Condensed answer: A rear BMX hub can be installed on some mountain bikes, but the procedure is not straightforward due to the dissimilar hub spacing.
It’s also possible to mount a front BMX hub on an MTB fork.
The conversion has dubious benefits and is often not worth the trouble.
MTB and BMX Hub Measurements
The main hub measure is known as Over Locknut Dimension (O.L.D.) and refers to the usable part of the hub or the distance between the outer sides of the two locknuts.
The O.L.D. of front and rear BMX and MTB hubs is presented in the table below:
|Front O.L.D.||Rear O.L.D|
|BMX (female hub)||100mm||110mm|
|BMX (male hub)||110mm||110mm|
|MTB (super boost)||110mm||157mm|
The data reveals the following:
1. The rear O.L.D. of MTB hubs is longer than that of BMX hubs.
2. The front O.L.D. of MTB hubs is equal to that of BMX front hubs when the fork uses a quick-release skewer or a standard thru-axle.
Installing a Rear Male BMX Hub On an MTB
BMX bikes do not use gears, and their rear hubs do not have to accommodate a cassette. As a consequence, BMX rear hubs are shorter than MTB models.
Strictly speaking, a rear BMX hub isn’t compatible with an MTB frame due to the different spacing.
However, there is a way to circumvent the issue. The solution is not perfect, and some people won’t like it, but it’s the only option.
You can put a rear BMX hub on an MTB by filling the gap with spacers.
The graph below illustrates the principle:
After sliding the BMX wheel into the MTB frame and adjusting the chain line, a gap on the non-drive side will form because the O.L.D. of the hub is too short (Fig. 1).
The only way to fix this issue is to add spacers. A standard male BMX hub has a 14mm axle and would require 14mm spacers to fill the gap.
The extra space isn’t the only problem, however.
The new location of the hub shifts the entire wheel towards the drive-side. The aftermath is a non-centered wheel.
The only way to center the wheel is to “re-dish” it” – a process during which the tension of the spokes is redistributed in a manner positioning the wheel equally close to each chainstay of the frame.
The procedure requires a spoke wrench corresponding to the size of your spokes, a wheel dishing gauge, and a truing stand.
If you don’t have wheel-building experience, consider bringing the wheel to a mechanic who has the needed skills and equipment.
What about a female hub?
Technically, the same procedure can be done with a female hub. The only difference is that the spacers will have to be placed on the female hub bolt which is 10mm.
However, the spacers will weaken the hub’s composure because less of the bolt’s body will be in the axle.
Therefore, it’s not recommended to do this experiment with a rear female hub.
The O.L.D. difference between an MTB and a BMX rear hub is 25mm (135mm-110mm).
Meanwhile, the total length of a hub bolt is about 30mm. The threaded part is even less than that.
If the gap that a BMX hub leaves when installed on an MTB frame is roughly 15-20mm, only 10-15mm of the bolt will go into the axle. This isn’t enough to ensure stability.
Quick Release or Thru Axle?
This conversion can work only when the bike uses quick-release skewers.
The reasons are as follows:
1. A quick-release MTB hub has a 135mm O.L.D. – much shorter than that of thru-axle and boost hubs.
The shorter O.L.D. minimizes the gap that forms upon installing a BMX hub and reduces the chances of a failed project.
2. Thru-axles do not have the necessary “architecture” and size to operate with BMX hubs.
Front thru-axles are thicker (15/20mm) and won’t fit through a front BMX hub even though the O.L.D. is the same (100mm).
Rear MTB thru-axles, on the other hand, are 12mm and may fit on a BMX hub with some tinkering, but the axle won’t be secured.
In addition, the gap between the hub and the non-drive side will be even greater because rear thru-axle hubs have a 142mm O.L.D.
3. Boost hubs have a ridiculously wider O.L.D. than BMX hub. Trying to install a BMX hub on such a frame or fork is highly illogical.
MTBs Have Narrower Dropouts
The table below contains the thickness of MTB and BMX axles:
|Front Axle||Rear Axle|
|MTB (super boost)||15/20mm||12mm|
Quick-release MTB hubs which are the only eligible models for this conversion operate with a 9mm front and a 10mm rear axle.
For that reason, 14mm BMX axles won’t fit into the dropouts of every MTB frame. The only way around this obstacle is to file a bit of material off the dropouts and increase the space. (Obviously, this solution makes sense only when the frame is made of metal.)
This procedure could void the warranty of the frame because you would be doing a modification that the manufacturer has not authorized under any form.
10mm axles shouldn’t require filing. Their downside is that they’re weaker. And in this case, you want the axle to be as strong as possible due to the gap that forms on the non-drive side.
Installing a Front BMX Hub On an MTB
A quick-release MTB front hub has the same spacing as a BMX hub – 100mm. Therefore, a BMX front hub will fit right away.
If the fork is made for a thru-axle, the conversion will fail because thru-axles are thicker and have a different structure than what a BMX hub is designed for.
Is it worth it?
Truth be told, putting a BMX hub on an MTB is a conversion that makes little to no sense due to the amount of work involved in the process and the questionable gains.
The only instance when this switch is somewhat logical is when you have a street MTB frame and you want an extra-strong BMX hub with it.
But even in that case, the investment is of dubious worth when you account for the downsides of this modification. Those would be:
- Compromised structural integrity
The gap that opens when you install a BMX hub on an MTB frame reduces the axle’s support and results in uneven stress.
- A new wheel
If your MTB works with 24 or 26-inch wheels, you can buy a 24 or 26-inch BMX wheel.
But if the wheel is larger, you will have to build a new wheel around a BMX hub.
A proper conversion requires de-installation of the wheel, chain line adjustment, spacer fitting, and re-dishing of the wheel.
- No warranty
If the BMX axle doesn’t fit, and you have to file the dropouts, you risk damaging the frame and/or fork and losing their warranty.
- Goodbye disc brakes
BMX bikes do not use disk brakes because the rotors can easily get damaged during grinding.
The rotors may also hurt the rider when performing tricks.
BMX hubs reflect that and have no holes for installing a rotor.
When you convert to a BMX hub, you lose the option to mount disc brakes on the converted wheel.
- MTB hubs are plenty strong
A decent MTB hub offers sufficient strength and can survive pretty serious landings. This negates the benefits of installing a BMX hub on an MTB frame even further.