· By Austin Duggan
Why we use MTB hubs on a BMX
BMX bikes have used the same dropout standards for decades; an "over locknut dimension" or "O.L.D." of 110mm in the rear, and 100mm in the front.
At Chimerabmx, we've purposely designed our frames and forks to have freestyle BMX geometry. But we use the MTB standards of 135mm in the rear, and a 110mm "boost" spacing in the front. Why do we do this, and how do we make it all work with pegs and BMX style dropouts? The short answer is that it helps us compensate for the loss of wheel strength that occurs when switching to disc brake hubs, and we make it all work with clever machining. The longer, more detailed answer starts with an understanding of how hub design relates to wheel strength.
What makes a wheel durable?
There are a bunch of factors. But generally speaking, a durable wheel has a relatively high degree of:
Radial stiffness: A wheel's resistance to flex against pressure in the vertical direction (the direction that the spokes are pointing).
Lateral stiffness: A wheel's resistance to flex against pressure in the sideways direction (perpendicular to the direction the spokes are pointing).
Radial strength: How much force in the vertical direction it will take to start buckling the spokes.
Lateral strength: How much force in the sideways direction it will take to buckle the spokes.
The difference between stiffness and strength.
Stiffness is a measure of wheel flex. Strength is a measures of the wheel's resistance against its spokes buckling.
Stiffness is important because the stiffer a wheel is, the more each spoke will share the force of a hard landing, hitting a pothole in the road, running into something, etc. Stiffness helps keep each spoke from taking too much of the load, and in turn helps keep the rim from flexing in one particular area.
Strength in the lateral direction resists against the wheel folding over (a.k.a. "tacoing"). That's important when you carve at the skatepark, do flatland tricks, or land crooked. Radial strength, on the other hand, helps keep the wheel from caving in, basically turning into an oval. That's less common though. As you'll see below, wheels are by design much stronger radially than they are laterally. So unless you land really hard exactly perpendicular to the ground, your wheel's more prone to taco rather than cave. For a good example of this, check this out. In general, lateral strength on a wheel is much harder to achieve than radial strength.
Stiffness doesn't always make a wheel strong. If you lower the spoke tension, your wheel will be about as stiff, but the spokes will buckle more easily.
Conversely, strength doesn't make a wheel stiff. If you use thinner spokes, your wheel strength will actually increase because it will be able to flex more without buckling. But it will be far less stiff.
How stiff and strong should a wheel be?
This depends on too many things to discuss here; the rider, the bike,...even the temperature matters. Here, we're just focused on the effect of hub standards on wheel durability. There are three crucial features of a hub when it comes to wheel durability; the flange-to-flange distance, the offset, and the flange diameter. To get a good basis for comparison, let's look at these specs for a popular BMX hub; the Odyssey Antigram V2. This hub has 49mm flanges with a center to driveside flange distance of 30mm, and a center to non-driveside flange distance of 30mm. This means that the flange-to-flange distance is 60mm, and the offset is zero.
These are pretty typical measurements for BMX hubs, give or take 2mm. Let's punch this info into Mathew Ford's wheel simulator to see what a 24" wheel with this hub would be like. To keep all our measurements consistent, I'll use some other common features of a well-built BMX wheel: 36h, 3cross lacing with 110kgf spoke tension on 500g rims. This shows us that...
A good 24" BMX wheel has:
- 38400lbs/in radial stiffness
- 1350lbs/in lateral stiffness
- won't begin to cave until it gets 655lbs of force in the vertical direction (radial strength).
- won't begin to taco until it gets 200lbs of force on the side of the wheel (lateral strength).
It turns out that these are roughly the numbers (give or take 2%) you get when you put in the specs of any decent BMX hub, front or rear, with all the other stats we put in. Since those stats are also standard, these ratings should be a good comparison base for any 24" bike maker or wheel builder - if your BMX wheel is far lower than these standards, then you need to redesign.
Wheel strength and E-Bike braking
We're trying to make wheels that can stand up to the abuse of moderate-impact BMX riding. So it would be great if we could just use standard BMX hubs. Unfortunately, we can't. Our electric BMX hits top speeds of over 35mph. If you want to stop without having to call ahead to make a reservation, you need powerful hydraulic disc brakes both front and rear. That requires using hubs that have a disc mount. A disc mount takes up a portion of the total width of a hub. It results in much less center to disc-flange distance than center to drive-flange distance. This is called an "offset" because one of the flanges is offset from the center by a different distance than the other. As you can see with this Paul Components hub (an excellent company by the way), a 100mm disc hub has much less flange-to-flange distance than a BMX hub, and a substantial offset.
