[yogiprophet]

I just did a calculation, comparing the attack angle for a 26" wheel with a 2.2 tire and a 29" wheel with a 2.0 tire (which is the setup I measured).
The angle of attack is the angle between the vertical line from axle to ground to the line from axle to bump.
The difference are as such(all bumps are square edge):

4" bump: 26"- 45.2 degrees, 29"- 43.4 degrees, difference - 1.8degrees
6" bump: 26"- 56.2 degrees, 29"- 53.9 degrees, difference - 2.3degrees
8" bump: 26"- 65.9 degrees, 29"- 63.1 degrees, difference - 2.8degrees

These differences do seem small.

Also, for suspension bikes, 26" bikes will have more suspension and the whole point of suspension is to reduce the attack angle by allowing the wheel to pick up and roll over an object. Try rolling over a curb with a 29" wheel with no suspension, and then do the same with a 26" wheel with suspension (without picking up the front-end).

Wanted to get some feedback on this forum regarding this.

I'm not trying to say 29" wheels do not have any advantages on a suspension bike, though I would like to understand the advantages from a scientific standpoint. In other words, without emotionally charged perceptions, which I know is asking a lot since we are so attached to our perceptions.

 

[unit]

So, you are asking for advantages from an angle of attack point of view only, or would you like other advantages discussed also?

Your numbers (for difference in angles) do seem very small, but any advantage is an advantage...especially that 1/10,000th of a degree difference between the point at which a tire augers in vs. rolls over an obstacle.

Hope you find what you are looking for.

 

[serious]

When I was running a 96er setup on my rigid SS, I did not think that the 29er tire made as much difference as many would have us believe (I am only talking about angle of attack now). It was a little "smoother", but only if the air pressure was lower than in the 26er tire. Otheriwse, I would be hardpressed to guess what I was riding.

But a 29er on rigid does not even come remotely close to 80mm front suspension on a 26er.

 

[JonathanGennick]

I just did a calculation, comparing the attack angle for a 26" wheel with a 2.2 tire and a 29" wheel with a 2.0 tire (which is the setup I measured).
The angle of attack is the angle between the vertical line from axle to ground to the line from axle to bump.

Is there a source for your definition of "angle of attack"? I don't mean to be difficult. But I've done a bit of Googling here, and I'm unable to find a rigorous definition of what "angle of attack" really means in reference to a wheel. I can find lots of discussion relative to airplanes wings, but nothing for wheels.

 

[yogiprophet]

Your numbers (for difference in angles) do seem very small, but any advantage is an advantage...especially that 1/10,000th of a degree difference between the point at which a tire augers in vs. rolls over an obstacle..

I am talking about bikes set up similarly. IOW, with the same headtube height, so this would mean that the 26er would have more suspension meaning a better ability to roll over objects(from a suspension standpoint alone).

Without suspension(where the 1/10000 of a degree would make a difference) the 29er rules without question so, there is no need to discuss that.

 

[yogiprophet]

Is there a source for your definition of "angle of attack"? I don't mean to be difficult. But I've done a bit of Googling here, and I'm unable to find a rigorous definition of what "angle of attack" really means in reference to a wheel. I can find lots of discussion relative to airplanes wings, but nothing for wheels.

I believe a gave a clear definition of what I mean by "attack angle" or "angle of attack" above. They are just words used to describe the angle the axle makes between the ground and an object(such as a rock). Call it what ever you wish...it is just a measure of the difficulty a wheel would have rolling over an object. The larger the angle, the more resistance to roll over the object simply put because the wheel would(at least initially) have to change directions by that angle.

So, a bike with a 26" wheel without suspension, when hitting a 6" curb, has to change directions by 56.2 degrees initially, and this angle decreases untill the wheel has complely gone over the curb.

 

[Rotmilky]

I just did a calculation, comparing the attack angle for a 26" wheel with a 2.2 tire and a 29" wheel with a 2.0 tire (which is the setup I measured).
The angle of attack is the angle between the vertical line from axle to ground to the line from axle to bump.
The difference are as such(all bumps are square edge):

4" bump: 26"- 45.2 degrees, 29"- 43.4 degrees, difference - 1.8degrees
6" bump: 26"- 56.2 degrees, 29"- 53.9 degrees, difference - 2.3degrees
8" bump: 26"- 65.9 degrees, 29"- 63.1 degrees, difference - 2.8degrees

These differences do seem small.

