Bumble Beemer Build Recap

Alex · Post by **Alex** » Fri Nov 30, 2018 11:42 pm

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The following are primarily observations that have no data or expertise to back them up.

Beemer was built to be COTS heavy, no frills, and reliable. We wanted to use as many go-kart intended parts as possible, fearing that lawn mower and hand-cart accessories can’t handle the speed and abuse, and that custom fab would be less reliable or not meet our build schedule.

Rims: We quickly found that good, cheap rims would be the key to unlocking a lot of go kart accessories, hubs, spindles, bearings, etc. Fortunately we found a lot of 27 various split rims for $50 on ebay. They are 5" Van-K type split rims and ranged form 2.5" to 4.5" but most were 3" to 4".

Rear Axle/ Powertrain:

We are powered by a 1800w MY1020 clone and the 1500w 48V china controller. Both have more extensive writeups on this site.

Our rear axle is mounted to the chassis with pillow blocks. The slotted holes of the pillow blocks afforded us adjustment for chain tensioning, although not always enough due to the angle our sprockets aligned.

We used 3/4” axle because it worked for us last year and we could find several hubs and accessories. There is a bit of concern that it is too cantilevered and not strong enough. I would like to upgrade to a bigger shaft diameter. The main issue is finding a brake rotor compatible with it, they exist we just need to commit to it and make sure it doesn’t mess with ground clearance.

We used aluminum go kart hubs from bmi karts. These are compatible with multiple axles, spindles, our rims, and a split sprocket hub that is offered on bmi karts.

Speaking of split sprockets, they allow us to easily change our gear ratio, however, we can only use 53 to 60 tooth sprockets which doesn’t quite get us the range we needed, it seems we always need to go faster.

The rigidity of our gearbox caused us a lot of trouble, I always underestimate the tension that goes on with chain. Not having direct support between the two sprockets caused us to reinforce it multiple times. We are still looking for a solution that gives us enough rigidity but still offers adjustability for the chain. For now this is getting bumped down the priority list. I think we have more performance gains in making driver changes and battery swaps easier.

Before:

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After reinforcement in Orlando:

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We started with a first reduction of #25 chain. After it heat treated itself and ate the aluminum sprocket we changed it to #35.

Slightly eaten sprocket:

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Our first gearing was about 9:1 and 16 mph. This screwed our qualifying time and we bumped it up to 18 mph which still wasn’t fast enough on many tracks. After changing to #35 chain we could gear as fast as 20 mph. I would like our range to be 24 to 18 mph.

A fan helped cool the motor. It ate itself at some point and lost two blades.

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Early stages laying out the chassis and rear axle

Chassis:

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The chassis was 1”x1/16” square tubing, nothing fancy.

The front end has some bends to be a cow pusher. It got destroyed at Detroit, we rebuilt it with better support.

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Rebuilt/reinforced front bumper

Being able to put the chassis on a mill made putting in mounting holes a breeze

Foot pegs. Use them. Seriously.

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The side rails are wood made from wooden deck balusters. These have been very nice. They are attached to the vertical steel tubes by wooden plugs shoved into the tube. When first put in they were tight but have since become much looser. Some sheet metal screws into the side should hold it in place fine. The intention was to facilitate holding down the body with wood screws but the body fit so well these weren’t needed.

Steering:

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The steering on Beemer by some miracle turned out really well. Adlai will tell you it doesn’t turn right well but that’s only a little bit true. It’s pretty easy to drive, no calluses or sore arms after a full weekend racing.

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We started by drilling a series of holes in the chassis so that we could adjust where the axle would go by mounting it with u bolts. With the u bolts we could adjust the caster angle to where we liked before adding a steering reinforcement to lock it in place. We ended up with a very aggressive 20 degree caster angle. It took a few spot welds to get the steering shaft in place. The handle bars are an offset cross pipe welded together from 7/8” OD chromoly tube. We should have welded our own handlebars much sooner, most COTs handlebars are not appropriate for powerwheels use.

Some decent grips are well worth it.

The tie rods are 3/8-24 threaded rods which we chose after seeing the 5/16 threaded rods bending on the 2017 TMNT gecko karts. The Ball joints really help make this feel smooth and go together easily.

Special shout out to Jason and Team Lazy Gecko for getting me involved in PRS and Adlai, the other guy who built this kart with me.