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Thread: 40mm rims?

  1. #1
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    40mm rims?

    When mountain bikes came out, I thought that the wheels were too small; shoulda started with a road bike rim I said. 25 years later it seems like I was on the right side of history. I also thought the long stems were roady leftover, the bars too narrow, suspension wouldn't be exactly difficult - see Motorcycles since 1953 - and the skinny rims were bizarre with those huge 1.95 tires on them.

    Recently the roadies have discovered that wider rims [29er?] and 25mm tires are better riding and handling than the traditional fare; better sidewall support. If you take the rim/tire width ratio of most two wheeled devices you'll find the rim is about 2/3rds the tire width. IOW it fits about halfway down the circle from the sidewall apex.

    Using standard operating practice a theoretical rim width for a 67mm 2.25" tire should be about 44mm inside. Instead, it's about half that. Wuffo? I get the concept that the tire needs some lateral give to deal with being leaned over, but so does a motorcycle tire.

    My guess is that the obsession with rotating mass has overridden a sensible sizing of rims and that we should just cut to the chase and make 45mm [carbon?] rims. Of course, as in all such things, the tires would then need to have their tread designs modified, or could we just use a 2.4 tread on a 2.1 casing?

    Or is there some actual functional reason why rims need to be proportionally skinny? I'm getting older and running out of time for the glacial pace of the obvious to wend its way through the product cycles. I don't mind lacing my own - usually do anyway - but I don't see any rims available near this dimension.

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    Weight is one of the main reasons. The other is that the first double wall MTB rims were simply re-rolled road rims. So there was a bit of a tradition there.

    That said there are more and more wide rims on the market these days. For a reasonable weight rim, the Velocity Blunt 35 is a nice wide 35mm width without being too heavy. Snow Cat rims from 616 Fabrications are 44mm wide and still not a terrible weight but significantly heavier than a XC rim.
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  3. #3
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    For the front yes, I run 25mm internal currently and 29mm which I sold, lower pressure less squirm it's all good, on the rear 29mm internal flattens the tread profile, great for grip but really does make the tire drag, running a cheap 321 currently but generally prefer a 19mm internal not much in it though.

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    Quote Originally Posted by bulerias View Post
    When mountain bikes came out, I thought that the wheels were too small; shoulda started with a road bike rim I said. 25 years later it seems like I was on the right side of history. I also thought the long stems were roady leftover, the bars too narrow, suspension wouldn't be exactly difficult - see Motorcycles since 1953 - and the skinny rims were bizarre with those huge 1.95 tires on them.

    Recently the roadies have discovered that wider rims [29er?] and 25mm tires are better riding and handling than the traditional fare; better sidewall support. If you take the rim/tire width ratio of most two wheeled devices you'll find the rim is about 2/3rds the tire width. IOW it fits about halfway down the circle from the sidewall apex.

    Using standard operating practice a theoretical rim width for a 67mm 2.25" tire should be about 44mm inside. Instead, it's about half that. Wuffo? I get the concept that the tire needs some lateral give to deal with being leaned over, but so does a motorcycle tire.

    My guess is that the obsession with rotating mass has overridden a sensible sizing of rims and that we should just cut to the chase and make 45mm [carbon?] rims. Of course, as in all such things, the tires would then need to have their tread designs modified, or could we just use a 2.4 tread on a 2.1 casing?

    Or is there some actual functional reason why rims need to be proportionally skinny? I'm getting older and running out of time for the glacial pace of the obvious to wend its way through the product cycles. I don't mind lacing my own - usually do anyway - but I don't see any rims available near this dimension.
    2.25 inches is about 57mm, not 67mm. Even then most 2.25" tires (nominal width) have a casing width of ~54mm, and it is the casing / rim width ratio that matters.

    Well, I do like somewhat wider rims, but they can be too wide and that adversely affects the handling. IME the sweetspot for most 2.00-2.40 inch tires (nominal) is around 28mm outside.

    I have used 2.0" tires on 45mm rims and they are a bit odd. 2.4-3.0" are good. And I am liking 3.8-4.7" (again, nominal width) on 65mm rims for trail use. The 36mm rims I have are OK with 2.1-2.2" tires, but the tires pinch flat more easily.
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    It's really pretty amazing how "normal" a bike feels even with extremely wide rims/tires. I've ridden one of those hacked "bike with car wheels" contraptions, and it's balance and cornering were still reasonable and "like a bike." I wonder how far the fat tire trend is going to go.

    Shiggy, what are the specific "adverse affects on handling" you notice? Too hard to pick a precise line between rocks/roots? Too much steering resistance/ground friction making tight turns at slow speed?

    Aerodynamics matter for road bikes, but not mountain bikes really. Maybe we're about to see a major design change as sudden and dramatic as the increase in standard ski widths over the last ~15 years.

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    Sorry, I blew the 57mm width bit; but it still makes the calculation about 38mm. Which is close to the Velocity P35 which is what I'll now consider for my winter wheel project. Thanks to the Whisperer for the lead. It's still only about 30mm internal but way wider than the typical 19mm that come with the bike. . And the 44mm rim would have been about 38mm internal so it was not actually an error for a 2.25" tire at 2:3 ratio - which was just an approximation anyway.

    It's pretty useless to say that a wide rim does X,Y,Z poorly when the tire was designed to have it's beads squished a whole lot closer. My point is that IF we had started with a rim width that is more customary for the tire width we might find it's a 'good thing'. Putting a narrow rim design tire on a wide rim doesn't really give very valid info.

    I can see the rim being more vulnerable and prone to pinching, but the trend seems to be inexorably wider and carbon will keep the weight down, if not the price.

  7. #7
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    Quote Originally Posted by sidewinder View Post
    It's really pretty amazing how "normal" a bike feels even with extremely wide rims/tires. I've ridden one of those hacked "bike with car wheels" contraptions, and it's balance and cornering were still reasonable and "like a bike." I wonder how far the fat tire trend is going to go.

    Shiggy, what are the specific "adverse affects on handling" you notice? Too hard to pick a precise line between rocks/roots? Too much steering resistance/ground friction making tight turns at slow speed?

    Aerodynamics matter for road bikes, but not mountain bikes really. Maybe we're about to see a major design change as sudden and dramatic as the increase in standard ski widths over the last ~15 years.
    Short version: in general, on too wide a tire flattens, making it resistant to leaning and turning and at the same time reducing straight line tracking stability.

    When you do lean the bike the tire can be very "edgy", like falling off a cliff. Also happens on off cambers and ruts.

    I find a "rounder" tire to track better, be more responsive, feel more lively and just handle better in most conditions and terrain.

    I have also used some models of tires that ride like c### on rims narrower than ~28mm (20-22 inside) and fine on 28-32mm rims.

    When I started mtbing 32mm wide rims with 1.75-2.125" (nominal, they were much narrower) tires were the norm. The migration to 28mm then 22-23mm happened for more reasons than just weight loss.
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