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  1. #1
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    School me on tires and rims... please

    I kind of lost touch with mountain biking for the last few years, and am now thinking of buying a new bike in a few months. To this end, I've been thinking about rim and tire choices. It seems that so much have changed in the past 3-4 years, compared to the previous twenty! Carbon rims, 650B, one by systems...

    Can someone explain to me what difference a rounded versus square tire profile makes, and how this would be affected by rim width? Would rim width affect bike handling, and if so, how?

    It seems to me that tires tend to be run at much lower pressures than back in the day (due to the popularity of tubeless systems), which then raises the possibility of a tire rolling off a rim during cornering. This in turn (ha) has led to wider rims, to address proper tire retention. Is this right?

  2. #2
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    Quote Originally Posted by joeadnan View Post
    Can someone explain to me what difference a rounded versus square tire profile makes, and how this would be affected by rim width? Would rim width affect bike handling, and if so, how?
    wider rims tend to square off tires. how much depends on the tire/rim combo.

    It seems to me that tires tend to be run at much lower pressures than back in the day (due to the popularity of tubeless systems), which then raises the possibility of a tire rolling off a rim during cornering. This in turn (ha) has led to wider rims, to address proper tire retention. Is this right?
    NO. tubeless tires don't roll off the rim in corners. you can burp a tubeless tire if you hit something too hard, but tires don't just fall off rims. your more likely to bend a rim or slash a tire than have the thing fall off because you went around a corner.

    the benefit of wider rims is more volume, lower pressure, more grip, more stable sidewalls, stiffer wheel (depending, these are generalizations) "squared off" tires sometimes makes a difference on certain tires because it helps the shoulder knobs dig in better at extreme lean angles. it's not one of the major factors IMO unless your having trouble with certain tires in certain conditions.

    wide rims and tubeless setup are awesome. just buy the appropriate rim for your weight, riding style, and tire you plan to use. ex. 140 lb XC racer on 2.1's doesn't need a 40mm DH rim. likewise, 250 pounder isn't going to be happy on a 21mm light-ish rim with 2.4's and aggressive knobs. (wide, aggressive tire on narrow rim under a heavier rider will roll over in corners and the weak rim will be too flexible.)

    general trail riding for most people running 2.2-2.4 tires are using 25-32mm inner width rims.
    Rigid SS 29er
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  3. #3
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    Thanks very much! The last wheels I built for myself were ZTR Olympics. I'll try out some wider rims soon to see if I can discern any difference in grip and handling. I can definitely tell the difference in handling and grip with larger volume tires running lower pressures, but I am not sure if I am sensitive enough to tell the difference between different profiles.

    I suppose the other variable is the stiffness of a rim: the wider rim would tend to be stiffer, and would flex less with high lateral loads, improving handling in corners.


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  4. #4
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    Wide rims make a big difference in how the bike feels. Many people like it. Many don't. Many have lots of issues with huge rims and relatively small tires. Many don't.

    It's by no means universally better. As a rough rule, wide rims work better if you want to use wide tires. Results with normal tires vary greatly.

  5. #5
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    Quote Originally Posted by *OneSpeed* View Post
    NO. tubeless tires don't roll off the rim in corners. you can burp a tubeless tire if you hit something too hard, but tires don't just fall off rims. your more likely to bend a rim or slash a tire than have the thing fall off because you went around a corner.
    Actually, they do. If you regularly use deep ruts as berms in high speed corners it's only a matter of time before you mess up an entry and rip a tire clean off the rim. It can also when you stuff a bike hard into catch berms.

    As for tire pressure, the main reason we're running lower pressures these days is because 1.9" tires are a thing of the past, almost everyone these days runs at least 2.2"-2.4" tires on their bikes. Tubeless put an end to pinch flats which then resulted in a dick-waving contest on who could run the lowest air pressure in their tires, which then resulted in wider rims, revised rim profiles to reduce dents from rim strikes and improve tire retention, and even larger tires & wider rims.

    With regards to tire profile, rim widths, and tire pressure, this gets really messy since different rider styles, preferences, and terrain will put things all over the place. For instance, there's a set of trails about 45 minutes from me where it's all low speed rock crawling over endless rocks which are often wet & slimy. Tires of choice there are large volume, rounded profile, run at the lowest possible pressure on wide rims for maximum tire conformance & traction. Fat bikes are quite popular there for that reason.

    Up at my friend's cottage it's high speed downhills with rutted corners, jumps into berms, and roots & rocks to keep things fun. Usual tires there are DHFs on about 25mm wide rims at a fairly high pressure, they give the best mix of strength, cornering grip, and predictability when smashing out the downhill sections while still being light enough to pedal uphill and cover ground on flatter sections. Speeds, impacts, and cornering loads are quite high so a lot more tire pressure is needed to both keep the tire on the rim and keep the rims from getting bashed to pieces on the rocks.

    As I said, it gets messy, and there is no hard & fast rule. I think the tires I'm using right now work best on a 24-25mm wide rim, there's others who think 28-30mm is the sweet spot. If it works for them and that's what they like, who am I to argue? I probably don't ride the same way on the same terrain, and even if I did I may not have the same preferences & perceptions.

  6. #6
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    Quote Originally Posted by aerius View Post
    Actually, they do. If you regularly use deep ruts as berms in high speed corners it's only a matter of time before you mess up an entry and rip a tire clean off the rim. It can also when you stuff a bike hard into catch berms.

    As for tire pressure, the main reason we're running lower pressures these days is because 1.9" tires are a thing of the past, almost everyone these days runs at least 2.2"-2.4" tires on their bikes. Tubeless put an end to pinch flats which then resulted in a dick-waving contest on who could run the lowest air pressure in their tires, which then resulted in wider rims, revised rim profiles to reduce dents from rim strikes and improve tire retention, and even larger tires & wider rims.

