Auto tires are typically designed to be inflated to lbs. psi in the mid-to-low 30's.

Bicycle tires typically 45 to 60 or 70 psi.

What is it about the construction of the typical dirt bike tire that it is typically inflated to much lower pressures - 8 to 15 lbs psi?

Do all dirt bike tire applications involve a tube? Why do bike wheels have spokes but not auto wheels? Auto wheels once had spokes; and wagon and carriages had spokes.

Why do bicycle tires require a tube & dirt bike tires, but not car or heavy truck tires? Is it because auto and truck rims are strong enough to withstand the pressure, but aluminum dirt bike rims can't? Why don't auto manufacturers reintroduce tubes into aluminum rims to save weight, or wouldn't aluminum casted rims be strong enough to support weight of car (they use alum. alloy now)?

Why is a dent in rim of dirt bike tire important to avoid (from what I've read on this forum) if the rim does not have critical function of containing air pressure like an auto rim? What matter a dented rim?

Edited by suppresst, 02 May 2012 - 05:15 PM.

Oh boy I don't know if I should touch this.
Tubes because the rims have holes for spokes.
There are tubless systems.
Cast and foreged wheels would have to be so heavy to stand up to the shock a spoked wheel can take.
Spoked wheels pull on the spokes so all the spokes around the wheel essentially hold the wheel up kind of like a spring
Metals are stronger in tension then compression.

Idk what else to tell you other than learn some engineering.

The answer to your first question is that it has nothing to do with the construction of the tire.  The surface media dictates the design of the tire and the optimum air pressure required for best traction and control.   Pressures are lower than you'd use on a sportbike because keeping the tire carcass from deforming for the sake of control at speed is not as great an issue what with speeds being as much lower, and the surface so uneven.

Dirt bikes have wire spoked wheels because they are lighter in most cases, more resilient and resistant to damage in the rough, and usually less expensive.

Because they have spoked wheels, they need tubes.  Air leaks around the spokes.  Even with cast wheels on road bikes, if the tire isn't designed to retain air on its own, you'll need a tube.

Dents in rims aren't a big deal until they get to the point where they affect stability at speed, or raise concerns about the amount of damage done to the rim in the affected area.

BTW, metals (especially steel) are essentially the same in compression and tension.  The reason you use spokes in tension is because of buckling.  A long, slender member (like a spoke) will try to natually straighten when pulled, but when compressed, any little eccentricity and the spoke want to curve.  Once it starts to curve, the eccentricity becomes greater and it takes less force than was applied previously to increase the curve, so the spokes would just crumple up (if left the same diameter/length).

Google "buckling"...

dirt tires run a low pressure so the knobs can move when in contact with the ground then release the dirt ... self cleaning.

Well, I'm getting a better response this time since I've revised my questions.

 englertracing, on 02 May 2012 - 08:08 PM, said:

Seems this person couldn't grasp why I would ask such elemental questions, but I'll bet not more than 2 people in 100  understand why certain wheels and tires are constructed as they are.

So, what I've garnered is that tires on spoked wheels need tubes because otherwise air would leak out the spoke holes, and I'm going to guess that aluminum rims on dirt bikes and bicycles might not be able to withstand the direct pressure of an inflated tire without a tube (a tire exploding off a car rim can seriously injure or kill, I understand, so the pressure must be immense).

Also, it seems that design and construction of rim and tire on dirt bikes are a function of the weight they have to support and the kind of surface media they typically run in.

Still not entirely sure why a bicycle tire is typically inflated so high compared to a dirt bike (many bicycles are operated off road), but I think I'm beginning to be able to reason it out. Thanks guys.

Edited by suppresst, 03 May 2012 - 04:11 PM.

compare the rigidity of the side wall of both a dirtbike and a mountain bike tire. The stiffer side wall on the dirtbike tire allows it to be run at lower pressures before pinch flats start occurring.

With the heavier bike, higher speeds, bigger jumps and harder landings, a dirt bike wheel has to be built to take serious abuse. While weight is defiantly a priority on dirt bikes, weight on bicycles is more important. Light rims and tires on the bicycle tire require the extra pressure.

Edited by Die_trying, 03 May 2012 - 04:34 PM.

 suppresst, on 03 May 2012 - 04:10 PM, said:

Bicycle tires use higher pressures as they get thinner / narrower because there is less surface area to spread the load.

spokes are just another way to absorb the impact a bunch of little bendable springs to protect you from bending rims.
have you ever lifted even a small car tire? they're pretty heavy (if they're not alum) imagine that on a dirtbike that already weighs 250 lbs, do you want more weight? and plus when they bend you have to buy a whole new rim instead of just lacing a new hoop.

think about the psi thing this way. (this is going to get weird just bear with me...) if you had a small plate and mashpotatoes a small amount of potatoes will fill the plate, but that same amount of potatoes wont fill that larger plate up as much, get it?

Some Trials bikes use tubeless tires on the rear with a spoke rim, xtra deep center section with a thick liner that seals the rim.