Also, for suspension bikes, 26" bikes will have more suspension and the whole point of suspension is to reduce the attack angle by allowing the wheel to pick up and roll over an object. Try rolling over a curb with a 29" wheel with no suspension, and then do the same with a 26" wheel with suspension (without picking up the front-end).

Wanted to get some feedback on this forum regarding this.

I'm not trying to say 29" wheels do not have any advantages on a suspension bike, though I would like to understand the advantages from a scientific standpoint. In other words, without emotionally charged perceptions, which I know is asking a lot since we are so attached to our perceptions.

Scientifically, 29ers have ~10% larger wheel diameter, so in some cases you might get a 10% performance improvement. The forces acting on the wheel go as the Sin and Cos of the angles you calculated. Depending on the bump size, etc., you may get close to 10% reduction in force imparted into the fork. But, that's a silly analysis in the first place. Who, while not being drunk or stupid, is going to try to ride over a 8" curb without pulling up on the front wheel...and then shifting weight to lift up the rear as it approaches? So, that pretty much nullifies and 'angle of attack' advantage on rocks greater than, say, 3" or so.

I'm not sure what, if any, reasonable comparison can be made between a rigid 29er and a squishy 26er. The 29er will ride rougher. Scientific...eh no. I currently have a 4" RIP9 and a 5+ inch travel Yeti and I can tell you the RIP9 has a hard time keeping up with the Yeti on downhills. How do I quantify that scientifically? I really can't.

There's a lot of hooey going around about why 29ers are better, etc. Most of it is just pure bull plop. I have both 26ers and 29ers and here is what I find to be the 29er advantages in order of most significant:

1) Size--If you're a big guy, they fit better. I'm 6'2" (not a hulk by any means) and my RIP9 fits me better than any bike I've had before. With the higher BB, I can run longer cranks.

2) Uphill traction--Going uphill, I can climb stuff with my 29er that I can't with my 26er. I think the 29ers have a larger surface of the wheel hitting the loam resulting in better traction.

3) They look cool and it's kinda fun to have an uncommon bike. :thumbsup: Sure, lots of people have them, but I still rarely run across one on the trail. I do answer lots of questions about the bike at the TH if someone notices.

The other 'advantages' of a 29er that I've heard seem to be insignificant. Get a bike you like to ride, and enjoy riding it. Stop thinking so hard about it. Both 26ers and 29ers will get you to the top and back.

 

[JonathanGennick]

I believe a gave a clear definition of what I mean by "attack angle" or "angle of attack" above.

Your definition was clear enough. I understand how you measured the angles that you did. What I was hoping to find via Google was a detailed explanation as to why those particular measurements are the "right" ones to use in quantifying the difficulty with which a wheel of a given size rolls over a bump. I would have thought to measure a different angle than you did. So I went out looking for more information. That's all.

 

[jbogner]

I agree that 2 degrees is meaningless. I mean, who can really tell the difference between a 71 and a 73 degree head angle?

 

[jddjirikian]

The one difference that really makes a 29er work for me is just the sheer amount of confidence inspiring handling those big wheels give on the trail.

I attribute that to the generally larger contact patch on a 29er wheel v. a 26er wheel.

Regardless, I like riding my 29er hardtail and haven't really ridden my 26 full suspension since I've had the 29er.

 

[yogiprophet]

Scientifically, 29ers have ~10% larger wheel diameter, so in some cases you might get a 10% performance improvement. The forces acting on the wheel go as the Sin and Cos of the angles you calculated. Depending on the bump size, etc., you may get close to 10% reduction in force imparted into the fork. But, that's a silly analysis in the first place. Who, while not being drunk or stupid, is going to try to ride over a 8" curb without pulling up on the front wheel...and then shifting weight to lift up the rear as it approaches? So, that pretty much nullifies and 'angle of attack' advantage on rocks greater than, say, 3" or so.

