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  1. #1
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    Why are road rims heavier than mtb rims

    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

  2. #2
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    Disc vs. rim brake perhaps. Compare disc to disc and what do you see?

  3. #3
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    Tire pressure. MTB rims don't need to withstand 100+ psi. (presuming we're comparing clincher rims).

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    They probably weigh more.
    Oh my, a troll who doesn't know the difference between your and you're. What will they think of next?

  5. #5
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    Quote Originally Posted by Peter P. View Post
    Tire pressure. MTB rims don't need to withstand 100+ psi. (presuming we're comparing clincher rims).
    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.

  6. #6
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    Quote Originally Posted by Peter P. View Post
    Tire pressure. MTB rims don't need to withstand 100+ psi. (presuming we're comparing clincher rims).
    Boyle's law.
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  7. #7
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    Quote Originally Posted by Peter P. View Post
    Tire pressure. MTB rims don't need to withstand 100+ psi. (presuming we're comparing clincher rims).
    I think you're right. Take an mtb XC rim, put some skinny tires on it and pump it to 100 psi, hit a pothole and the tire will probably blow itself cleanly off the rim.

  8. #8
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    Quote Originally Posted by aclinjury View Post
    I think you're right. Take an mtb XC rim, put some skinny tires on it and pump it to 100 psi, hit a pothole and the tire will probably blow itself cleanly off the rim.
    Nope...lots of guys used to use 26 x 1.0 tires to train on the road using their mtb. Conti made a tire that got used a LOT for that.
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    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.
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  10. #10
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    Shouldn't this thread be in Wheels/Tires? CX, you're becoming a softy.
    Last edited by Lombard; 5 Days Ago at 03:22 AM.
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  11. #11
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    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.
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    Quote Originally Posted by cxwrench View Post
    Nope...lots of guys used to use 26 x 1.0 tires to train on the road using their mtb. Conti made a tire that got used a LOT for that.
    Yep. I recall that some did max at 100.
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    Quote Originally Posted by Fajita Dave View Post
    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.
    More volume would create more pressure at the same PSI. And at lower pressure to a certain point.

    Are you just taking a wild guess with your example or do you know the mtn bike rims would equal the stress of the road rims in your example?

    If your presenting that as fact could you please share how you calculated that? I use a lot of difference tire sizes for various types of rides and would be curious.

  14. #14
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    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.

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    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-ho...ompatible.html

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

    Here is a U shaped MTB 29er from them:
    https://www.lightbicycle.com/U-shape...ompatible.html

    395g for a 25mm depth x 24mm ID

    Now their road wheels:

    https://www.lightbicycle.com/Carbon-...g-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.
    Last edited by yourrealdad; 5 Days Ago at 06:07 AM.

  16. #16
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    Quote Originally Posted by Jay Strongbow View Post
    More volume would create more pressure at the same PSI. And at lower pressure to a certain point.

    Are you just taking a wild guess with your example or do you know the mtn bike rims would equal the stress of the road rims in your example?

    If your presenting that as fact could you please share how you calculated that? I use a lot of difference tire sizes for various types of rides and would be curious.
    You sure about the bolded statement above? PSI is pressure per square inch. The definition doesn't take volume into consideration. So how can increasing volume increase pressure? Or am I not understanding you right?

  17. #17
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    Quote Originally Posted by cxwrench View Post
    Nope...lots of guys used to use 26 x 1.0 tires to train on the road using their mtb. Conti made a tire that got used a LOT for that.
    I remember November Dave has said that at 100 PSI is when they start to notice that road rims start to expand at the brake tracks. Then when you factor in heat from braking on road rim (rim braking), then the PSI could get over 100 quickly on descending. MTBs have disc so they don't have to deal with the tire possibly blowing out during braking. The other factor that mitigate a tire blowing out on an MTB rim is that MTB rims are wider and thus enable a higher volume for the tire, and this wider volume serve as shock absorber when hitting a pothole. A narrower road rim will have a smaller volume, so when a road wheel hit a pothole (assuming same speed) the impact PSI is necessarily higher. So when we factor in the braking heat and smaller volume, the road rims could experience a higher impact PSI force in some situation, thus their brake tracks have to be made with more material to withstand such impact?

  18. #18
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    Quote Originally Posted by yourrealdad View Post
    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-ho...ompatible.html

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

    Here is a U shaped MTB 29er from them:
    https://www.lightbicycle.com/U-shape...ompatible.html

    395g for a 25mm depth x 24mm ID

    Now their road wheels:

    https://www.lightbicycle.com/Carbon-...g-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.
    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.

