Dad vs The Soapbox Derby Wheel

I’ve been keeping a collection of emails sent by my Dad, as he works in his shop to get a disk brake attached to our (small) wheels. I think it’s pretty amazing that he’s got the energy and persistence (at 85!) to keep working away at a problem until he solves it.

Two days ago:

I have been chasing my tail for several days trying to get the brake discs to rotate without ‘runout’ when installed with the wheel on my support brackets. I have gotten as close as +/- .005″; not good enough.

Each time I lapped the face the disc bolts to, and reinstalled the disc, I got different results. I tried adding shims to the bearing outer races – no luck.

Finally, tonight I removed one of the spacers from between the bearings and measured the length at 4 points with a micrometer. 1.0303, 1.0302, 1.0295, 1.0288. That calculates to an angle of 0.1836 degrees.

Now I wonder how to true up the sleeve. I don’t know which end is off. And I don’t know if my lathe and chuck are accurate enough. I can also chuck it in my milling chuck. If I grind off a little I will need to add a hard stainless shim type washer to compensate and keep the length to 1.030. I have shim stock in various thicknesses.

Yesterday was:

1) I checked out the lathe; it is pretty accurate.
2) I trued up the sleeves by chucking in the lathe and grinding with my Dremel. One end was off on each of them.
3) Brake disc run-out, and the wheel still wobbles!!

And today, victory!

Today it finally dawned on me that the top hat we made to mount the brake disc to the wheel really forms a backbone for both. The disc is pretty flexible; the wheels are made from fiberglass reinforced plastic which has a low modulus of elasticity. In between is the rigid metal top hat.

By adjusting the bolts to the wheel, I have the one wheel running less than +/- .002 instead of +/- .020. Now I am working on the face of the hat against the disc. If need be I can shim and get it accurate. The gap in the brake clamp is .090 with a little drag on the feeler gauge. The disc is .070 thick, leaving less than .020 total.

I’m glad he’s on our team :)

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2 Responses to Dad vs The Soapbox Derby Wheel

  1. Glenn says:

    where did he get the wheel and what kind of bearing is in it? Trying to make my son’s cart faster and cannot use official soapbox wheels for this race.
    Thanks
    Glenn

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