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Why are road rims heavier than mtb rims

8K views 98 replies 21 participants last post by  aclinjury 
#1 ·
What don't I understand about carbon layups and strength, weights, etc?
Why are mtb rims that are 22mm wide internally, 25mm depth over 100g lighter than road rims that are narrower in width?

I have to think that mtb wheels take more of a beating, but does it have to do with the required higher pressure of the road tires/tubes?

Just curious?

Thanks
 
#5 ·
I'm pretty sure the stress on a rim increases with volume. Maybe someone else can confirm that? A 30mm ID rim with 2.4in tires at 30psi is very similar to the stress put on a 19mm ID rim with 23mm tires at 100psi. So I'd assume they're built to about the same amount of strength.

I haven't looked at mtb rim weights since I always go for the more downhill oriented rims. I would still think any wider mtb rim will weight more but 22mm is pretty narrow. Rim brake vs disc would make a big difference. What rims are you comparing?

It might have something to do with stiffness. Mtb rims are pretty flexible compared to road otherwise they ride like absolute crap.
 
#9 ·
Not to redirect the thread, but...
I used to use Specialized Turbo/S 26x1.0 on my Gary Fisher rigid. They were actually more like 22-23mm. It looks like Continental still makes a 28-559 in the original Grand Prix and the Gatorskin.
 
#10 · (Edited)
Shouldn't this thread be in Wheels/Tires? CX, you're becoming a softy. :D
 
#11 ·
I am not convinced pressure plays a role. I am not unconvinced either.

A couple other points to consider that have not been mentioned:

Make sure you are comparing the same sized rims by the same manufacture when pondering this question. There's generally a 20-30 g difference 27.5 to 29 in an mtb rim.

Also, don't forget that road rims often have some aero shaping, which does add material.

Given that MTB is a risky proposition, I would guess legal exposure for a rim blowout on the road is higher (given the rigors of mtb the trail would likely be blamed). So maybe extra fudge factor due to lawyers.
 
#14 ·
MTBs generally have suspension and more air volume to soak up impacts. I suspect hitting a pothole on a road bike subjects the wheel to higher forces than hitting a rock on an mountain bike.

Also keep in mind that MTB wheel weights have wide variations based on purpose. Downhill-oriented wheels are much heavier than featherweight XC race wheels.
 
#15 · (Edited)
So here is an example, but I have found it almost across the board with most companies. Although I think maybe rim vs disc brakes could be part of it, I don't know if it accounts for all of it.

So LB's 29er flyweight and regular xc rim:
https://www.lightbicycle.com/bead-hook-less-rims-carbon-29er-light-bike-rim-tubeless-compatible.html

280g/360g for a 24mm depth x 22mm ID

Here is a U shaped MTB 29er from them:
https://www.lightbicycle.com/U-shap...MTB-bike-carbon-rims-tubeless-compatible.html

395g for a 25mm depth x 24mm ID

Now their road wheels:

https://www.lightbicycle.com/Carbon...s-compatible-with-basalt-braking-surface.html

405g for a 25mm depth x 18mm ID

Both MTB rims are lighter than the road rim with a much wider ID. In the case of the flyweight you are looking at 125g difference for a 4mm wider rim.

Is all of that on brake surface?

Edit: Just looked at ENVE's site as well and similar findings like I said, although they don't have a disc brake specific climbing wheel. But their xc rim is much lighter and wider than their road climbing rim.
 
#18 ·
good info! what would be even more telling is to see a disc-mtb rim compared to a disc-road rim (preferably from the same manufacturer). I suspect the disc-road rim would still be heavier due to the requirement of a beefier "brake track area" (sort of speak) because a road bicycle going at 40-50 mph hitting a pothole with sharpe edges will impose a higher impact PSI than an mtb wheel hitting a big round boulder.
 
#31 ·
I'm not seeing the Boyle's Law theory. Boyle's Law says pressure changes as volume changes. That's how a bike pump works. You compress the air chamber volume to increase the air pressure.
But there is no volume change in a bicycle tire. It's static.

A 26" x 4" fatbike rim has ~326in^2 of surface are.
A 29" x 1" road rim has ~91in^2 surface area.

The surface area of the fatbike rim is 3-1/2 times that of a road rim. The volume is irrelevant. PSI is a ratio of pounds to in^2, not in^3. You use a lower pressure because the surface area is larger, not the tire volume.

It would be like saying if you had a 1 Gallon air compressor filled with 100psi, and a 10 Gallon air compressor filled with 100psi, that the wall thickness of the 10 Gallon take would need to be 10x thicker.
 
#45 ·
BTW I went to Carbonfan's site and their CX 700c rim that is 28mm depth x 21mm ID weighs in at 400g. They make a mtb rim that is about 290g for a 29er 25mm depth x 22mm ID.