Tubeless tires have an extra rubber coating on the inside to seal the casing, but I've run tube type MC tires without a tube and use the the Tubliss system to seal the rim, Tubliss also replaces the rim lock and clamps the tire bead against the rim.

Bicycle tires run high pressure to reduce rolling resistance, dirt bikes run low pressure (more casing flex) for bump absorption, traction,  and lug cleaning.

Edited by chuck4788, 03 May 2012 - 05:52 PM.

Humans are 1/4 HP, my 4-fiddy gots 50+   Whole new ballgame.

I fully understand englertracing's post. You covered a lotta ground with those questions.

The recommended amount of air in a tire for a particular application is dictated by desired effect, design of the carcass, and intended use.  The evolution of manufacturing capability certainly has a little to do with the design.  If you look back many years ago... the automobiles had tubes and spokes which you mentioned.

You asked why dirt bike tires run 8-15 psi...  desired effect in correlation with design.  You can put much more or less psi in the dirt bike tire, and when you get to the extremes of each end (2 psi and 25 psi) you will see these are undesirable effects.

You asked... "Do all dirt bike tire applications involve a tube?"  No.   "Why do bike wheels have spokes but not auto wheels? Auto wheels once had spokes; and wagon and carriages had spokes."    Once upon a time we had to do our business in an out house too.  Basically comes back to intended use.  Imagine an auto type wheel for a dirt bike... how heavy would it be?

Why do bicycle tires require a tube & dirt bike tires, but not car or heavy truck tires?   Technology.  Tire design and wheel design eliminated the tube.
Is it because auto and truck rims are strong enough to withstand the pressure, but aluminum dirt bike rims can't?   No.
Why don't auto manufacturers reintroduce tubes into aluminum rims to save weight,  or wouldn't aluminum casted rims be strong enough to support weight of car (they use alum. alloy now)?  Adding a tube would add weight. They cast aluminum wheels now.

Why is a dent in rim of dirt bike tire important to avoid (from what I've read on this forum) if the rim does not have critical function of containing air pressure like an auto rim? What matter a dented rim? It's not that it is important to avoid, it is undesirable.  A force induced bend or dent after the manufacturing process in a mild form will at the least affect the concentric effect of the wheel and give you a wobble or out of balance effect in addition to compromising the strength.  See earlier post on buckling.  In most cases if the rim is mildly bent (no more than .125") and the spokes are kept with proper tension the rim will survive for most normal riders.

I responded to those questions to get me to this... a tubeless option for the dirt bikes.  They are already stamping the holes and bevels in the rim.  What if they designed the nipples into the rim via stamping or forging?  I know you now have one less replaceable part but I think I could live with that for the advantages it may bring.  I have not thought it through entirely but if possible now the rim would be sealed, no tube. (I know the shelf of the rim would need to be redesigned along with the tire to seal the bead at relatively low pressure.)  Then the hub could be either threaded or replaceable nipples designed in.

I know this would be a daunting task for the manufacturers of the rims, motorcycles, and tires, but think of the advantages it would bring in grip and un-sprung  weight savings for dirt bike applications.

Edited by kx450f63, 04 May 2012 - 06:03 AM.

 suppresst, on 03 May 2012 - 04:10 PM, said:

Using tubes has nothing to do with the fact that the rims are aluminum. Trucks use aluminum rims that are tubeless and they are inflated to 110 psi. Tubes are used in dirtbike wheels completely because the spokes would leak air. Like others have said, there are kits to seal the spokes so you don't need a tube on a dirtbike wheel, but those kits are 10 times more expensive than a tube, so most people do not go that route.

Bicycle tires are much thinner since they do not have to hold up to the forces that a motorcycle applies to a tire. Because the carcase of a bicycle tire is so flimsy, more pressure is needed to keep the tire from squishing down. A dirtbike tire is built much sturdier to hold up to the power and not rip the knobs off, as well as to not get holes from sharp rocks and roots. Because the carcase of the tire is so stiff, less pressure is needed to hold keep the tire inflated.

Tires being used in dirt are typically run at lower pressure to get a wider footprint for better traction, while tires intended for the street use higher pressure to reduce rolling resistance. This is why offroad motorcycles use 12-15 psi and streetbikes use 35-40 psi. Same thing for bicycles, mountain bikes use 25-30 psi while road bicycles use much higher pressure.

