Go Kart Ideas - Would Love Feedback!

[mikegrundvig]

Hi all; I've never made a go kart before but I have a good bit of mechanical/electrical experience and am very interesting in building one over the next few months. I was exhibiting at the Maker Faire in Kansas City when I saw these guys race:
http://powerracingseries.org/
I watched this in person and I'm hooked. I'd been planing on building a kart for a while but this has given my project focus and direction. The rules are simple:

1) Must have a Power Wheels body
2) Must be electric and use no more than 36 volts
3) Must cost less than $500 not counting safety equipment (brakes/kill switch, etc. are considered safety related)

So those are the basic requirements. I own a good-sized CNC milling machine and really want to make this thing awesome. This is a great learning experience for me and so I want it to be a bit over-the-top design wise. I figure it will take me a good long time to design and make.

The motor and speed controller I'm looking to use is this pair:
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=5142
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8920
That motor is a 6500 watt monster while still tiny. At 36 volts, it'd be doing ~4600 no load RPM so obviously some gearing is required. There are people using this style of outrunner motors on bicycles and producing incredible performance (crazy acceleration and insane top speed). The electronics of this are not the problem - I'm quite familiar with how to get all that working just fine.

I want the cart to have four wheel independent suspension and I'd love for it to be 4-wheel drive because that'd be a fun challenge but for the moment, I'm working on the assumption it will be rear-wheel drive only. I'm thinking it should use a differential combined with a pair of universal joints on each side.

I guess I'll stop here. Before a single piece of metal is purchased, cut, or welded, I'm going to have it drawn in CAD. I know this irritates some of you who just wing it but my projects have been far more successful when I engineer them out first.

With all that said, I'm interested in any feedback and ideas people have as you are all experts and I'm the new guy. Thanks!

-Mike

 

[DCProductions]

This sounds like a cool project, but I have some concerns about the scope of it. I don't really know what kind of race this is, but I doubt the kart needs a suspension. The pictures on the racing site seemed to suggest that it was an onroad only race; in that case you should try to mimic a racing kart frame. Using some of the $500 budget toward quality batteries, tires, or other performance mods would be a better investment, in my opinion, than 4wd or suspension components (though a F1 suspension would be killer!). Just an opinion so far, and I'm no expert.

Excellent motor choice by the way! How many axes is your CNC? Custom gearbox would be cool, but I'm thinking chain and sprocket would be cheaper.

 

[number 37]

You'd be surprised at how strong some of these are in factory form.

We pulled the motors and had a soap box styled race (with bumping allowed) down our 350' asphalt driveway last New Years Eve. Started with 5 cars, ended with two. The damaged ones were the cheaper barbie type cars.

We got all our donor cars from CraigsList for free, and my Cadillac X-calade (lol) was the overall winner. Was tons of fun and slightly dangerous as my drive is S curved and has a couple fairly steep sections. We ended up across the road and 40' up the neighbors driveway when we made it all the way to the bottom.


In your project, I'd look for a "big" model, and actually have it in hand prior to ordering anything. You may be able to save a lot of $ by not upgrading things that don't need upgraded

 

[mikegrundvig]

Thanks for the comments!

Excellent motor choice by the way!

The people racing these are doing <=20 mph tops so my goal is high torque and a top end of 23 mph or so. They are using at best the MYXXXX line of scooter motors (topping out at about 1000-1200 watts) and moving on down from there into less ideal motors. I know these RC motors work and are absolute monsters if you can gear them properly. They can also be very efficient if you are careful.

How many axes is your CNC? Custom gearbox would be cool, but I'm thinking chain and sprocket would be cheaper.

My CNC is four axis with a working envelope of 24x by 15y by 20z. Certainly big enough to machine most anything this project will need. To save money, I'm likely going to machine the gears for the differential using the http://www.gearotic.com program. This will let me machine bevel and spur gears using my mill no problem. I've also already started the design for some DIY milled U-Joints - found some easy ways to do it and that will save a ton of money.

though a F1 suspension would be killer!

Agreed and that is what I want to produce, I just think it will be a fun design and manufacturing project. This site has been incredibly helpful in me getting a good understanding of what it will take to do:
http://www.serpent.com/product/300000

Thanks!

-Mike

 

[mikegrundvig]

After some research, it looks like I'm going to go with a "spur gear differential" - they appear both simpler and cheaper to make than a bevel-gear design. Below is a link to a version someone else already made. I quite like the simplicity and design. It's very machinable for sure which is a big plus. Here is a post with lots of pics of it:
http://scolton.blogspot.com/2010/07/cap-kart-summer-rebuild-3.html

While researching that project, I ran into this nifty trick too! Never heard of spur gear stock. Slick stuff, looks like this raw:

and it's sold by the foot. It looks like this when you make a gear with it (not my work - just a photo from the differential above):

One thing down - about 1000 to go

-Mike

 

[number 37]

that spur gear stock is PIMP.

please do a step by step when you build your diff that one in the blog looks awesome