suspension design feedback - independent swing arms

[DAGAD]

Aloha - can I get some expert suspension critique from you guys?

Fully independent limited to a width of 12.5" or 320mm because it's going under a pedal car from 1960s that only allows for that width
goal is 3" of suspension travel. Only room for 1" up travel without cutting out original seat and raising it up.

max speed of 20mph
Intended terrain. driveway, lawn, park, tennis courts, parking lots, and long bumpy rarely mowed grass at the park.
This is for a four year (<40lbs) old but I want to be able to sit on it and mess around as well. I'm 180lbs

I joined the forum in order to get feedback on this because I'm just not sure it will work well. quite limited on suspension design because it's so small.

 

[DAGAD]

A little more information:

tires are 6" tall and 2" wide pneumatic
shocks in back are 6.5" long with 1.5" travel - shocks in front are 9" (greater than 3" travel in front but limited to 3" in rear)
the brown cylinders in back are electric hobby motors - attached to 1:5 gearbox (blue)

biggest questions:
- Do the swing arms need to be connected with some sort of anti-roll bar?
- Is the rear shock compressed angle of ~40degrees OK? I can't seem to get the angle any less
- Do I need to redesign the front suspension arms so they leave the frame at an angle (semi-leading arm)
- are those cheap coil shocks for bikes and scooters any good? My experience with them on walmart bikes makes them seem like junk. I based this build around mountain air shock from rockshox or fox but they are very expensive.
- After I figure out suspension I need to spend time with steering - I just eyeballed it but I'm sure there's a science to it. camber, caster, ackerman angle, etc. -also not sure if I should set up steering behind or in front of the spindle?

 

[mckutzy]

Hello and welcome.
If this is just for a kid.. this is way overbuilt...
You could use die springs, and save alot of space instead of using the mtb shocks.. Use it in something like this setup..Spindle setup goes on this...
https://www.gopowersports.com/springer-spindles/

The rear end can be on a swing arm, and maybe use on of those shocks... if you really wanted it....

 

[DAGAD]

Thanks for the helpful reply. Yes. I agree it’s overbuilt. I’ve had the idea in my head of an independent rear suspension build because the two separate motors actually make it possible. Also - I’m having fun trying to figure this suspension out as originally I wasn’t sure it was even possible in such a tiny package.
I also like the idea of those springy spindles in front. I had considered them however the front suspension and steering seems to fit ok (Though I’m unsure the characteristics of a leading arm independent suspension). It’s the rear suspension I’m most worried about. I remeasured the rear shock angle and it’s >50 degrees at full compression. So there’s that issue. . . Not to mention the other questions I wrote above.

 

[Functional Artist]

Kool project ya got there

Maybe check these karts out
...the Damien kart has a custom "swing arm" rear suspension

http://www.diygokarts.com/vb/showthread.php?t=40084

…so does the Atom kart

http://www.diygokarts.com/vb/showthread.php?t=40167

…& both rear suspensions are kinda compact designs

 

[DAGAD]

Thanks so much for the feedback! That Damian kart specifically has a gloriously simple and compact rear swing arm. I had originally pictured in my head for this build a setup almost just like yours - A small coil (undamped) with only ~two inches wheel travel. Somehow it has since morphed into something more.

On my design I lowered my lower shock mounts even lower in the back which reduced the shock angle to 35 degrees at full compression so I think I’m gonna stick with the 165mm air shocks or (coil overs if I can find super soft springs).

The front of your Arial Atom cart has single a arms which is what I wanted for this build but I have ~2” total per side between the spindle and the centerline of the car so the suspension arms have to run front to back or nothing at all. (Considered old style Ibeam suspension but didn’t see an advantage).

Anyway - nice to know I was thinking similar to such a great builder and thanks again for the feedback.