When you have an offset like that, you have to tension the spokes of one side of the wheel more than the other. The asymmetry in spoke tension hurts the overall lateral strength. So does the decreased flange-to-flange distance. All else being equal, using BMX width wheels with disc brakes decreases lateral strength by 30%. Wheels like that would be far more prone to taco! Surprisingly, we see a lot of ebike companies, including electric "BMX" makers, using the 100old standard for their front disc hubs. Not good, especially for riders who want to jump their bikes or carve at the skate park.
Our rear hub/wheel
To avoid the loss of lateral strength that a 110old rear disc hub would have, we increased our rear dropouts to the MTB standard of 135old. This allows our rear hub to have a 34mm center-to-drive flange distance, and 34mm center-to-disc flange distance. It's wider overall than a BMX hub, with no offset. All by itself, this would have resulted in wheels with 8% more radial strength than a regular BMX wheel, while also offering roughly the same amount of stiffness and lateral strength. But we also chose to use hubs with large 68mm flanges - far bigger than usually found in the BMX world. So if we stayed with the same stats as above (36h, 3-cross, 13g spokes, and 110kgf tension) our rear wheel would wind up with around 15% more lateral strength and stiffness than a normal BMX wheel, while also being about 2% stronger and stiffer radially. That's a tough wheel!
Our front hub/wheel
To avoid the loss of lateral strength that a 100old front disc hub would have, we increased our forks to the MTB 110old "boost" standard. This allows our front hub to have a 43 mm center-to-drive flange distance, and 25mm center-to-disc flange distance. It has a lot of offset (18mm!), but it's much wider overall than a BMX hub and has the same large 68mm flanges as our rear hub. This results in a front wheel that is roughly as strong as a common BMX wheel (2% more radial strength and 2% less lateral strength), but is 30% stiffer laterally. So by switching to this wider mtb standard and using fat hub flanges, we're able to achieve the disc braking that gives us the proper stopping power for an ebike without sacrificing the wheel strength that a BMX needs.
A note about spokes
All else being equal, our front hub will result in a wheel just as strong as the BMX wheel we looked at above, and our rear hub will result in a wheel that's even stronger. All else is not equal, though. We make our wheels with the 36h, 13g spoke setup cited above, but only by request (at no extra charge!) As so many snarky internet reply guys have pointed out, our bikes instead come standard 32h with 14g double-butted spokes. Why would we do that? Doesn't that change everything?!
It does, but in many ways for the better. A 32h, 13g setup on our hubs shaves 5oz off the bike, yields wheels that are noticeably less stiff, and actually strengthens the rear wheel radially by 10% without dropping its lateral strength below our stated goals. This is great for the riders who are just using the bike to race around, commute, and do the occasional light BMX trick (bunny hop, wheelie, curb jump, etc). It's a lighter, more comfortable ride with plenty of rear wheel strength. Granted, the front wheel lateral strength will be about 10% less than the ideal stated above, but it'll still be at least 20% stronger than the 100old front wheels offered by other electric "BMX" ebike companies. So if you're not planning on doing any moderate to high impact BMX tricks, the weight savings and cushier ride is probably worth it. We leave it up to you.
Wait...will MTB hubs fit BMX pegs?
Most MTB hubs use thru-axles, which aren't compatible with pegs. Fortunately, there are a couple of high-end hub makers who offer 135old single-speed hubs with 10mm bolts to fit BMX-style dropouts and pegs. They include HopeTech, Industry Nine, Profile, and Onyx Racing (not to be confused with Onyx Motorbikes). When we were sourcing hubs, we followed their lead and worked with our hub supplier to design a hollow steel axle for our rear hub so that it slides into the dropouts with 10mm bolts, and can be used with pegs.
We could have done the same thing with our front hubs. But that would have been unfair to our riders. There are no hub makers who offer a 110 front boost hub that can be used with 10mm bolts. We think riders should be able to easily customize their bikes using their favorite off-the-shelf brands/parts. It's part of what makes bike ownership fun. You should enjoy your bike, not just your bike ride. That's a philosophy at Chimera BMX. So instead of making a totally proprietary front hub that you can only get from us, we made a conversion axle. This axle is made of 7075 aluminum. It's 110mm wide, 15mm in diameter, and has 10mm threads on either side. Here are some photos:
With our conversion axle, you can turn any 110/15 thru axle hub into a 110/10mm bolt-on hub that will work with pegs. Virtually every major hub maker on the market offers 15mm thru axle front hubs. So, in summation, we use the MTB standards on a BMX bike because it gives our riders the best of all worlds; the strength and stopping power of MTB disc hubs with the peg-grinding compatibility of BMX hubs and the ability to customize their bike with many of their favorite off-the-shelf parts.
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