few psi just isnít going to make a noticeable difference under normal, good conditions on the street. As long as you stay close to the bike manufacturers recommend settings. Modern street tires have an amazingly wide operating temperature range.

I will add though that under cold wet conditions an over inflated tire can be hazardous to your health.


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post #7 of 17 (permalink) Old Feb 19th, 2019, 10:51 am
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I do think finding the sweet spot for your road tire air pressure is worth some expirementing. Too hard and it feel like you're riding on struts, too soft and it fell like you're riding on a bowl of jello...there are certainly a few sweepers on the Cherahola where you're on the tires edge for some time , but that would be the exception for me.( only get there once a year , if lucky!)
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post #8 of 17 (permalink) Old Feb 19th, 2019, 11:12 am
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The other part of the equation is us - most of us just aren't "tuned-in" enough or honest with our own abilities/limitations to really know what it feels like to ride a bike at the limit of adhesion. Guys that can - are the aliens.

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post #9 of 17 (permalink) Old Feb 19th, 2019, 11:48 am
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Mike, you should have been with me at the track last week. It was 40, misty, and total overcast. I learned what the edge of adhesion felt like about 3 or 4 times per lap.

Now you want to talk about difficult condition to find the proper air pressure!!!


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post #10 of 17 (permalink) Old Feb 19th, 2019, 3:28 pm Thread Starter
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I totally agree about not riding on the critical edge, not me anyway.
The thing is underinflated will heat up too much on a long trip which might be all in a straight line anyway. Then the tyres will wear badly and it's even possible to have a blowout. Overinflated wears badly in a different way and never heats enough causing less adhesion. It can make a big difference if extreme. However I also agree if you use a sensible pressure like somewhere between 30 and 36 it probably isn't going to be an issue and personally I like them at 32 front and back with Metzlers.

However there was a different reason why I started this which is I'm just getting involved with an old Trumpy which isn't even a quarter the bike that the Duke is of course, but it seems that there's some serious ignorance about tyre pressure over on the Triumph forum.
For instance under my seat there are a few 'specs' about oil capacities etc. and tyre pressures, where it recommends to use 25psi.
This may have been OK in 1973 using crossply tyres, though I doubt it, but it's certainly a very bad idea today with modern much softer radials. I just wanted to illuminate the subject for some of the old school dudes over there. My original quote came from a Duc forum written by a bloke called Larry Kelly and when Strega replied it sounded almost paraphrased which gave me the impression that he may be the same guy. Apologies Strega, I'm probably way off the mark and out of line there. But to legitimize the theory I was hoping for something official that could be used just to get a ballpark starting point that would work for most of 'em.
I have Bridgestones on mine set at 32psi and they feel good. I'm not looking for seat of the pants edge of adhesion stuff, just a sensible medium way.
Thanks for the discussion guys, very interesting and I agree with all of you in one way or another.
Cheers.

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