I'm not sure what, if any, reasonable comparison can be made between a rigid 29er and a squishy 26er. The 29er will ride rougher. Scientific...eh no. I currently have a 4" RIP9 and a 5+ inch travel Yeti and I can tell you the RIP9 has a hard time keeping up with the Yeti on downhills. How do I quantify that scientifically? I really can't.

There's a lot of hooey going around about why 29ers are better, etc. Most of it is just pure bull plop. I have both 26ers and 29ers and here is what I find to be the 29er advantages in order of most significant:

1) Size--If you're a big guy, they fit better. I'm 6'2" (not a hulk by any means) and my RIP9 fits me better than any bike I've had before. With the higher BB, I can run longer cranks.

2) Uphill traction--Going uphill, I can climb stuff with my 29er that I can't with my 26er. I think the 29ers have a larger surface of the wheel hitting the loam resulting in better traction.

3) They look cool and it's kinda fun to have an uncommon bike. :thumbsup: Sure, lots of people have them, but I still rarely run across one on the trail. I do answer lots of questions about the bike at the TH if someone notices.

The other 'advantages' of a 29er that I've heard seem to be insignificant. Get a bike you like to ride, and enjoy riding it. Stop thinking so hard about it. Both 26ers and 29ers will get you to the top and back.

Yes about 9.3% bigger diameter with equal size tires, but I measured a smaller tire on the 29er because that is what my 2 friends have on theirs. A 10% larger tire size though does not equat to a 10% performance gain.

By the way, I enjoy rolling curbs (without pulling up)on my bikes just to see the action of the suspension. call it dumb if you like, but sometimes when you bomb through a rock garden at speeds, you don't have time to lift over every one and you just have to take a few hits along the way. How does a 29er with less suspension compare with a 26er. that is what I am getting at I suppose.
Not trying to consider any rigid bikes.
You don't have to answer any questions on this forum - you can be out riding your bike too, but I am glad you did. Thanks for the input. By the way I enjoy thinking about this stuff, and I ride a lot I love to ride very fast....

What about climbing technical stuff?

 

[yogiprophet]

When measuring angle of attack, identical tires are part of the equation. Giving the 26er a larger tire decreases accuracy. The other part of the equation which has not been brought is decreased rolling resistance for the 29er. This occurs because the lower angle of attack causes the tire carcass to flex at the contact patch with a less severe crease. I believe 29 inches posted a German article that identified a 14% decrease in rolling resistance with the big wheels.

Better traction also allows you to carry more speed through turns. This means less acceleration of a 200lb bike and rider package as you exit a turn. It all adds up - but in the end it has to be your choice.

If I want to have better traction in corners I can either go to a bigger tire size or I can go to a bigger wheel size or I can do both but then I am adding a bunch of weight where I want it the least, so what is fair?

Thanks for telling me about the German article. Wow!! 14% does sound like a hefty savings. Can anyone attest to this?

 

[Schmucker]

One also has to consider the higher axle position. This directs the force from the bumps up into the frame rather than back against the fork.

 

[yogiprophet]

I believe 29 inches posted a German article that identified a 14% decrease in rolling resistance with the big wheels.
Better traction also allows you to carry more speed through turns. This means less acceleration of a 200lb bike and rider package as you exit a turn. It all adds up - but in the end it has to be your choice.

Here is the conclusion of the article:
The so-called gyroscopic forces of the 29er are higher than that of the smaller wheels due to more mass placed further from the center of rotation. The question that begs to be answered is how much energy is required to accelerate a 29 inch wheel? We used the same test stand that we used for the wheel test in this issue (marty's note: another test carried out in the same issue). With a pendulum we can measuer how much energy is required to accelerate the wheel from zero to 30 km/h. In order not to compare apples and oranges we used the new Mavix "Crossmax" 29er front wheel and the Mavic "Crossmax-ST" 26er front wheel. Tires: two identical SChwalbe "Little Albert" in 29 and 26 inch versions. Result: The 29 inch wheel, requiring 92 Joule, is clearly worse to accelerate than the 26 inch wheel (83 Joule). If you compare these values with those of our wheel test (marty's note: another test in this issue) then the 29er wheel ranks amongst the worst 26inch all-mountain wheels. Also the stiffness of the wheel does not favor the 29er. It places, with a value of 36 Newtons per mm , amongst the softest wheelsets of our test.

