A Sander Called Bruce: Lessons Learned


Yeah, I know this seemed like kind of a silly thing, but I actually did learn a bunch of stuff.  Here’s some of it.

First, my hack brazing held up remarkably well.  I am using the Bernz-O-Matic brazing rig I talked about over here, and the steel is pretty standard square tubing from Home Depot, and I cleaned it up pretty good.  I also clamped it properly, and did one thing – made sure that both surfaces being joined were red-hot before putting the rod in there – which made a huge difference.  I also pre-heated the parts with a mapp gas torch to save oxygen on the brazing torch.  In spite of all the crashes, I still have good brazes.

On the physics of drag racing, one basic thing, and that’s that you are transferring torque to the ground.  Once the wheels spin, you’re going to get essentially zero torque being transmitted.  That’s a basic law of drag racing that is so basic people think you’re pulling their leg if you even ask.  What you want is for the thing to grab, lift on to the wheels, and then settle back down as the car picks up speed.  If it hits the wheelie bars too hard, you’re wasting traction.  If it doesn’t lift, you either don’t have enough torque or you’re not getting traction.

Tire pressure controls the size of the patch, and consequently the friction, but also a rear suspension (springs, basically) allow the wheels to stay planted and avoid chattering, giving you more traction.  Also, the conventional wisdom is that 60grit paper is ideal, rather than the 36grit I’m running.  That is something that’s pretty easy to test, and I’ll be setting that up soon, but with my wheel configuration, not on a conventional belt sander – which I suspect has some effect on the grit/traction equation as well.  The wheelie bars also want to have springs, so it’s more of a gentle transition on to them and off.

The motor is clearly overkill.  At 10kW and 23lbs, it spins the wheels to maximum RPM in about 0.06 seconds.  That’s about as close to instantaneous as imaginable.  The trick now will be to dial the size of the motor down to the smallest, lightest possible that will still have enough torque to spin up the wheels and enough power to pull it at about 2500RPM (at the wheels).  The lighter the motor, the easier job it has to do everything, but the less the total power.

The other part of that motor equation is the overall weight of the machine.  At 60lbs, it’s simply not safe to run.  At half that, if it could go just as fast, it will be twice as scary…  but safer.

Part of the secret sauce for next year will be an inductor/capacitor setup to ramp up the voltage.  Here’s what that would do:


Essentially that will get the thing moving slightly before ramping up the voltage to the maximum.  Think driving on ice.  You feather the throttle, and as soon as it spins, you’re lost, right?  Same thing happening here.  I actually got a better launch once I turned the voltage down to 36V from 50V.  At 50V I couldn’t keep belts on it anyway – it’s spinning the wheels at about 3600RPM and the belts just blew apart.

I’d also, finally cut the pack down to two 4s packs.  Right now I’m running 4, so they’re in parallel and then in series to give me 36V.  I just don’t need the extra two.  Simplify and lighten.



2 responses to “A Sander Called Bruce: Lessons Learned

  1. Actually, torque can and will be transferred during wheel/belt spin. It all depend on the dynamic coefficient of friction. That’s why sandpaper works, lol.

    When are you going to join NEDRA and take your bike to the track? It’s time to jump in Buddy. You think this is addictive?


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