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The wheel strength issue

697 Views 11 Replies 7 Participants Last post by  El Caballo
DT is currently getting flamed on the 29er board for suggesting that 29ers have heavier, weaker wheels by comparison with 26ers. Physics says he's right.

However, front wheels have already lost nearly twice as much lateral strength going from regular hubs to disc hubs (21%), due to the tighter flange spacing, than they lose going to 29" from 26" (11%).

Yet I haven't seen DT, or anyone else, mention this as a problem. Why not?

Right now, *road bikes* can build up stronger front wheels than even the gnarliest DH rigs, because the hub flanges are so much farther apart. This, to me, is the retarded brother of bicycle engineering that we keep in the assisted care facility over the hill and never visit.

We had two opportunities to fix this: once when first building disc hubs and forks, and a second time when first building 20mm axles. Yet despite the constant, ongoing engineering efforts to make better rim extrusions, stiffer fork stanchions, higher hub flanges, straight pull spokes, 1.5 steer tubes, and other fork and wheel innovations, apparently no one noticed this basic issue.

Why not?
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whoa....like....you invented the V brake?
I read the thread and the only flame I saw was from our very on Gonzo. Seemed like a very gentlemanly and level-headed discussion to me.
Horsebreath,

anyone who's spent a fair amount of time building and riding both 26ers and 29ers will tell you that the 29er wheel will be heavier than the equal-strength 26er wheel.

I suggest you page Mike Curiak.

the rest of your post is the sort of pseudo-science that denies reality -- do you work for Tony Ellsworth?
gonzostrike said:
I suggest you page Mike Curiak.
I'm weeks ahead of you. He said "Amen, hallelujah, you have my vote for Prezident."
http://forums.mtbr.com/showpost.php?p=1849034&postcount=36

Back to the topic: I'm expressing my surprise that people get really worked up over losing wheel stiffness/strength on 29ers, but no one seemed to notice when we lost twice that much by going to disc hubs.

Here's the science, since you asked:

The wider the hub flanges are, the more triangulation there is to the spokes, the more that a lateral force acts against the tensile strength of a spoke, and the less it acts against the bending strength. Spokes have nearly infinite strength in tension and nearly zero strength in bending. Therefore, the lateral strength of a wheel is basically a sine/cosine relationship.

Damon Rinard's measurement data (http://www.sheldonbrown.com/rinard/wheel/data.htm) confirms that, though this is a simplification, it's quite close to the truth. The simple sine/cosine model predicts a 139% difference in stiffness between front and rear wheels built with identical rims and equal spoke count: Rinard's measurements show 137%, 146%, and 148%.

This is pretty damn spot-on for real world data applied to such a simple model. Does anyone else have an explanation that fits the data better and conforms to basic physics? I'm all ears.

The original thread, with all the gory details, is over here:
http://forums.mtbr.com/showthread.php?t=190125
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El Caballo said:
Back to the topic: I'm expressing my surprise that people get really worked up over losing wheel stiffness/strength on 29ers, but no one seemed to notice when we lost twice that much by going to disc hubs.
No one forced anyone to switch to disc wheels. You can still buy rim-brake wheels and use them if you want.

I think part of the issue now is that since disc wheels are already weaker than rim wheels, going to a 29" wheel makes them even weaker.

Even though 29" wheels are weaker, maybe nobody will notice... just like 26" wheels going to disc.
Blue Shorts said:
No one forced anyone to switch to disc wheels. You can still buy rim-brake wheels and use them if you want.
In general, the more you need disc brakes, the more you need strong wheels. It's a poor choice. What we really need is a wider standard for front hubs and fork leg spacing. (150mm and 165mm rear hubs already exist.)

Blue Shorts said:
I think part of the issue now is that since disc wheels are already weaker than rim wheels, going to a 29" wheel makes them even weaker.
You are correct. I was actually prompted to look into this by two things:

1) A post from Mike Curiak, saying that he wasn't sure he could build 29" wheels that would stand up to a Clyde on a long-travel bike, no matter what rims he used.
2) A post from Alex @ bcdracing about wheel issues with his 29" downhill bike. He actually went so far as to fabricate a 120mm front hub (WB forks use this spacing, the idea being you can use a standard 110x20mm hub with a spacer and build a dishless front wheel.)

