Building a Go Kart - geometry questions
 

So I started a Go Kart project w/my kids (ages 5&7) this weekend. I bought a kit that includes all the parts except the frame and engine. It included the wheels, steering linkage, rear axle, brake etc... and we're building the frame ourselves. We made some good progress this weekend, starting from scratch, we now have a roller! We're excited to say the least.

I'm kind of winging the design and frame dimensions based on photos of old karts and a few dimensioned drawing I have from other kit karts and instructions.

We've been pushing each other around the driveway and I noticed a few things right away. First, it pushes (understeers) really badly. Second, the front wheels don't quite align like they seem like they should when turning - ie: the inside wheel is turning a shorter radius, but isn't lined up to do so (ackerman angle?). I think I might need to shorten the wheelbase too to allow it to turn in a smaller radius.

The kart has a live rear axle - ie: basically a locked diff where both wheels turn at the same rate all the time. I'm wondering if some of the handling issues are due to the live axle and will be cured when we put some power to the rear wheels?

I know only enough to be dangerous about this type of geometry! But I do own a welder, grinder, saw etc.... so I can make changes if they will help down the road. Can anyone offer some help?

Thanks,
Tom

Here's some pics of the current status:

http://forums.pelicanparts.com/uploa...1285036525.jpg

http://forums.pelicanparts.com/uploa...1285036541.jpg

http://forums.pelicanparts.com/uploa...1285036554.jpg

http://forums.pelicanparts.com/uploa...1285036565.jpg

http://forums.pelicanparts.com/uploa...1285036579.jpg

http://forums.pelicanparts.com/uploa...1285036591.jpg

It looks to me like the steering linkage from the center pivot needs to be in a straight line with the tie rod ends.
I would think the tie rod ends should have a straight shot with the steering linkage and also be at the same hight.
I`m no expert but it looks like by pulling one wheel at one angle and pushing the other at a different angle may let the wheels turn at a different rate.

I could get intensely technical with this, or I could just boil it down to something simple. We will try simple first:

Usually the steer links are near-straight, and the outboard points are further inboard than located on your kit. The inboard points are usually not one on top each other, but instead on a tree, apart from each other. Look up any racing kart (not 'go' kart) pictures and copy it.

You need front caster. This means the angle at which the front spindle is at needs to be tipped top-backwards. 9-13 degrees is normal. This allows for tire tires to jack the kart and take weight off of the inside rear tire which helps the car slip that tire in order to make the solid axle not push the car straight. You kart currently looks to have 0 degrees of caster.

Heres a site that shows these basics with some illustrations
Steering Geometry and Setup for Go-Karts at Kartbuilding Blog

Also, for the caster to do it's job with jacking the chassis, the chassis has to be sufficiently stiff to not twist an absurd amount when you turn. As of your current design, it does not meet these stiffness requirements.

Schumi - thanks for your help and simple is good. I am familiar with most of the terms, but not necessarily how changes directly affect the outcome and handling in the end - like camber and caster. I have set up my 911 with specific alignment settings based on what I've read works and from reading tire wear, but I haven't had the time or resources to try all sorts of different adjustments to see how they affect the handling. This is the ultimate learning experiment for me and my boys in that I have no time deadline, the frame is malleable (to a point) so if something does not work correctly, we can try a different design.

I will try to straighten the tie rods. Does it matter if they attach ahead of the front wheel pivot point or behind? I did build in some caster and some neg. camber in the front, but not nearly as much as you suggest. I also did a bit more research on Ackermann angle which is also something I can try to correct. Thanks for your link too.

Here's a few links that helped me w/the Ackermann concept:

Ackermann steering geometry - Wikipedia, the free encyclopedia

YouTube - Neurotikart II - Steering setup

At the end of the day, this is just a fun learning project w/my kids. We have no other worldly intentions w/it. It would be nice if it drove really well when it was done though!

I'm sure I'll be back with more questions after the next round of refinements!

Thanks,
Tom

Uhoh... Mike spent several years building gokarts on steroids. This could get interesting....

Tom

I can't offer any technical help but I just wanted you to know....

You're a really good dad for doing this with your boys.

Ackerman is good, My shifter kart as well as my son's clutch kart have tons of ackerman.

It is the solid axle. With this setup, the inner and out tires cannot turn at different speeds as needed when you turn. This leads to huge turning radii or massive understeer. Cut the axle and drive only one wheel or find a place to cannibalize a small differential.

Arcsine is correct in my limited experience. All karts I've see drive one rear wheel or have a differential.

Err... No

Where did you get the kit with all the parts?

On the other hand, both rear wheels are driven on racing karts. And there is no differential. The front suspension is designed to jack the inner rear wheel off the ground when turning.

In short, karts and cars are set up differently.

When the kart is at slow speeds and especially when being pushed, the rear scrub effect is hideous. Get ahat puppy under power and you won't notice a thing.

I too can go over board on technicalities as I raced karts most of my life. Until you line up to the next person with a kart, there is nothing complicated about them. :D

When I used to race karts ( a while ago!!) we used to set the front end up with 2mm of toe out, completely against what you would think, but when you had the weight of the driver in the kart plus the dynamic camber changes due to the flexing of the frame it all worked out!!

I also used to use a carbon fibre floor pan that was thicker at the back than the front......but thats another story!!