The guy how wrote this article is not aware of physics, that is for sure. What he is calling "gyroscopic forces" and its affects are bogus. What he means is - Moment of Inertia. Nevertheless, these affects are only considered during acceleraction. Once you get going (under ideal circumstances anyway) rolling resistance is the only force to consider(if you are going in a straight line). I say ideal because the reality is that we are not machines and noone has a perfect spin. Even if you are going at a "constant" velocity, you accelerate twice per revolution of the cranks. Try this on a trainer if you are not convinced. Even the best of us can hear this acceleration. This would be the major energy consumer that would offset the lower rolling resistance other than pure acceleration(increasing speed).

I think the coolest features noted is the 29rs ability to corner better and go through sand better. I mean I corner like a friginn' bat out of hell already these days(it took me a long time to get this right mostly because I'm so tall). And we have a lot of sand around here. Being a physicist and a mountain biker, I looooove momentum. I put a larger tire up front of my racing bike(tubeless of course) and man I am cornering noticably faster - keeping the momentum and riding faster overall, even with the higher moment of inertia. I can imagine a 29r with a fat tire would accelerate quite sluggishly, but it would probably be a blast to ride anyway - probably not fun to race though.

 

[El Caballo]

The angle of attack isn't what's making the difference here. There are two major contributions.

The first major contribution is how far in front of the axle the tire first hits the bump. Imagine two bicycles hitting the same bump, going the same speed, but with different size wheels. A larger front wheel will hit the bump sooner and roll off the back of it later than a smaller wheel -- but both wheels must move up and down the same amount.

This means that the larger wheel will take more time to rise and to fall the same distance as the smaller wheel, even though their forward speeds are the same. In the frame of reference of the rider, the bump has effectively been "smoothed out" by the larger wheel.

The second major contribution, which may be larger, is that for bumps which are close together, a larger wheel doesn't fall between the bumps as much as a smaller wheel, and therefore won't move up and down as much. It's the reason that a skateboard can get stuck in pavement cracks that you don't even notice on a bicycle.

 

[chequamagon]

you forgot to factor in the following principles:

 

[yogiprophet]

The angle of attack isn't what's making the difference here. There are two major contributions.

The first major contribution is how far in front of the axle the tire first hits the bump. Imagine two bicycles hitting the same bump, going the same speed, but with different size wheels. A larger front wheel will hit the bump sooner and roll off the back of it later than a smaller wheel -- but both wheels must move up and down the same amount.

This means that the larger wheel will take more time to rise and to fall the same distance as the smaller wheel, even though their forward speeds are the same. In the frame of reference of the rider, the bump has effectively been "smoothed out" by the larger wheel.

The second major contribution, which may be larger, is that for bumps which are close together, a larger wheel doesn't fall between the bumps as much as a smaller wheel, and therefore won't move up and down as much. It's the reason that a skateboard can get stuck in pavement cracks that you don't even notice on a bicycle.

Yes, good point, but suspension also allows the wheel to be picked up and move over a bump and the 26r I am considering has more of it.
And the angle of attack is a measure(the sine of the angle times the wheel radius) of exactly what you had mentioned.

With that said, if you have a 26r and a 29r, both without suspension, the 29r will roll over obsticles(ranging from 1" to 8") with about 5% more efficiency. But if you add suspension to both with more to the 26r(because you can fit more in the same space), then the efficiency is not as clear because of the complexity of damping. One thing is for sure, it would level the field at least somewhat if not even it out all together. Wouldn't you think?

 

[Fulton]

One thing you forgetting in your theory is that compressing the suspension absorbs energy, that energy comes from forward momentum hitting the bump.

 

[beenawhile]

WOW to much math :thumbsup:

 

[yogiprophet]

One thing you forgetting in your theory is that compressing the suspension absorbs energy, that energy comes from forward momentum hitting the bump.

Suspension is supposed to absorb energy. The impact that a bump would otherwise be translated vertically.

And what theory am I propossing here??? I'm only trying to get input as to others experiences and technical knowledge with both a 26" and a 29" suspension bikes.