A Question Of Wheel Strength

Question: Why don’t they make the hubs for 29″er wheels with bigger diameter hub flanges so the wheels can be stronger?

Answer: The matter of wheel strength for 29″ers has been a concern ever since the modern 29″er hit the scene almost ten years ago now. Longer spokes are often times pointed at for being the culprit here. However; is that really the case? Let’s take a look at somethings that may clear up this matter for us.

Wheels have several things that can affect their strength. The way the materials are used, the way they are assembled, and how these two work together are the basics of wheel strength. Particular things such as the spokes, their shape, number and length can affect wheel strength to a degree. Rim diameter, profile, and design are other critical factors. Hub flange strength is important to anchor those spokes well. Let’s assume we are using high quality, well made parts in our wheel assembly and we have the best wheel smith putting it all together.

So, in this scenario, why would a 26″er wheel be stronger than a 29″er wheel? Lots of you are going to say it’s the long spokes. In reality, it’s the geometry of the wheel that is the issue. Let’s take a rear wheel as an example.

Back in the 80’s, when wheels had but 6 speeds, wheels were pretty much symmetrical in nature. That is to say, the angles at which the drive and non-drive side spokes left the hub flange were identical, or nearly so. Now with the addition of each speed up to the 9 we have today in a mountain bike cassette, compromises in wheel geometry had to be made to accomodate the space taken up by the cassettes. Take a look at any rear wheel with a multiple speed cassette. You will notice the drive side spokes leave the hub at a much shallower angle than the non-drive side spokes do. Front wheels are not exempt either. Disc brake rotors cause the same compromises in spoke angles. This is called “dish and causes wheels to be weaker than they would be if they had symmetry of spoke angles from drive side to non-drive side.

This is the biggest influence on wheel strength beyond materials design. In fact, hub flange diameter, and thus spoke length, are much less of a factor. Increasing flange diameter has not been proven to show much, if any, increase in wheel strength.

So, in terms of our original question, the hub flanges have a nominal effect on wheel strength. Increasing the distance between the hub flanges and having a dishless wheel has a much more dramatic effect on wheel strength.

What can be done then? Wheel overlock dimensions have been set since the onset of the 90’s for mountain bikes at 135mm for a rear wheel and 100mm for a front. Is this as good as it gets? Well, some things are pointing to changes in this area.

Niner’s W.F.O 9, an all mountain 29″er that is currently in developement, has a 150mm OD rear hub, a width commonly used for tandem bicycles and some down hill specific machines. (Guess why!) Lenz Sport’s Lunchbox full suspension 29″er also already is using this rear over lock dimension to get a dishless, thus stronger, rear wheel. But what about the front?

Well, that is being looked at by Paul Components who showed a 120mm OD front disc hub aimed at 29″ers at the recently held NAHBS in Portland, Oregon. Word is a suspension fork manufacturer is also looking at this standard as well.

For now it’s best to use a rim, spoke count, spoke guage, and hub design best suited to you and your riding style and to have a competent wheel smith put it all together.

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No Responses to “A Question Of Wheel Strength”

  1. George Krpan Says:

    Are there any compromises to having 150mm rear spacing?
    The rear wheel of my 29er definitely needs more maintenance than on the 26ers that I have owned. But, I haven’t noticed this about the front wheel.
    I have read where some people have tacoed their front wheels but it is usually a Clydesdale tying to use 400 gram rims with a low spoke count, duh.
    A 120mm hub would, of course, need a 120mm fork.

  2. Davidcopperfield Says:

    For non-super light rider with race only purposes a 36 spoked rear 622mm wheel is a must. 150mm hub is a good idea however in XC and light AM would add too much weight. For DH 29ers the tandem super wide 165mm hub with tall flanger ought to be used if not you get weaker wheel which needs more attention, trueing etc.

    In my opinion all those 32 holed 29er rims out of XC race field should be removed, because 32 is a 559mm- 26″ standard.

  3. Evan Says:

    All other things being equal, you can increase the number of spoke crossings (3X or 4X) to get a more laterally “rigid” wheel, but you also add weight by using longer spokes. If you have room, you can “race lace” the wheels by running all the spokes facing out from the hub instead of alternating in and out. And don’t forget the old-school method of tied and soldered spokes. There is no guarantee of a massive increase in strength, but for heavier riders it certainly can’t hurt.

