Building a Go Kart - geometry questions

[Tom '74 911]

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:

[Two Rivers]

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.

[Schumi]

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

[Schumi]

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.

[Tom '74 911]

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

[porsche4life]

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

[sc_rufctr]

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.

[Joe Ricard]

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

[arcsine]

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.

[Hugh R]

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

[Langers]

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

Err... No

[BGCarrera32]

Where did you get the kit with all the parts?

[Dixie]

Originally Posted by Hugh R

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

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.

[Zeke]

Originally Posted by arcsine

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.

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.

[jimrs2000]

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!!

[arcsine]

Originally Posted by Langers

Err... No

Care to enlighten us with what I do not understand?

[porsche4life]

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...

[Schumi]

Originally Posted by arcsine

Care to enlighten us with what I do not understand?

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

[Bill Douglas]

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.

[WolfeMacleod]

Originally Posted by Bill Douglas

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.