Derby Rims Ups Options & Inventory, Adds 40mm Wide 26″ Carbon Rims

Why do we have another hookless rim when we don’t have a hookless tire? I was excited about this until the hookless part.

“now if you took a 2.35 tire and mounted it on the same rim the profile would be 2.35 . take that same tire and mount it on a rim that’s 40mm wide and it will spread the bead essentially making the foot print almost as wide as the 2.5 tire but using less rubber to do so”

No it won’t. It will make the carcass wider but won’t change the footprint at all. These tires have knobs on them and those knobs don’t relocate themselves with rim width.

“the main benefit of running wide tubeless wheels allows the bead to be spread further apart making burping a tire much more difficult to do.”

Also not true. The main benefit, of you ask fans of wider rims, is improved handling.

“the only downside of width is weight and user error for using a rim to wide .you can essentially stretch a tire so fat making it useless . ie. a 2.0-2.10 on a 40mm rim”

Again, not true. 2.0-2.1 tires will work fine on a 40mm rim. maybe not optimal but better than on a narrow rim.

It’s this kind of uninformed thinking that leads to the Derby claims in the first place.

Hit submit too soon: lengthening the 2d line is what you’re doing when you switch a 2.35 tire to a 2.5 tire.

Spreading the endpoints of the line farther apart ends with the same absolute width as using a longer line, in these circumstances.

Most of the advantage of wider rims has to do with increasing effective sidewall stiffness for the same pressure and increasing air volume, allowing for low pressure while maintaining the same flat resistance. There is a point of diminishing return though – at some point the wider wheel decreases volume, and physcially the tire wont be able to remain its bead.

Pinch flats are likely reduce because the marginal gains of stiffer sidewall and increased volume act similar to increase pressure (on a narrower rim). Greatly eliminating “snakebite”. Case in point, a 2.35 tire is a good 20mm wider than a 40mm rim and will easily snakebite at low enough pressures. Wider rims just make it much lower than most will go or for the normal impact forces seen.

Auto tires manufactures have long given recommended rim widths for their tires. Wider wheels typically result in a higher performing tire envelope for the same reason. I can easily see bike tire OEM starting to optimize tires for rim widths, if they aren’t already doing so. case in point, a wider rim may improve contact pattern, but is the 2.35 tire was design for use on a 24mm rim, this wont be the case.

“you’re saying the tread pattern doesn’t become wider. Absolutely not true. the tread width, simplified to 2d geometry, is a curved line.”
The tread pattern doesn’t become wider. The tread is molded onto the carcass and would have to spread out to become wider.

Sure, the tire profile is curved. It also flattens under load. This changes nothing.

A wider rim increases the circumference of the carcass. The tread on that carcass is the same.

“also, because you’ve reduced the curvature of the tire, you increased contact patch size”

No you haven’t. The contact patch size is governed by the load and the tire pressure. carcass width only affects the shape.

“sad that basic geometry is hard for people.”

Well, grog, on that we agree.

Why don’t you try an experiment where you use the same tire on two different width rims. Using the exact same load and tire pressure, take an imprint of the tire contact patch. You’d be astonished to learn that everything you’ve said is wrong.

@groghunter,
With repsect to sidewall stiffness – tires roll and deform due to lateral loads. Your correct on the cause of tire burping, but wider wheels for a given tire will reduce lateral deflection, resulting in better contact patch management and better response from the tire. This is well understood in both the automotive and motorcycle world. That doesn’t mean its without tradeoff though, and it is not as simple in a dynamic tire load like a bicycle experiences (as opposed to a car)

@craigsj – I largely agree with your last post, but I am not convinced larger air volumes are worse than small ones. It really depends on what your riding as well as your suspension tune and rider weight. Low speed technical riding may benefit from larger air volumes as the tire can deform more. Lower air volumes will increase internal pressure more for a given amount of tire deflection and this may not alway be desireable as it will prevent more rubber from hitting the ground (i.e. low spring rate). Higher speed riding may benefit from lower air volume for reasons you allude to.

I’ve run conventional aramid fiber folding bead tires on the Derby 650b rim, both tubed and tubeless. At low tire pressures high volume tires use, if a tire is not designed “hookless” as J would like to see, it’s a non-issue on Derby’s hookless rim. I would prefer less elastic beads such as carbon, agreed.

Another non-issue is mounting relatively narrow tires on Derby rims. I used 38mm nominal width tires on the 40mm rim with all positive results.

For a given nominal width tire, air pressure & load, increasing rim width alters the shape of the tire’s contact patch, not contact patch area. The contact patch shape becomes less elongated on a wider rim, which reduces pneumatic trail, resulting in reduced slip angle. A desirable effect for performance handling.

@HillDancer: As you mention, that’s all true for a given air pressure, but the increased volume allows you to run a lower air pressure, & in fact almost requires it, in practical use, due to reduced tire deflection at a given air pressure & increase in volume.

Thus why fatbike tires are usually running 6-10 PSI, & thus why in practice, a larger volume increases contact patch.

@craigsj: If I’m so wrong, & wider rims don’t improve performance, why does every other sport with spinning wheels use rims that are much closer to tire width than MTB? even road bikes. Heck, even Fat bikes, at least compared with MTB.

Wouldn’t spreading a tire too wide cause the cornering knobs to be useless?
I don’t want to corner on a bare sidewall.

Automobiles don’t really have that issue I guess; sort of a weird analogy for a mountain bike tire, imho.

Note: I lean when I turn, though I believe some XC riders do not.

I’ve seen three of these break/crack due to rock strikes – fragile and a ton of money. Riders running between 30-35psi will DHR II 2.4 or WTB tough casing. Beaware