knurledflanges

Hi there, can anyone provide any resources that show how to actually do the math for calculating the torsional stiffness/strength needs of a hub brake wheel with a given ERD, tension, spoke count/dimension, and expected maximum brake/drive loads?
In other words, the math you would have to do if you wanted to take a non-"feel" based approach to choosing a cross pattern in a big hub/small rim situation where you're trying to avoid the spoke line at the nipple being unnecessarily bad while ensuring the wheel is adequately robust.
Any help is super appreciated. Thanks!

Andrew R Stewart 

I guess one could hire a mechanical engineer and come up with the answers you seek. But something tells me that the effort if far more complex then you think it is. Perhaps you could read J Brandt's book "The Bicycle Wheel" or AR Sharp's for some initial insight.

Or you could do what most do. experiment with actual stuff. Document and base line wheel construction then stress and measure the resulting changes. You know after a 150 years of bicycles there's not much that hasn't been explored or tried. Over the years the designs that don't work well get boiled out of the soup, until some one comes along and says that they will be different.

This forum, as well as others, have real smart guys reading. I'm looking forward to their replies. Andy.

chriskmurray

Big hub small rim, is this an ebike project?

If you are worried about torsional loads just go with the most crosses possible and run spokes with thicker elbows like Wheelsmith DB13's if this is an ebike. Really big hubs it is not uncommon to only be able to do 1 cross which has worked fine for countless riders but if it is a smaller hub motor (again assuming ebike) you could probably do 2 or even 3 cross if you have high enough spoke counts.

If you are worried about how the spoke may be forced to bend when it meets the nipple, again this has never been a problem in practice for me even with rohloff hubs which have 100mm flange diameters on 26" wheels. If your hub is bigger than this or the rim smaller you may run into an issue but there are companies who design rims with drilling for very large hubs like this Products | Ryde

Sapim also makes nipples that claim to help this although I have never built with them so I have no first hand experience.

One thing to remember, a big part of ensuring you wheel lasts for a long long time is build quality such has high and even spoke tensions along with thoroughly stress reliving the wheel.

FBinNY 

This is a highly overworked concern. The torsional considerations depend on the right angle distance (moment arm) from the line of the spoke (if continued past the axle) to the center of the hub. Flange size matters to the extent that it determines the maximum possible, though in practice we rarely build to the max. If you look at modern bikes, even super strong sprinters are able to live with relatively small moment arms, so you shouldn't have any issues with reducing crosses on a large hub.

Based on experience, and depending on the rim, I try to keep moment arms under two inches, because that's more than I need, and any more tends to cause spoke/nipple alignment issues at the hub.

fietsbob

As good a reason to start a College Career leading to a MS in Mechanical engeneering Degree as Any .

knurledflanges

Thanks for the replies all.
The situation I was in was building someone a Nexus 8/Rhynolite 36h 26in wheel to be used with an IM80 roller brake (which is one of the powerful, upper-end ones). Looking at the Wheelcalc models for this combination at 3x vs 2x, 2x had much better line at the nipple so I went with that, also thinking about how Rohloff hubs with their mandatory 2x are just a little bigger at 100mm vs 93mm, so the angle at the hub would be similar but a little further from tangent. Then after the wheel was built I noticed that Shimano tells you to only use 3x or 4x, and my personal policy with ignoring Shimano's guidelines on technical things is only do it if you basically know exactly what you're doing. In this case I think it should be fine, but I don't know exactly how well within the limits I am with this wheel at 2x, which is bugging me.

I know it's a complex problem, especially once you're also looking at dynamic loads on the wheel. Nonetheless it does seem like something where once one knew how to do it, one could just make a spreadsheet that lets you figure it out for all the wheels ever. And if that spreadsheet hasn't been made yet but the formulas for it are out there in published form somewhere, I was thinking about making it myself.

FRANK CANNON

Not sure I know what you are talking about here, but I think you want to build some strong drum brake wheels.

If so hit this guy up, he's put such things to the test.

Charlie Kelly's Home Page

FBinNY 

It really isn't that complex if you don't over think it. First of all abandon the notion that the brake makes a difference, because it doesn't really. The maximum braking torque is determined by the brake or by the friction between the tire and pavement, whichever is lower. Since just about any brake is capable of locking and skidding the rear wheel what matters is the axle load and the tire friction on the toad surface. Even that number is lower than one might expect because braking moments lower rear axle load, so it's self limiting.

As I posted earlier, wheels with spoke/hub moment arms as small as 1" or so, have survived the strongest sprinters and brakes all along I(look at mtb disc brake hubs), so anything beyond that is a bonus. Focus your attention on factors that matter, most notably the alignment of the spoke and nipple at the rim, and don't fret over what doesn't matter.