Front steering geometry

[lizard]

I noticed in most or all of the cheaper kart for kids, specifically the ones with no suspension, the front steering geometry is horrible. They push like a dump truck, even worse with live axles, steering is usually too quick, steering wheels too small, etc.
Im not against cutting up the front end and adding castor and camber and designing a roll center migration pattern. Any one ever
look into what can be done to improve the front steering response on solid suspension karts? If so, have any ideas, tips and tricks or experiences?

 

[madprofessor]

Working on that very thing with my overly large kart right now, never mind that it has full A-arm long travel front suspension. Comparison on the steering works out the same.
Homemade rack and pinion steering to get 320 degrees of steering wheel had to be cut off, started over, ended up making a pitman arm setup like average karts, with the usual 10" steering wheel bought for leg clearance. A pitman arm limits steering wheel to about 150 degrees max, and coupled with a little 10" steering wheel makes a very twitchy setup.
Because my steering shaft ended up several inches too high to make a direct pitman hookup, I put a sprocket on the end of it, and a sprocket on the pitman's pivot point, hooked up a chain around it with a solid tensioner. Here's the difference..............
I used a 13-tooth on the steering shaft and a 32-tooth on the pitman arm. Hoped it would give me a lot more steering wheel, and it did help but still fell short of what I really wanted, the math looked right but didn't apply right. Point to you is that the sprocket differential and the 13" steering wheel I'm ordering are both answers toward your goal of less twitch. Pitman arms and rack-and-pinions come equipped with fixed limitations you can't change, as does a tiny steering wheel. The sprockets and the wheel size are things that aren't fixed, you can do plenty of things with that.
NOTE: Doing a center-drive/left-drive convertible setup. Will chain-drive the left to the center, where steering shaft will remain, only steering wheel will swap around. Will do another sprocket differential there, much greater. Left-drive should get about 540 degrees of steering wheel.

 

[madprofessor]

About your "fixed" frame setup on the kart...............Good thing you're willing to cut. Whack those spindles off and weld them back on with even more than the degrees of caster people like to recommend. Ideally you can let go of the steering in a full turn at speed and it returns immediately to center, extra caster will give you that.
The "push" you refer to is an ackermann angle problem. With the wheels pointed straight ahead, your tierod attachment point on a spindle arm needs to be in a straight line with the spindle kingpin bolt's head and the exact center of the rear axle. A string tied there on the axle and stretched tight directly over the kingpin bolt head should also pass directly over the bolt hole for tierod attachment. Weld 'er up there.
Note: All store-bought spindle assemblies I've seen, including mine from BMI, come welded as 90 degree (arm angle to axle) setups, no ackermann possible, modification required as described above.

 

[lizard]

This is going to get interesting. I'm used to designing the front pick up points based on the rear roll center and panhard bar mounts but with a fixed rear axle, the RC is at the rear tire so I seem to have something peculiar to work on here. About what Castor are people talking about? 10' positive?

 

[madprofessor]

Youtube videos I've seen have set for anything from 2-5 degrees. Personally, on a light weight fixed suspension kart running on pitman arm setup, I'd radicalize it beyond what others do. Something like 8 degrees. (Not sure if pos. or neg., talking about the top of the kingpin being tilted toward the back of the kart 1/45th of a circle, or 8 degrees.)
That would make a heavy kart get heavy on the steering, kind of the opposite of no caster. A light weight kart wouldn't suffer any on the steering with it, and the stability would be worth it to me. Definitely reduces twitch.

 

[lizard]

Karts have huge scrub radious as well.
im thinking around 8 positive castor and decrease the scrub to help keep the front end from weight jacking the opposite corner on corner exit. I think, they would increase turn in feel, keep the inside corner tire loaded for mid corner stability and keep the back end from passing up the front just after apex.

 

[madprofessor]

Sounds like you're planning on "going fast and turning left"!!! If you're making a circle track kart, is the motor sitting beside you instead of behind you? If behind you, have you considered staggered tire sizes? How about a side-sliding seat rack?
Post pics if you have them.

 

[lizard]

I have a big car for round track stuff. A USTMS car and also building a big car for SCCA stuff.
I just can’t for the love of Baby Jesus leave things alone hahaha. This is the same one mentioned earlier post.
I also have a Manco 2 seater. It’s a lot better but also single tire drive. It has some Ackerman built in as well. No castor though. A little camber because the front axles are bent lol. At least they are even on both sides though.

 

[madprofessor]

Can't figure out why folks put 5/8" spindle axles on karts when 3/4" cost the same. I'd bet even money that any kart with what I call "wheelie camber" has only 5/8" axles.
Looks bad, but actually helps the loaded tire in a turn, though dragging the inside tire.
Is it an optical illusion that kart above has front wheels closer together than the rear wheels do, a nudge toward the trike configuration?

 

[lizard]

5/8 weigh less.
The front track width is something around 4” narrower than the front. The rear tire width makes it look more prominent as does the stance of the front RH nearly 2” lower. Well, it was but I fixed that already in favor of ground clearance.

 

[Denny]

Actually on a solid suspension, live axle gokart you want weight jacking. It helps getting it to turn both on and off road instead of just pushing the front of the kart just straight.