  19. #19
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    Quote Originally Posted by aclinjury View Post
    You sure about the bolded statement above? PSI is pressure per square inch. The definition doesn't take volume into consideration. So how can increasing volume increase pressure? Or am I not understanding you right?
    How many times do I have to say 'Boyle's Law'. It's why fatbike rims have pressure limits of around 25psi and many mtb rims have lower pressure limits for larger tires, higher pressure limits for smaller tires. As the volume of air goes up the pressure needed for a similar feel goes down. A 3" tire at 100psi would probably destroy any rim made.
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  20. #20
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    Quote Originally Posted by aclinjury View Post
    You sure about the bolded statement above? PSI is pressure per square inch. The definition doesn't take volume into consideration. So how can increasing volume increase pressure? Or am I not understanding you right?
    You are correct. Additionally, pressure does not increase or decrease (short of a hole somewhere) on impacts. Pressure is exerted evenly in an enclosed space (inside of the tire), unless there are baffles on the inside to restrict the flow of air. So on an impact, the tire itself will deform, shifting the same amount of air within the tire into the form the tire takes at the time, however the pressure will remain constant.

    Now reducing the interior area of the tire while keeping the same volume of air initially would increase the pressure..

    As for why one is lighter than the other.. I have no clue

  21. #21
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    Quote Originally Posted by aclinjury View Post
    You sure about the bolded statement above? PSI is pressure per square inch. The definition doesn't take volume into consideration. So how can increasing volume increase pressure? Or am I not understanding you right?
    Yes I'm sure about it.

    I don't really know how, I'm not the scientist who figured it out and a forum about bikes probably isn't the place to tackle that. But try getting 100 psi into a car tire, or feel a road bike tires at 32 (which is about what cars take) and I think it becomes obvious that it's true.

  22. #22
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    Quote Originally Posted by Jay Strongbow View Post
    More volume would create more pressure at the same PSI. And at lower pressure to a certain point.

    Are you just taking a wild guess with your example or do you know the mtn bike rims would equal the stress of the road rims in your example?

    If your presenting that as fact could you please share how you calculated that? I use a lot of difference tire sizes for various types of rides and would be curious.
    No calculations on my part. A while ago I was looking at what rims to buy for my last mtb wheel set I stumbled across an article where someone blew up a carbon mtb rim and blamed the manufacture for defects. He was commuting to work and rode with 40psi with a 30mm rim ID and fat tires. Someone who seemed to know Boyle's Law made the comparison that you quoted in my first post.

    So just like you said more volume creates more pressure on the rim at the same PSI. Meaning a wide rim/tire at low pressure will require the same amount of strength as a narrow rim/tire at high pressure. Which wouldn't explain why the OP's comparison is showing mtb rims to be lighter.
    Last edited by Fajita Dave; 5 Days Ago at 07:29 AM.

  23. #23
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    Quote Originally Posted by ljvb View Post
    You are correct. Additionally, pressure does not increase or decrease (short of a hole somewhere) on impacts. Pressure is exerted evenly in an enclosed space (inside of the tire), unless there are baffles on the inside to restrict the flow of air. So on an impact, the tire itself will deform, shifting the same amount of air within the tire into the form the tire takes at the time, however the pressure will remain constant.

    Now reducing the interior area of the tire while keeping the same volume of air initially would increase the pressure..

    As for why one is lighter than the other.. I have no clue
    But hitting something does reduce interior area so half of what you said has to be wrong.

    And I just proved it in about 10 seconds by putting a psi gauge on a tire (a 33mm that's around 30psi) and watching the needle move up when I pushed on the top simulating what happens when you hit a pot hole or whatever. Doesn't change much, but it does.

  24. #24
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    Quote Originally Posted by Fajita Dave View Post
    No calculations on my part. A while ago I was looking at what rims to buy for my last mtb wheel set I stumbled across an article where someone blew up a carbon mtb rim and blamed the manufacture for defects. He was commuting to work and rode with 40psi with a 30mm rim ID and fat tires. Someone who seemed to know Boyle's Law made the comparison that you quoted in my first post.

    So just like you said more volume creates more pressure at the same PSI. Meaning a wide rim/tire at low pressure will require the same amount of strength as a narrow rim/tire at high pressure. Which wouldn't explain why the OP's comparison is showing mtb rims to be lighter.
    Got ya. Definitely agree with what you're saying in principal.

  25. #25
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    Quote Originally Posted by aclinjury View Post
    I remember November Dave has said that at 100 PSI is when they start to notice that road rims start to expand at the brake tracks. Then when you factor in heat from braking on road rim (rim braking), then the PSI could get over 100 quickly on descending. MTBs have disc so they don't have to deal with the tire possibly blowing out during braking. The other factor that mitigate a tire blowing out on an MTB rim is that MTB rims are wider and thus enable a higher volume for the tire, and this wider volume serve as shock absorber when hitting a pothole. A narrower road rim will have a smaller volume, so when a road wheel hit a pothole (assuming same speed) the impact PSI is necessarily higher. So when we factor in the braking heat and smaller volume, the road rims could experience a higher impact PSI force in some situation, thus their brake tracks have to be made with more material to withstand such impact?
    That all sounds pretty reasonable
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