What determines tire pressure

[Pauly]

So I get that everyone has a preference for tire pressure.  I just changed from the stock speedmasters(crap) to AM26 s.  the mechanic who installed them ran them up to 35.5 psi according to my new digital tire gauge(also crap).

So my question, the factory recommended tire pressure, is that determined by the bike or the tire?  Is it the bikes geometry that determines that number or is it the design of the tire?

Also here a. Story.  2 weekends ago the lady and I were headed to the hot springs to soak.  We stopped for gas and she said she need to add some air to her car's tires.  I asked where her gauge was, and was told she just "guesstimates" them, added air whenever she feels like they have used some up as. If it's a commodity to be used up.  I went straight in and bought the afore mentioned tire gauge. And what do you think I found? The sticker said 33 psi.  I found a 35, a 45, a 47, AND A 62 psi tire.  She said it's the first time she's ever actually seen me shocked. I'm pretty easy going but apparently my face was one of horror when the "guesstimate" came out

[Scottie J]

Women..... Ugh...  LOL

I'm running my tires 24psi in front and 28psi in the rear.

Scottie

[Bulletman]

Women..... Ugh...  LOL

I'm running my tires 24psi in front and 28psi in the rear.

Scottie

What TYPE/Make/Size of tires are you running on your bike?
Cheers

[Scottie J]

I have an old school Goodyear bias-ply ribbed tire on the front and an old Dunlop K81 on the rear.

Scottie

[Ducati Scotty]

In a two words: tire temperature.  As a tire rolls its temperature increases.  This is due to both external frictional forces from accelerating, braking, and cornering as well as internal frictional forces.  The body of the tire flexes under the weight of the load it carries.  As a section of the tire goes from nice and round to squashed under the load and back again, that creates a little friction in the body of the tire and that creates heat.

You want enough heat to get the tire warm so the tread of the tire sticks well but you don't want too much which would cause the body of the tire to fail.  The most extreme example is racing.  When you see a race bike come into the pits during testing you'll see two techs jab something into the front and back tires.  This is to take the temperature of the tire body.  They're looking for a perfect temperature but these tires are also being pushed to the absolute limit.

How does all this relate to air pressure?  If you put higher pressure in a tire it flexes less and heats up less.  Put less pressure in and it flexes more and heats up more.  So you can effectively control the temperature of your tire under any riding condition by varying the pressure.  Other things factor in as well.  Higher loads (heavier rider, passenger, cargo) will flex the tire more.  Higher sustained speeds will exert more force and heat it up more.  You will notice in many owner's manuals that there are different pressures listed for solo riding and passenger riding.  My RE manual has this.

So stepping back from racing to regular street riding the same priciples apply.  You want enough air pressure to make the tires warm up and stick well but not so little that they overheat and fail.  Since we don't have pyrometers to measure temperature we can use a less precise method.  Start with your tires dead cold and check the pressures.   Then go for a good ride of at least 20-30 minutes.  Stop and check the pressures immediately.  For a race bike (high power bike pushed to the limit) I think it's something like a 2-4# maximum increase in the front and a 6-8# maximum increase in the rear that is acceptable.  Again, that's for a race bike being pushed hard.  I would expect less for a street bike under normal riding conditions.  Another quicker and even less precise method is to just lay a bare hand on the tire right after a ride.  It should be warm to hot but not so hot it's uncomfortable to touch.

If you're getting way less of a pressure increase you're too high and maybe not getting the traction you could.  If you're getting a higher pressure increase you're too low and in danger of your tire failing from excessive heating.   There are other things to consider too.  Pressure increase and heating aside, the tire can't be so low that it would easily slip on the rim.  And off road is a whole other kettle of fish.

I run 18/24# in my C5 tires and even after long freeway rides (probably the most stress I put on tires) I've never had them too hot or too high a pressure.  The stock tires are very stiff so don't flex much with these 'low' pressures.  I think that so far the BT45s are the same but they're still new and I'm still getting used to them.

Hope this helps.

Scott

[Bulletman]

As Usual +1 Scottie   

[singhg5]

The manufacture recommendation of tire pressure is determined by the bike load AND load distribution between front and rear tires and the tire geometry itself.

Same tire can be used in two different makes of bikes but they will have different pressure.  e.g., Avon roadriders can be used in RE and also in Suzuki GS 450 but the pressure in RE is less than that in Suzuki.

Besides the load, there are other factors such as bike handling, tire longevity, road grip etc. based on all of which the bike manufacturer recommends tire pressure.

Higher temperature just expands the air inside the tire (or tube) which affects the tire shape, contact patch, wear and tear, and increases pressure. However for riding the bike the tire pressure is kept the same at 28 PSI for rear whether it is 50F outside or 90F outside. In that sense the outside temperature does not determine what air pressure should be used in the tire.

The RE manual clearly states that rear tire pressure should be 28 psi for solo but increased to 30 psi when load is increased by additional rider on the pillion.

[barenekd]

If you look at the vehicle's (Car or Bike) specs, that is the one you should use. They don't specify tire differences, they just show pressure. Part of it is handling, part of it is comfort. You guys who are running high 20s or more in the front tire, your are beating yourself to death. The bikes handle great at the 18/28 too.
If you don't notice any difference between the 18 and 28 pressure, you need to get the sludge out of the forks and get some 10wt fork oil in them. You will definitely notice the difference in comfort then! The bike has a very nice ride with a decent suspension under it!
Sometimes people have a specific reason for using a different air pressure. That's  fine. If it works, more power to them!
If you have to ask, use the owner's manual
Bare

[Ducati Scotty]

In that sense the outside temperature does not determine what air pressure should be used in the tire.

I didn't mean ambient temperature, I meant tire temperature.  I've ammended my post.

Thanks,
Scott

[Royalista]

As you wisely changed your tires from the stock Speedmasters to the Avon 26 you might be interested in the recommendations by Avon for its Roadriders AM26. You can download avon_mc.databook.pdf from avonmoto.com with all the specs. They recommend for Roadrider AM26 27psi front and 29psi rear. There you will also see Avon's alternates for bullets, with their respective specifications.

As stated by others these are baseline values. According to load, type of ride, conditions (weather, surface) they may be increased or decreased. You can find guidelines in dualsport riding.

[Ducati Scotty]

I should mention too that in almost 12,000 miles of running 18/28 I did not have any problems or odd wear on the tires.  When I replaced them the front was still in pretty good shape with plenty of tread.  The rear had just the faintest hint left of the center groove and some feathering on the outside edges of the tread, normal wear.

Scott

[ace.cafe]

I think it needs to be stipulated which wheel size and motorcycle is involved in this question.

On the Iron Barrel models with 19" wheels, 28 front and 30-32 rear is about right for AM26 Road Riders on that bike.