I think I've found the latest innovation...can't wait to get some






Hey, and that 3 wheel idea looks to be a goody

Future so bright, gotta wear shades

 suppresst, on 02 May 2012 - 05:13 PM, said:

  It doesn't have anything to do with the construction of the tire (although the construction of the bead of the tire is less critical, since it can be designed without worrying about it sealing air).  Dirt bike tires are inflated to much lower pressures due to the application.  If you run that dirt bike tire on the asphalt, you better run it at a much higher psi than 8, or it will feel ridiculously unsafe at speed, and you'll rip all the knobbies off.  If you inflate the tire to 28psi for the street, then when you get in the dirt your traction will suffer and the bike will be much harder to ride in the rough.

 suppresst, on 02 May 2012 - 05:13 PM, said:

  As far as I know, all dirt bikes are sold from the factory with a tube setup, but there are a couple of dual sport bikes that have wheels with a special flange that the spokes attach to, instead of intruding into the tire area.  Those spoke wheels don't need tubes, but are undoubtedly more expensive to make.  There are also a couple of different ways to seal up a dirt bike tire so it doesn't need a tube.  Even just adhesive and duct tape over the spokes would probably work, but it would be a pain to do, and if you adjusted the spokes you would probably lose your seal.

Auto wheels are under significantly different demands than dirt bike wheels.  A wheel constructed like an auto wheel would likely not hold up to severe off-roading, unless it was WAY too heavy for a dirt bike application.  But it does look better, and you don't have to occasionally tighten spokes.

 suppresst, on 02 May 2012 - 05:13 PM, said:

  Again, it's because automobile wheels don't have holes in them.  Holes would let the air out.

 suppresst, on 02 May 2012 - 05:13 PM, said:

Like somebody else said, a small dent, no biggie.. but it weakens the rim, could cause it to have a side to side wobble, and if the dent was big enough, the tire would no longer have a round shape.  By the way, car wheels can have dents and still hold air, but it's still something to avoid.

 bg10459, on 03 May 2012 - 04:38 PM, said:

Mountain bike tires are NOT run this high. Or at least they shouldn't be if the rider knows what they are doing. DH tires can be run as low as 16psi and the average XC tire is from 25-35psi. If mountain bike tires are run a 70-80psi your ride would be very rough and the rider would have no control on rough terrain (just like a dirt bike). Road bikes DO run high pressures, around 110-120psi because they have high pressure csaings and they want very low rolling resistance.

Well, all the above has been very helpful. I had no idea about the TUbliss system, and just studying it (at TUbliss website) was a great education. Everyone else contributed so much about application and construction that I feel I have a pretty good grasp of the dirt bike tires and rims now, and I really appreciate the feedback.

One reason this has been very helpful is I ride alone a great deal of the time, and tire, tube or rim problems occurring far from help are a concern. Understanding all three better help me to prevent trouble and cope with it when it comes.

The only area where i feel I lack understanding is in inflation - I am struck by how just a few pumps with a hand pump, or slight release of air from tube makes significant variations in psi. It's hard to wrap one's mind around the reality that such minute psi adjustments can have such big impact on performance.

I began riding with a man who lent me a 125cc bike with a flat front tire! We were mostly in sandy soil but covered 21 miles and he didn't seem to think it was a big deal. Since that initial experience I have never thought psi mattered all that much. Lately however, I've been riding trails with more roots and rocks and I was advised to attend to tire inflation (just in time, apparently, from what I've learned here).

Now, Michigan (where I ride) has over 100 public trails covering over 3000 miles of terrain. When new to a trail one never knows exactly what to expect. One cannot add air to tube on the trail (in most circumstances no service stations around, or not legally accessible for off-road equipment) so at this level of knowledge the best I've come up with is to start out at factory recommended psi (15psi) and lower from there. 15psi is good for roots and rocks but can be devlish in sand, so one avoids pinch-flats at risk of murdering oneself against a tree in a sandy stretch. When I ask about psi I get nonchalant responses with recommendations all over the spectrum from 9 psi to 15. If psi is so critical then one cannot be nonchalant, can one?

Maybe TUbliss is best system for me, considering varying terrain and riding alone a lot?

Thoughts?

Run what works for ya in the terrain you ride. After awhile you'll have preferences...but then you most likely have a budget, so you will make compromises. It's all part of the journey.

Just wait til oil fascinates ya.

 suppresst, on 02 May 2012 - 05:13 PM, said:

 Krannie, on 05 May 2012 - 07:31 PM, said:

Why is a dent in rim of dirt bike tire important to avoid (from what I've read on this forum) if the rim does not have critical function of containing air pressure like an auto rim? What matter a dented rim? IT DOESNT' MATTER, IF YOU DON'T WANT CONTROL OF YOUR FRONT WHEEL. BENT RIMS ARE A SYMPTOM TO A COMPROMISED WHEEL STRENGTH.

Krannie, maybe I'm misreading you, but it does seem you are bit (or a lot) incredulous by what you perceive to be my naiveness. Permit me to share some perspective. In watching footage of the Baja 100 I saw that world class riders routinely suffered major, high speed crashes and still managed to finish the race. Mouse McCoy crashed and suffered a separated shoulder, broken finger and broken ribs and his bike was described as "junked". Nevertheless, he still managed to carry on the last 30 miles at speeds in excess of 100 mph and finish the race. I think its safe to say he had a bent rim. With so much information floating around out there, obviously that which is a disaster (bent rim) to one rider, is a minor inconvenience to another. I like to get at the truth.