Sunday, December 4, 2016

Wheels Part 1: Honda Trail 90

Honda CT-90
This is the first post in a long line of posts dedicated to wheels. If you are following the Stevenson formula, it says to use 4 Honda CT-90 front wheels, and tire of choice. The CT-90 was a moped/pit-bike/trail bike from the 60's and was prevalent in asia/pacific and the western states until the mid 80's.

To find these you simply look on ebay for a set, or scavenge them from a local motorcycle graveyard. They will come in various states of distress depending on how they were stored. Sometimes the rubbers are still intact, but I recommend getting newer rubbers with an all-terrain compound, and the fewest steel belts possible. I will talk about tires later. For now any 17" x 2" tire will work.

Honda CT-90 wheels have the following dimensions:

  1. 1.4" width (35.5mm)
  2. 17" outside diameter (432mm)
  3. 0.590551 inch axle (15mm)
  4. 6002-2RS bearings (15 x 32 x 9)
Because the axle is metric (15mm) and the bearings are also metric (6002-2RS) it will cause problems if you try and use go-kart spindles. The 'hardware' and 'running gear' description of the Stevenson formula is to use 'Azusa' parts when possible. The issue doing that is the Azusa spindles are SAE (standard) 5/8" axle.

Azusa Spindles have the following dimensions:

  1. 5/8"-18 axle
  2. 6" length
  3. 5/8" kingpin (thrust bolt)
  4. 1623-2RS bearings (5/8" x 1-3/8" x 7/16")
  5. Between the CT90 wheels and the Azusa spindles the size difference is = -.875mm, or -.0344".

    I dont recommend shimming, taping, or sizing here. The only reasonable option is this:

    If you replace the 5/8" axle with a metric 15mm 'thrust bolt' from a pit-bike, it will be dimensional again. It takes some welding and fixturing, I recommend a Lincoln Square-Wave TIG ($1800.00 USD) or a professional helping out.

    There are also several ways to make a standard bearing fit inside the CT90 hub, but none of them are pleasant. I don't recommend self-drilling the aluminum hub for the larger OD of the 1-3/8" bearings. Nor do I recommend 'aluminum taping' of either the ID of the axle or the OD of the bearing. Things tend to wiggle when driving, and wiggle translates to broken welds and trips to the emergency room.

    The other problem is weight. the CT90 wheel is roughly 10 lbs by itself, and after applying a tube and rubbers, it ends up weighing 25-30 lbs (depending on the steel in the tire) which adds up quick. 4 wheels and tires can weigh as much as 100 lbs, which is basically 1/2 of your weight on the cart. Add to that the 'unsprung weight' of a 25lb wheel, and the inherent twitchyness of a hand-built chassis, and it spells NO FUN, at least to me.

    Here's a detail image that shows where the chassis is weak. I was forced to consider adding weight here via zinc plated flat-stock from a local hardware store. This stuff is flexible and light, and is easily attached to the wood with thru bolts and wood screws. Once fixed in position it stiffens even the most flimsy construction.

    The area shown in the picture (green stripe) is the area that was weak. After adding the metal panels to this area the chassis is very robust. It became clear to me that having a straight section here would not offer any real benefit other than keeping the axle bearings square to the wheels - which again is not necessary. For the purposes of style the back end tapers. There is no real engineering or vehicle dynamic penalty for doing this, other than adding a slight amount of flex to the axles on the rear end - an inherent benefit for driving dynamics and ride quality. -CW

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