    With regards to tire profile, rim widths, and tire pressure, this gets really messy since different rider styles, preferences, and terrain will put things all over the place. For instance, there's a set of trails about 45 minutes from me where it's all low speed rock crawling over endless rocks which are often wet & slimy. Tires of choice there are large volume, rounded profile, run at the lowest possible pressure on wide rims for maximum tire conformance & traction. Fat bikes are quite popular there for that reason.

    Up at my friend's cottage it's high speed downhills with rutted corners, jumps into berms, and roots & rocks to keep things fun. Usual tires there are DHFs on about 25mm wide rims at a fairly high pressure, they give the best mix of strength, cornering grip, and predictability when smashing out the downhill sections while still being light enough to pedal uphill and cover ground on flatter sections. Speeds, impacts, and cornering loads are quite high so a lot more tire pressure is needed to both keep the tire on the rim and keep the rims from getting bashed to pieces on the rocks.

    As I said, it gets messy, and there is no hard & fast rule. I think the tires I'm using right now work best on a 24-25mm wide rim, there's others who think 28-30mm is the sweet spot. If it works for them and that's what they like, who am I to argue? I probably don't ride the same way on the same terrain, and even if I did I may not have the same preferences & perceptions.
    Thanks for the alternative perspective. As part of "catching up" with the state of the play in the Mtb world, I watched with fascination the videos from the Enduro World Series. These guys are really ripping it up. There was at least one occasion where a rider had a tire roll off the rim. Of course, these guys are really pushing the limits of their equipment in a way that I would not be able to, at least not without risking life and limb.

    As an interesting aside, part of the attraction of the EWS to me is that it replicates the same constraints that we as amateurs face on trails everyday: long climbs and technical descents. Hence it's easy to relate and aspire to that type of riding. The bikes they use would also be the kind that we would, or would like to, use. Efficient climbers that are equally adept at descending.

    Pure downhill racing may be interesting as a spectacle, but very difficult to relate to, as the downhill bikes are built only for its specific task of going downhill fast. I can't see myself using one on the local trails, and to buy one just for downhilling would be an extravagance for most.


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  7. #7
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    Yeah, like he said - aerius
    Last edited by Miker J; 11-09-2016 at 06:55 AM.

  8. #8
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    The 29er fully I bought a while back came with rims that were 19mm wide. The bike had 2" tires. It was certainly better than the 26er I had been riding but it was not spectacular.
    Since I was breaking spokes on the stock wheels, I decided to try something else. I built a pair of wheels with 30mm rims and double butted spokes. I also went to 2.2 and 2.3 inch tires. Huge improvement. Then, I built a pair of wheels with 35mm rims and used the same tires front and rear from the 30mm rims. This turned out to be exactly what I was expecting from the bike back when I got it.
    I'm a big boy so 35mm may be too much rim for some people. Maybe if I rode more cross country style stuff I'd stick with the 30mm rims but 35 suits my needs perfectly.
    I like turtles

  9. #9
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    Where'd op go?

    I wanna know what happened!

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  10. #10
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    Still here, and appreciative of the responses received.

    The technology and standards have certainly moved on since I last built a wheel for myself. One bike has ZTR Olympics on it, which would be viewed as narrow cross country rims these days. My rim-braked hardtail has a pair of Mavic 517/717 ceramic, which in my view are the best rim brake rims ever made.

    This discussion has gotten me thinking about getting a wider rim for a new build that I am planning, possibly in the 30mm region. I like the idea of asymmetric rims, to balance the tension on the spokes. The ZTR Olympics were built with a Hope singlespeed hub, running the 6 largest cogs from a 9-speed cassette. This makes the wheel dishless. It's strong and durable, despite the lightweight rim.

    So the next question is the choice of rim. Carbon or aluminium? My preference is to try an asymmetric rim, as explained.

    Any recommendations?

    I looked at some of the carbon rims, and balked at the asking price for some of them. The Chinese branded carbon rims are less extravagantly priced, but are they just as good?

    The other consideration is whether I should stick to aluminium for now, just to feel how the wider rims feel when shod with wide tyres. If I changed to carbon, it would be difficult to know whether any perceived change in handling was due to the stiffness of the carbon, or simply due to the wider rims.

    I've been looking at asymmetric rims and saw that WTB makes some in the desired width. Anybody has any other recommendations?

    Current bikes are a Maverick Durance from 2009 (which is shared with my wife), an 18-year old Seven Sola with BTCs, and a dirt drop equipped 1994 Obed. I love the challenge of technical trails but I have a cross country oriented background. 70kg dry weight.


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  11. #11
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    Just an update to this thread. Some photos to start with.



    I ordered a pair of Lightbicycle carbon rims. Asymmetric with 28mm internal width.





    The rims were matched with a pair of Tune King/Kong boost hubs.


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  12. #12
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    Both hubs are 28h. Laced cross three with Sapim Super CX-Ray bladed spokes.








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  13. #13
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    I had to get a new truing stand, as my 20+ year old Minoura couldn’t handle 15mm axles.





    Cat was not impressed.


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  14. #14
    turtles make me hot
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    They never are.

    Nice wheels and nice stand.
    I like turtles

  15. #15
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    I definitely feel the difference between the carbon rims and the stock aluminium ones, which came as a big surprise to me. I changed the tyres from
    Ardents to DHFs, which I probably ought not have done if the object of the exercise was to feel the difference between the narrower Al rims vs the carbon ones. The ride is perceptibly stiffer, which leads me to think that I can drop a psi or two from my previous setting.


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