Monday, December 5, 2016

Wheels Part 2: Why So Heavy?

The first section on wheels was dedicated to the Honda CT90 trail bike moped wheel. The truth is that the CT90 wheel is no longer easily discovered, its slightly rare these days. And the ones available at a scrap yard are typically of scrapyard level quality. If you are lucky you might find a set for under $200 that aren't totally rusted, bent or damaged, but the weight is still a problem. And there is no easy solution to adding a 'key-way groove' to the hub - it was never designed or intended to be keyed.

CT90 wheel does have some advantages though, including shapes intended for braking surfaces, should you choose to add a brake at any point the mech is already there. And a well kept front hub includes an odometer gear that can actually be used to measure speed if you find an odometer cable long enough. Also the rear hub cush-drive is a reasonable answer to chain chop and torque steer.

But there are some glaring problems that lead me away from this wheel. This section is really about what happens after you realize that the CT90 wheel is not for you - it is most certainly not for me.

CT90 Wheels are:
  • WAY too heavy at 27 lbs with wheel and tire EACH for a total of 108 lbs.
  • Hard to make SAE bearings fit with an Azusa spindle accurately
  • Difficult to attach keyway hubs, or machine a key-way
  • More rare these days - unless you live near a giant motorcycle graveyard
  • Almost the same dimension as a BMX wheel and tire...or even a buggy...

Moving on.

A few of our key group members brought up the point that Mopeds in history used 'mag' wheels, or cast alloy wheels. I researched this and found some nice examples like this Puch 'Snowflake':


One problem with this type of wheel (from an Italian Moped) is that they are not cheap. Basically for a pair (front/rear) of these it cost around $320 on myronsmopeds.com. They look very cool, and would work perfectly but are simply too pricey for this type of thing, and they only work with 12mm axles.

This is a problem. At 12mm this axle will not support the weight of turning given the overall weight of our vehicle. If we are able to make a lighter chassis, and shorten the length of the spindle it may be possible in the future - but I have yet to see a 12mm axle with the strength required for the types of forces involved in a Cycle Kart.

That being said, I believe there is a solution out there that employs the use of a machined hub to replace the center section of the Puch Snowflake. This is an advanced theory, so I will just mention it here briefly and not go into detail.

In summary the Puch Snowflake 'looks cool' and 'fits the bill' but is not a good answer. -CW


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