# Thread: Tire weight and different sized wheels

1. ## Tire weight and different sized wheels

Can anyone tell me how to figure out what a 29er tire would need to weigh to need the same power to spin as a 27.5? Hope i asked that well enough,with all else being equal same rim spokes hub but how much weight difference in the tire to take the same power to spin up. The reason i ask is cause i've been playing with tires on my 29er and can really notice the difference of a couple hundred grams and since i'm thinking bout a new bike i'm leaning toward 27.5 and was wondering ?

2. no physics majors not even a try?

3. 27.5/29. It's linear, I think. The bottom line is, different treads and sidewall flexes would probably have more affect unless you're just spinning it on the stand. Put it up there and turn the crank by hand. It's not a huge force.

4. Originally Posted by t0pcat
Can anyone tell me how to figure out what a 29er tire would need to weigh to need the same power to spin as a 27.5? Hope i asked that well enough,with all else being equal same rim spokes hub but how much weight difference in the tire to take the same power to spin up. The reason i ask is cause i've been playing with tires on my 29er and can really notice the difference of a couple hundred grams and since i'm thinking bout a new bike i'm leaning toward 27.5 and was wondering ?
I believe you're asking what would the mass of a 29er tire need to be to take the same amount of energy as a 650b tire to accelerate to a given tangential velocity.

The problem isn't worth solving because the answer is that the 29er tire would need to weigh less than the 650b tire to have the same rotational inertia, and this simply isn't possible. But you're looking at it from the wrong angle. Yes, the 29er wheel might take a little more energy to get up to speed, but once you hit a climb and start decelerating, the 29er wheel will maintain its momentum a little better (i.e. the energy is conserved). The questions you should be asking yourself are A) Which wheel size do I feel the most comfortable riding, and which one is the most fun?, and B) Which wheel size is faster for the trails I ride (if racing is in your future)?

5. Didn't we go through a long thread that determined that it was the weight out at the rim that determined the acceleration and that the diameter was irrelevant? IOW, if you consider that the plot is to get the mass at the rim up to ground speed, whether it's a large rim rotating fewer revolutions or a small spinning more the mass to accelerate is the same as is the peripheral speed.

An 800 gram tire on a 500 gram rim will be the same regardless of 26 or 29. Of course a given tire or rim design will usually be proportionally heavier in a 29" size, but the diameter is otherwise irrelevant.

What I notice is more of what I'll call 'swing weight'. The tire is further away from the steering center and takes more to swing side to side; this effect also makes it less squirrely and easily deflected.

6. Originally Posted by bulerias
Didn't we go through a long thread that determined that it was the weight out at the rim that determined the acceleration and that the diameter was irrelevant? IOW, if you consider that the plot is to get the mass at the rim up to ground speed, whether it's a large rim rotating fewer revolutions or a small spinning more the mass to accelerate is the same as is the peripheral speed.

An 800 gram tire on a 500 gram rim will be the same regardless of 26 or 29. Of course a given tire or rim design will usually be proportionally heavier in a 29" size, but the diameter is otherwise irrelevant.

What I notice is more of what I'll call 'swing weight'. The tire is further away from the steering center and takes more to swing side to side; this effect also makes it less squirrely and easily deflected.
I'm not sure about the other thread, but you're right. If the mass of 2 tires and rims of different diameter are identical, it will take the same amount of energy to get both up to identical tangential velocities. But of course the mass of the rims won't be identical, and the tires likely will not be either.

For those interested, the Kinetic Energy, E, of a rotating point mass is equal to 1/2 I * w^2, where I is the rotational inertia (equal to m*r^2) and w is the angular velocity (equal to V/r).

What you refer to as 'swing weight' I call 'gyroscopic effect'. Definitely noticeable with heavier tires & wheels.

7. Originally Posted by bholwell
I'm not sure about the other thread, but you're right. If the mass of 2 tires and rims of different diameter are identical, it will take the same amount of energy to get both up to identical tangential velocities. But of course the mass of the rims won't be identical, and the tires likely will not be either.

For those interested, the Kinetic Energy, E, of a rotating point mass is equal to 1/2 I * w^2, where I is the rotational inertia (equal to m*r^2) and w is the angular velocity (equal to V/r).

What you refer to as 'swing weight' I call 'gyroscopic effect'. Definitely noticeable with heavier tires & wheels.
Actually, I was referring to just the static inertia of a larger rim/tire of the same weight. It's like shaking a longer stick. Even if the gyroscopic effect of larger/smaller wheels cancels out [??] there's still a longer lever at play. Probably why heavy tire 29 guys like wider bars.

8. Originally Posted by bulerias
Actually, I was referring to just the static inertia of a larger rim/tire of the same weight. It's like shaking a longer stick. Even if the gyroscopic effect of larger/smaller wheels cancels out [??] there's still a longer lever at play. Probably why heavy tire 29 guys like wider bars.
Oh, I see. Yes, the larger wheelsize has a greater moment of inertia about the axis of the steerer tube, even when the wheel is stationary.

However I don't think the gyroscopic effect of different wheel sizes are the same at the same tangential velocity, even if they have identical masses. This is because the gyroscopic effect is directly related to the moment of inertia multiplied by the angular velocity, whereas the Kinetic Energy of a rotating mass is directly related to the moment of inertia multiplied by the angular velocity squared.

9. so you guys are saying that a 1000 gram rim and tire in 27.5 will need the same power to turn as a 1000 gram 29er rim and tire? i don't hardly
see how that is possible, especially if you guys think it has so much affect on latteral motion.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•