Care to enlighten us with what I do not understand?

Both of my karts have had a live rear axle... One had an uncanny ability to roll over as well. I think that was more due to the narrow wheel base tho. Who builds a cart that is wider in front than the rear, and then puts a 150cc engine on it with a live rear axle?

It did NOT do doughnuts well at all...

Caster jacking. If you paid attention, I shortly explained this in my first post.

Whatever you do it will be excellent.

I grew up without a dad so when I was 12 I built my own go-cart. It had a wooden frame and geometry and caster were words I hadn't learnt yet. The cart was FAST and fun.

When I was 9, I nailed 2x4's to plywood, and lawnmower wheels to the 2x4's....
Steering was done with a rope, as the front 2x4 had only one nail. In the center.

Sounds good. I'll race you.

Mine had a lawn mower engine and belt drive :cool:

This is a great piece of advice Milt. True with most things in life.

On the Kart - I flipped the spindles side-to-side and moved the tie rods under/behind the steering shaft. This really straightened them out. Based on some additional reading and posts above, in addition to making some modifications to help the Ackermann angle, I think I will cut off the front spindle brackets and reweld them with more caster. That's about as far as I'm willing to go on Kart Version 1.0. I don't think my boys will give a rat's a$$ quite honestly whether it steers correctly or not. They're just excited to drive the thing! It's mostly me driving the design bus, and I want it to work well for when I have to "test drive it" for them.

After that, it's time to hook up the brake, work out a seat and then on to the MOTOR!

Fun stuff.

Thanks for the help,
Tom

I bought the kit from this place:

Go Kart Galaxy- Go Kart Parts and Minibike Parts at Discount Prices

I could've bought a frame as well for less than $100 more. . . but where's the fun in that? Tubing cost me something like $30, I have a welder, drill press, a borrowed tubing notcher, metal band saw, grinder etc... and need the practice. Of course the priceless component is time w/the kids building and engineering and testing etc....

I'm sure I could do it cheaper by fabricating more parts myself and buying an old junkyard snowmobile for parts etc..., but I decided this was my threshold for $$, time and knowledge etc... If I drag the project out too long, my boys will lose interest.

Tom

Cool site. With what engine do you power something like this and where would you buy it?

You can get an engine from Harbor Freight for like $200 bucks.

It's a standard engine for a lot of organized racing series.

EDIT: They are on sale, http://www.harborfreight.com/65-hp-horizontal-shaft-overhead-valve-gas-engine-66015.html

I think you could probably adapt just about any engine you have lying around w/a little fabrication work. One w/a horizontal output shaft would be the easiest to directly connect to a sprocket on the rear axle.

I am eventually planning to make it electric. I'd like to learn a little about the systems and having it quiet in my neighborhood would be a bonus too. I have a few old chainsaw motors that a friend gave me sitting under my workbench. One of them is pretty big - I have no idea if they have the torque to get a kart like this rolling, but since they are free, I thought I'd give them a try first. My boys are young, so speed is not the ultimate goal for Kart Version 1.0. They will get a kick out of the fact that it will move under its own power - even if it's only moving at 5mph!

Check this beast out:
YouTube - Neurotikart II - First Ride!

Tom

Yeah, I wanna be sitting in one of those when it crashes and the batteries break open spewing acid everywhere.

Optima! No acid.

2nd yes, 1st don't think so.

Maybe an engine out of an old Honda 125. That way you aready have a clutch and sprocket. And as the boys are quite young don't mention it has more than one gear.

Here is my 100cc race kart
http://forums.pelicanparts.com/uploa...1286992833.jpg

Well, It's alive! It's not quite finished, but it moves under it's own power. After some head scratching with the chain saw motor, I decided to spend $109 at Harbor Freight and get their 6.5 HP clone engine. It was simple to set up and install. Here's a few pics of the inaugural test w/my 5yr. old. It was just about dark when I finally got it running, but we couldn't wait for the next morning. . . video to follow as soon as I have few minutes to do some editing. It's fast as heck!

Tom

http://forums.pelicanparts.com/uploa...1289882540.jpg

http://forums.pelicanparts.com/uploa...1289882571.jpg

http://forums.pelicanparts.com/uploa...1289882642.jpg

I have some kart side pods and I think a nose cone if you want...

Free if you pay the freightSmileWavyhttp://forums.pelicanparts.com/uploa...1289883091.jpg:cool:

Ps. Very cool thing for you do do for your son!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !

I'm thinking of making a fiberglass body like this as a winter project:

http://forums.pelicanparts.com/uploa...1289883241.gif

:eek: WOW!!!!!!!

( the side pods and nose cone will absorb a TON of energy in a crash. I've tested them at speed)

Karts become very addictive. My lads started on a fun kart like that then progressed to a 100cc two stroke fun kart, then to a full blown race outfit. Oldest son is state title holder and youngest son is #2 in the sate in their respective classes. Both use Yamaha KT100S engines. Over 17.5HP after blueprinting. Excellent family bonding times.
Good on ya :)

Very nicely done! I like the use of the child booster seat as a racing seat.:) That's a super cool father/son project.

put a hpv 100cc on it!

Get a roll bar on that thing.... it's only a matter of time now.

Awesome project BTW. Wrenching on my go kart with my dad led directly to buying a 73 VW to restore with my dad which led to my car obsession. Thanks, Dad!