So I decided to do the math, and the results were alarming. We need 125x20mm on a 26er (559mm) just to match old non-disc front wheels, and 135mmx20mm on a 29er (622mm).

Even this standard wasn't engineered. We just used road bike spacing -- which is designed to be as narrow as possible for low air resistance. What we really want is to be as strong as a BMX wheel. This would require about 152x20mm for 26ers, and 165x20mm for 29ers. Hmm.

Interestingly, Charlie Cunningham advocated 118mm front hubs back in the day. These were non-disc, and would be equivalent to a 143x20mm TA hub.
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El Caballo said:
In general, the more you need disc brakes, the more you need strong wheels. It's a poor choice. What we really need is a wider standard for front hubs and fork leg spacing. (150mm and 165mm rear hubs already exist.).
No arguments here. :)

Bicycle usage has gottn much more demanding in the last 10 years or so. I'm sure that a lot would be changed if bike builders could start from scratch and design wheels (and other components, including frames) that would be better for todays uses.

Currently, you have to wait for things to evolve... meaning..Don't hold your breath.
El Caballo said:
Yet I haven't seen DT, or anyone else, mention this as a problem. Why not?

apparently no one noticed this basic issue.

Why not?
Just because you haven't seen DT mention it publicly or in print doesn't mean that he hasn't given it any thought. I don't know either way, but I don't share your assumption without evidence that "no one has noticed this basic issue." DT is a frame builder. As far as I know he doesn't design forks or hubs or tires or sit on any committees responsible for their respective specifications (although I think he had a hand in ISCG mounts). Like most framebuilders he has to work with what's available. But who knows maybe behind the scene he's developing a whole new line of forks, hubs, and tires?
White Horse

Wow! those are some serious #'s. One of the topics that will be discussed at the Decline syposium in Whistler next month are mtb standards. If you don't mind I will bring up your #;s and points. At what model point do you think these new standards are to be applied?
5 Spot or RFX ? Do you think they should be applied to 4" frames as well?

DT
El Caballo said:
1) A post from Mike Curiak, saying that he wasn't sure he could build 29" wheels that would stand up to a Clyde on a long-travel bike, no matter what rims he used.
so despite your first reply TO ME in which you tried to say Curiak disagreed with me, you are saying here that he agreed with me. no surprise there. he's the source I would cite.

your "physics" analysis reminds me of Steve from JH doing the Peebag Egghead (P.E.) analysis of suspension designs.

I don't care if "on paper" there is a fix, if in the field it still fails.

and that's what Curiak is talking about.

but go ahead, consult your slide rule and other text-oriented "authorities" if it helps with your quest for superiority.
DT:

I have a spreadsheet and attached documentation that lets you run hypothetical numbers quickly and look at all the measurement data. PM me an email address and I'll be glad to send you a copy.

As far as the model point, everyone benefits, but I think it's an easier sell at the 20mm TA fork boundary: 5" and up for 26ers, and 4" and up for 29ers. If you need (or think you need) a TA fork, you obviously care about stiffness and strength.

The tradeoff is that you're gaining weight at the hub and the fork crowns, but you can use a lighter rim. Trading rotating weight for non-rotating weight is usually a good trade. (Frames may need some extra clearance at the downtube -- or those high-rise lower headset cups that do the same thing.)

Physics says this trade still makes sense at the QR level, but it might be a tougher sell to the XC crowd. On the other hand, if you can show big guys that they can run stupid-light rims... What do you think?

turnerbikes said:
Wow! those are some serious #'s. One of the topics that will be discussed at the Decline syposium in Whistler next month are mtb standards. If you don't mind I will bring up your #;s and points. At what model point do you think these new standards are to be applied?
5 Spot or RFX ? Do you think they should be applied to 4" frames as well?

DT
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