Demystifying The OS20 Wheel Size

by Paul Knox.

OS20 is an oversized pro size 20″ rim and tyre combo that’s close to the 22″ wheel size available to our freestyling counterparts. The OS20 standard was co-created by Tioga and Alienation for UCI World Cup time trial specialist and 2008 Olympic Silver medallist, Mike Day, specifically for BMX racing. OS20 Wheels have been around for a while now (*see the sidebar below for a quick explanation on OS20), but there still seems to be a bit of a mystery surrounding them. OS20 has been slow on the uptake, there’s no doubt, and it seems to be mostly older racers that have adopted them, looking for more stability from their ride.

One of the most common questions I get asked is “do you need an OS20 specific frame, or can you run a regular 20” current frame”? There are many debates on the answer to this question; there is a variety of OS20 specific Frames & Forks available, but they may not suit everybody; conversely, some want to keep the frame they have out of either preference or maybe necessity.

The general summing up pretty goes like this – an OS20 specific frame and fork gives you the same bottom bracket height and rear end length as (an average) 20” with a brake mount position to match the bigger wheel; a conversion of (an average) 20” results in a higher BB (because of the bigger wheel) and either your back wheel stretched out on the dropout to make the brakes line up or needing brake post extenders.

Rather than enter a treatise on Pros and Cons of either choice, here’s a primer for anybody considering a conversion:

I’ve done a few OS20 conversions, and here’s what I’ve found worked for me personally:

  • Answer Dagger forks can run an OS20 1.85 with no clearance issues, and match up with 20” geometry. Yess have an alloy 20” Pro fork that’s notched to clear an OS1.85, and there are one or two CroMo forks that will just clear too. I use the Answer for all my conversions -they’re a great match for any frame.
  • I’ve run the same Alienation OS20 rims forever. Lace them up to your choice of hubs and you’re all good. They are bulletproof.
  • There’s a gearing calculator on ? for Tioga Powerblock OS20 1.6 & 1.85 tyres to sort out your gearing. 43/16 is good for me, but my local Aussie start hills seem to be on average a bit taller/steeper than US gates (that I see)?
  • I run the same height bars on my OS as I did on my old 20’s. No need to change the height like on a cruiser – your BB to bar dimension isn’t changed.
  • I prefer to run a long back end to match my cruiser at 15.25”. I tried running shorter, but for me the OS20 feels better run long. For example, on my Chase (RSP2), this equated to the axle right at the end of the dropout. Like the very end. I had no issues at all, but you can take a single link out of the chain and use a single 1/2 link (I’ve had to do this an another frame) if you want to get a bit more bite on the dropouts
  • With the back end stretched out, I didn’t need any brake adapters on any of my conversions for my brakes, Pryme V2s.
  • Bottom Bracket height on the Chase after conversion was around 12 3/8”. Personally, I had no issues that. I got out the gate OK, and it felt great for jumping.

As a point of reference, I have also been running a Chase OS20 to compare with my RSP2.0 conversion. Here’s a photo to compare; as you can see, visually it’s not a great difference! Unfortunately, with all the tracks (still) being closed, I haven’t been able to check any technical performance difference between the 2 (gate times, track times, etc), but for me personally I think these will be negligible. For me, thus far, it’s been what do I prefer- taller or lower BB height?

*What’s an OS20? Basically, it’s the bigger wheel & tyre combo than a standard 20 x2.125 that is still “20 inch” within the UCI guidelines. The OS20 rims are 451 diameter (a regular 20” is 406), which is the same diameter as a 20 1 3/8” rim. There are only a couple of tyre options currently available, Tioga Powerblocks (in 1.6 or 1.85) or Vee Speedboosters (1.6 or 1.75).


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