Both 700c, both asym, both disc rims, but a difference of over 100g.
Would the small difference in depth/width make up for that? Doubtful.
 
#90 ·
A lot of that has to do with the construction of the tires as well. a car tire.. not mounted on a rim is still pretty damn hard.. especially run flats which have insane side walls and a crappy harsh drive (my Mini has them). MTB and Road tires are much softer and very flexible off a rim, MTB tires with zero air in them mounted on a rim will feel softer because there is much more material to flex than a road tire with no air.

Think of a long and short piece of plastic, trying to flex the short one is much more difficult than trying to flex the longer one. That has to do with the amount of leverage you can apply as well as the material it is made of.
 
#75 ·
Hey OP, I’ve asked the same question of carbon rim makers and a local wheel builder. They give the same kind of answer: road rims have to be stronger than mtb rims because they have to withstand more stress from the higher air pressures with smaller volume, so more carbon material is used.

Whether or not this is scientifically accurate or valid, I don’t know, but that’s the explanation they give. Intuitively it makes sense to me, but again, they may just be trying to explain something more complicated to a layman in terms I can understand.

And they may be blowin smoke up me arse, but I’m ok with the answer.
 
#76 ·
As to the OP's original question, I think there are a lot of factors at play that may contribute to making it hard to perform a true apples to apples comparison between road and mtn bike rims. Some thoughts that come to mind:

Rims designed for disc brakes don't need extra material (weight) for a brake track which wears down over time.

Disc brakes usually need more spokes than a rim brake wheel, especially on the front to transfer the braking forces from the disc through the spokes to the rim and tire. More spokes means one can go with a less strong rim, as there are shorter unsupported sections of rim between each pair of spokes and the rim is transferring less impact force through to each spoke.

Road bike wheels have got into marketing the absolutely lowest spoke count wheels possible, whereas I think that craze hasn't caught on to quite to the same degree in the mtn bike world. Having said that, I do have some very low spoke count disc mtn bike wheels. As I previously mentioned, fewer spokes often requires more material (weight) in the rim. A 32H rim may in fact have been designed to support an 18H option - they have to design for the lowest spoke count the rim may be used for.

Road bikes are generally more into aerodynamics, so more road bike rims have a deeper section than mtn bike rims. The deeper section usually requires more material (weight).

Mtn bikes have generally used disc brakes for 20 or so years, whereas road bikes are still mostly rim brakes. The result is mtn bike rims are likely all designed and intended for disc brake only applications. Since road bikes are only just starting the (attempted) transition to disc brakes, or at very least having to now support both rim and disc brakes, I suspect most road bike rims are generally still making a single rim that works for both rim or disc brake applications. In time, if rim brakes go away, they will redesign rims to cater to only disc brakes, and thus be able to remove brake track material (weight).

I can think of at least one more reason why a mtn bike rim can save weight, but it will throw us back off track with the high school math PSI/surface are side track that I suspect is still confusing some. I'm not going there again. ;)

To make this a viable apples to apples comparison, you would need to compare a like road & mtn bike rim (same rim section), where both are designed for disc brakes only and for the same spoke counts. In other words, about the only differences would be in diameter.
 
#92 ·
The main reasons why mtb rims tend to be lighter are the shallowness of them and being disc only. With disc only rims the sidewall can be much thinner than a rim brake counterpart. Aerodynamics also don't matter too much so you can get away with super shallow rim depths.

The 360g Stan's rims I have are only 16mm deep and the sidewalls are incredibly thin.
 
#94 ·
It may contribute a bit but it’s not the main reason given by manufacturers and wheelbuilders. I order carbon rims from Chinese makers like Light Bicycle all the time...they give you the option to buy disc or non-disc. The weight differences between the two for any given rim are 10-20g. We are talking much bigger variances here.
 
#95 ·
remember this video from years back? these are the Dura Ace 9000 wheels.
Looks like road wheels are sturdier than we think huh?
If we put skinny tires (not high volume) on a set of lightweight XC mtb wheels, they might not be as tough as these Dura Ace wheels!


That kind of riding with no suspension would have beat a Stan's Crest rim flat. I've bent 3 different rims on my mountain bikes and none of them were particularly hard hits. It was just the angle of the impact or an object that I hit.

I guess road rims need to stand up to impact at high speeds. You sure as hell do not want any risk of a rim failing at 50+mph because of a small pot hole. I have hit some substantial sticks, rocks and holes at high speeds on my road bike and I couldn't believe the wheel was still true. I've stopped just to check if there was any damage before.

On my mountain bike speeds are kept under control by terrain and the tires / suspension absorb the vast majority of the impact. Usually a rim failure is caused by some weird side load in a rock garden or corner. The wheels definitely have more lateral flex as well or they'd be a pretty miserable ride for anything that isn't smooth. The lateral stiffness of a typical road wheel would probably require a lot more material.
 
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