Tire pressure question

[Texpaul]

I know that when you take a 1200 GS off road your can improve traction by lowering the tire pressures. The figures I have heard are 26 to 28 lbs front and rear (with spoked rims). But those are cold pressures (I assume). My question revolves around lowering the tire pressure on warm/hot tires. If I have been riding for 2 hrs on tires at hiway pressures (say 36 and 42 lbs respectively) and I need to lower my pressure for off road work, do I just deflate to 28 lbs or do I have to do calculate some compensation for the increased temperature?

 

[36654]

I know that when you take a 1200 GS off road your can improve traction by lowering the tire pressures. The figures I have heard are 26 to 28 lbs front and rear (with spoked rims). But those are cold pressures (I assume). My question revolves around lowering the tire pressure on warm/hot tires. If I have been riding for 2 hrs on tires at hiway pressures (say 36 and 42 lbs respectively) and I need to lower my pressure for off road work, do I just deflate to 28 lbs or do I have to do calculate some compensation for the increased temperature?

I would guess-timate a relative reduction. New low hot pressure = ( Low Cold Pressure / High Cold Pressure )* High Hot pressure.

Beyond that, leave the dirt roads to the folks without teeth.

 

[PGlaves]

I would deflate a hot tire to my desired cold pressure plus 4 or 5 psi for a radial tire. So to hit 26 cold I'd look at 32 or so hot. This assumes the original cold pressure was correct giving a 6 psi or so increase cold to hot - for radials - 10% for bias ply.

 

[dukekaboom]

Tire Pressure

This is a bit of a challenge unless you know the temperature of the air in the tire after two hours of riding in which case you can use the Ideal Gas Law which is PV = nRT!

Kent

 

[RYD1WD]

I'll have to research this to be absolutely sure of what models/generations it applies, but...

On my '14 RT the TPM measures internal temp and gives a readout based on calculated cold pressure (68F). I can make adjustments to my tire pressures when I roll into the garage after several hours of continuous running (or while on the road), using the TPM values, and they will be no more than +/-1 lb off the next morning when actually stone cold. If I'm correct, this is global across all current, and relatively recent models... but I will confirm with details. If I'm correct, you could just use the TPM values and call it close enough.... at least I would given my experience of accuracy with mine.

 

[dunc723]

This is a bit of a challenge unless you know the temperature of the air in the tire after two hours of riding in which case you can use the Ideal Gas Law which is PV = nRT!

Kent

Don't forget the temperature must be absolute (Kelvin or Rankine). And you're making the fundamental assumption that volume is not changing.

Pete

 

[WalterK75]

I've never done this but, logically, it seems the point is to lower tire pressure to get traction. Maybe a pound or two would be enough depending on what the surface is like on which you are riding. I'm not sure lowering it to a predetermined pressure is the answer without taking surface conditions into account.

 

[f650624]

This is almost as fun as the "what oil" and "what tire" threads.

 

[PGlaves]

I've never done this but, logically, it seems the point is to lower tire pressure to get traction. Maybe a pound or two would be enough depending on what the surface is like on which you are riding. I'm not sure lowering it to a predetermined pressure is the answer without taking surface conditions into account.

Probably correct - except Paul was recounting advice from some serious experienced off pavement riders. Sometimes experience trumps.

 

[rangerdog]

This is almost as fun as the "what oil" and "what tire" threads.

So it's a different pressure if you're using synthetic air?

 

[dannyleo]

You can go a lot lower

I've had BMW off-road trainers tell me they've gone down to 18psi with no adverse affects (effects?) and that was still safe for the ride home. I've also had one who confided to me that while recommending lowering the pressure, he usually just kept his street pressures in. Since he was quite a bit more experienced and qualified than I was, he could get away with that. It does make a difference depending on how rough the terrain is. Even on just gravel you will feel some difference, but it really depends a lot on both the rider's abilities and the terrain; gravel, dirt, mud, steep ravines, through the woods with lots of tree roots and fallen logs/branches? Etc, etc. My two cents, probably worth about a cent and a half with the current economy. Maybe a cent and a quarter.

 

[rich]

Why do you want to take a GS off road anyway? You silly boy.

 

[36654]

And you're making the fundamental assumption that volume is not changing.

Isn't that a pretty safe assumption

 

[36654]

I've had BMW off-road trainers tell me they've gone down to 18psi with no adverse affects (effects?) and that was still safe for the ride home. I've also had one who confided to me that while recommending lowering the pressure, he usually just kept his street pressures in. Since he was quite a bit more experienced and qualified than I was, he could get away with that. It does make a difference depending on how rough the terrain is. Even on just gravel you will feel some difference, but it really depends a lot on both the rider's abilities and the terrain; gravel, dirt, mud, steep ravines, through the woods with lots of tree roots and fallen logs/branches? Etc, etc. My two cents, probably worth about a cent and a half with the current economy. Maybe a cent and a quarter.

You need to spend some time with Good Ol'Boy loggers. They'll fill you in on all the details of this arcane science.

As someone else said, why are you taking that behemoth off the road anyway?

 

[visian]

I've had BMW off-road trainers tell me they've gone down to 18psi with no adverse affects (effects?) and that was still safe for the ride home. I've also had one who confided to me that while recommending lowering the pressure, he usually just kept his street pressures in. Since he was quite a bit more experienced and qualified than I was, he could get away with that. It does make a difference depending on how rough the terrain is. Even on just gravel you will feel some difference, but it really depends a lot on both the rider's abilities and the terrain; gravel, dirt, mud, steep ravines, through the woods with lots of tree roots and fallen logs/branches? Etc, etc. My two cents, probably worth about a cent and a half with the current economy. Maybe a cent and a quarter.

Spot-on advice.

You'll probably not notice a lot of difference if your tires are in the high 20s, but you will notice it a bit on gravel. I don't bother lowering pressure for gravel because I am comfortable with the loose feeling.

The tougher the going the lower you want *except* if the going features excessive sharp-edged obstacles like rocks or roots. This holds especially true for tube-type tires.

As one of the lazier riders you'll meet, if I am going to take the time to lower my tire pressure, I set both in the low 20s.

You need to spend some time with Good Ol'Boy loggers. They'll fill you in on all the details of this arcane science.

Yeah, and it's not a super-exact science, I don't think you need to use mathematical formulae!

Ian

 

[36654]

Yeah, and it's not a super-exact science, I don't think you need to use mathematical formulae!

The Amish don't even use the air........

 

[pffog]

The bigger question is why are BMW riders so anal about tire pressures? Tire construction makes any chart invalid, and in reality tire pressures are like shooting a sawed off shot gun, precision is not needed or even necessarily beneficial, just shoot for the general direction and stop worrying.