Asymmetric rims...why?

Iain C
Iain C Posts: 464
edited September 2008 in MTB workshop & tech
It's only today when I ordered my "asymmetric" tubeless rim tape and had a look at my front wheel that I noticed that it is actually asymmetric in cross section. I was very surprised, I usually notice stuff like that and I'd missed it, with a lot of riding, wheels on and off putting it into cars, changing disc rotors and flipping loads of punctures!

Why are they asymmetric then? I'm guessing it's to compensate for rotor clearance and give you a rim that's centred without having to put in too much "dish" on the spokes?

Are all rims like that?

Comments

  • nicklouse
    nicklouse Posts: 50,675
    the idea is even spoke tension by using the same length spokes.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • Iain C
    Iain C Posts: 464
    Hmmm, but if I look at my front wheel it does have dish built into it, would the "flat" side spokes therefore not have to be shorter by default?
  • nicklouse
    nicklouse Posts: 50,675
    but imagine what it would be like with the spokes closer to the disc side.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • nicklouse
    nicklouse Posts: 50,675
    from Bonties pages

    bontrager_478901ab89caa_280.jpg
    Offset Spoke Bed

    If you look at the spoke bed of a low-profile Bontrager rear road wheel (like the Race X Lite or Race Lite), you'll notice that the spokes don't join the rim at the center. Rather, they're shifted slightly toward the non-drive side. Bontrager calls this design feature OSB, or Offset Spoke Bed. And here's why we do it.

    Over the years drivetrain manufacturers have added more and more gears to rear cassettes, first seven speeds, then eight, then nine, now ten. To accommodate the wider cassette body that came with more gears, most wheel builders simply shoved the spokes in toward the center, shortening the spokes and increasing spoke tension on the driveside of the wheel to add what is known as "dish" to the wheel. Problem is, adding dish reduces the bracing angle of the driveside spokes, which in turn reduces lateral stiffness and weakens the wheel against lateral loads. The spoke tension is way up on the driveside, and way down on the non-driveside. That's a recipe for durability issues.

    To avoid the pitfalls associated with traditional, center-drilled designs trying to accommodate expanding cassette dimensions, Bontrager uses OSB, which preserves spoke bracing angles and makes a much strong, stiffer wheel. Thanks to OSB, which comes with no additional weight penalty, spokes remain evenly tensioned on both sides, for greater longevity. OSB's an expensive feature. It basically doubles the amount of engineering we spend on every wheelset we make. Instead of sharing a single extrusion for the front and the rear of each wheelset, Bontrager invests in separate designs and extrusion for one simple reason: it makes for stronger, longer lasting wheels.

    http://bontrager.com/technology/
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • Iain C
    Iain C Posts: 464
    Right, I geddit. Picture is worth a thousand words and all that (although picture and words together make even more sense!) That example is talking about rear road wheels and I'm looking at a front MTB wheel but I guess the reasoning is the same and we're substituting a casette for a disc.

    Thanks for posting and explaining, that, I appreciate it! :)