A-Track musings

The one feature of the early (’93 and before) ATKs that I have the most misgivings about is the “A-Track” system. For those of you not familiar with this, it’s basically two idler sprockets positioned near the swingarm pivot, with the purpose of keeping constant tension on the drive chain at all times. The goal being to allow the rear suspension to move as it should, totally uneffected by the forces imparted by the powertrain (no squat under acceleration or rise during braking). Sounds like a brilliant idea, right? Well, yes it is a great idea. However, the execution of that idea is what worries me. 

Early idler sprockets rolled on a single 6002 ball bearing and were by all accounts rather short lived. The redesigned idlers roll on a pair of 6202 ball bearings and from what I’ve heard last longer. Trouble is, nobody seems to be able to tell me how long. It’s one thing if you’re a racer and only run a couple of motos a weekend before the bike gets torn down for service. It’s quite another if the bike is used as I intend to – long dual-sport trips into remote areas. A shagged idler sprocket bearing could be a serious issue out in the middle of nowhere. I plan to carry a complete spare idler sprocket with me whereever I go…

New ones cost just under $100 complete with bearings and hardware. It seems to be a proprietary design, made specifically for ATK back then and American Dirt Bike now. Fortunately, I’ve bought a couple used ones – one good double bearing type and a pair of salvageable single bearing type. My plan is to install new bearings in the newer type and attempt to convert the older type to use two 6002 bearings, hopefully making them more durable. I’ll use the new type on the top position (since it will be under the highest loads) and the modified older type on the bottom.

The revised, double bearing idler sprocket is at top and the older single bearing type at the bottom. The triangular plates to the right hold the idlers in position and the round objects at their ends are to adjust the idler position up and down.

The revised, double bearing idler sprocket is at top and the older single bearing type at the bottom. The triangular plates to the right hold the idlers in position and the round objects at their ends are to adjust the idler position up and down.

Worst case scenario, I’ll remove the whole A-Track system from the swingarm (mounting tabs and all) and make it like a “normal” bike with plastic chain sliders and such.

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2 Responses to A-Track musings

  1. Dale says:

    Removing the A-Track may not be a bad idea (even though I never had any problem with mine, but it was fairly new and on the much lower power 350). There are many off the shelf chain tensioners or guides that can do what you need as you know. Guess it depends on how close to original you want it to be.

    • rotax504 says:

      I’ll run it for a while first and see how durable it is (or isn’t). If I see that it’s going to eat bearings and rollers, driving me to the poorhouse, then an A-Trackectomy will happen. I won’t really need off the shelf tensioners or guides – McMaster-Carr has suitable plastics to make my own out of.

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