    I think it was Jeff Jones at NAHBS who had new 9sp hubs reduced to 8 or even 7 cogs, and then shifted toward the drive side for less dish.

  4. jncarpenter Says:

    Actually, I believe JJ uses a SS cassette hub with 5/6 gears & little/ no dish. I have 3 sets of I9 wheels, 2 being 29ers…1 SS & 1 9 Speed. I definitely notice the ss wheel is a touch stiffer. In both cases the same spokes & rims are used.

  5. Marz Says:

    I already have spoke to caliper clearance problems with my standard flanged Hugi front hub. it requires 2 spacer washers on the hub to maintain good clearance through hard riding…. A taller flange will only agrravate this problem…

    any tips to remedy this – the larger the wheel the smaller the angle away from the hub to the rim?

  6. Mike Says:

    “Are there any compromises to having 150mm rear spacing?” – Not in terms of the wheel itself. There are compromises, however, in terms of what 150mm rear spacing does to front chainline. With 150mm spacing, which is used on freeride bikes, you would use an 83mm wide bb shell to get a chainline of 56mm that matches up with the rear spacing. For a DH type bike, this is okay as widening the entire drivetrain nets you more clearance for big tires. For XC type riding, it alters your body geometry (to borrow Specialized’s term) with regards to where your feet are pedaling beneath your pelvis. Kind of a bow-legged feel.

    I believe there are resources available that make 29″ wheels sufficiently strong. Choosing the appropriate rim, spoke gauge, and, most important of all, spoke tension should be sufficient for 99% of the folks. Don’t go all weight weanie on the rim and spoke if you are a bid dude or have an aggressive riding style.

    Asymmetric rims used in the rear to get less dish due to the gears and used in the front to reduce dish due to the brake are a good choice to increase wheel strength and to more equally balance tension in the spokes between the right and left.

  7. Mike Says:

    Almost forgot to add, when WTB introduced their Phoenix, you could get a frame made with 140mm spacing to eliminate dish. Coincidentally, WTB also produced 140mm spaced hubs. I’m surprised that the industry didn’t adopt 140mm spacing when 9-speed came out. There was some polling by Shimano among the industry to ask this question, but it came to naught.

    I like the wider, lighter, XC front ends that are coming.

    And 32 spokes is more than sufficient for XC wheel strength.

  8. Desert9r Says:

    I was just think the other day, I have never seen anyone actually using a 4x wheel. I would think the longer spokes needed would add to a 29er “passive” suspension, atleast to some degree.

  9. Mike Says:

    I’m not sure you would feel the passive suspension of a 4x wheel with such a large volume of air trapped by the tires. Maybe with road wheels and 100psi.

  10. Cloxxki Says:

    Hasn’t chainline been increased over the years as well?
    Chainline will only be an issue when on a smaller cog on a small ring, a rare occasion.

    150mm would be a start, but I’d say : in the least go for the smallest that is dishless in 9spd, what ever that come down to. Bend chainstays to give heel some space, no biggie.

    Wider forks will be stiffer also. Possibly a weight-saving change.

  11. Mike Says:

    Cloxxxi – Chainline has increased from 47.5mm 51mm without widening the rear spacing because of frame manufacturers needing more space for linkages, more popular use of fat tires, and because of increased seat tube diameters limiting how far the derailleur can be dropped down. The issue caused by this isn’t on the small ring (which would make small/small an easier gear to cross over to), but to the middle and large rings. When in the middle or large rings, the chain angle to the larger cogs will be greater and promote increased chain/cog/chainring wear.

    Again, WTB has also produced wider forks years ago with 118mm spacing for their Type II fork. I’d love to have a 29″ version of this fork/hub combo.

  12. George Krpan Says:

    Mike, thanks for the input.
    Some people might find that a wider bottom bracket may actually fit them better.

  13. Cloxxki Says:

    Yes Mike, I meant that with wider hubs, only the smaller rims might become an issue. The current issue of large/large goes away of course.

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