A-arm lengths and mounting points help

[Lavender Sidebottom]

I understand why the upper a-arm needs to be shorter, but how do you know how much shorter? And also, why would the mounting points of the upper a-arm need to be further outward than the lower mounts? Isn't it enough to just have a shorter upper a-arm?

Thanks!

[nutz4sand]

http://www.racingaspirations.com/suspensiongeometry.php

Mess with that a bit. See if it helps you understand.

[chuckorlando]

If they were unequal at the spindle it would cause the spindle to lean in at the top and out at the bottom. If every thing was the same it would lean in when raised and out when dropped.

[yoshi]

Originally Posted by Lavender Sidebottom

I understand why the upper a-arm needs to be shorter, but how do you know how much shorter? And also, why would the mounting points of the upper a-arm need to be further outward than the lower mounts? Isn't it enough to just have a shorter upper a-arm?

Thanks!

how much lean varies between builders, but I have a lot more camber change in my SXS than my SR2 for the reason that the SXS is for dirt and needs better contact to the ground, which works different in the sand.

If the upper a-arm needs to be 4" shorter than the lower, and you only have a 1 or 2" difference at the spindle for kingpin inclination, the extra 2 or 3" would have to be made up at the chassis..

Originally Posted by chuckorlando

If they were unequal at the spindle it would cause the spindle to lean in at the top and out at the bottom. If every thing was the same it would lean in when raised and out when dropped.

That's incorrect, majority of arms are un-equal at the spindle for proper kingpin inclination. And if everything was the same/equal, the tire wouldn't lean in or out as it cycles up and/or down...

[chuckorlando]

I typed that wrong. I should have said in line with the spindles not equal. I was trying to say what you did

[tannere]

so why would you not want the Aarm mounts the same hight at the spindle and on the frame and top and botom same lenght

that would keep the wheel strait up and down throughout the motion

thats what you want right????

[nutz4sand]

No Tannere.

You want the tire to lean inward as it nears the top of its travel.

As the chassis rolls to the outside and the tires is pushed up it needs to lean in to counter the body roll. If it did not then the tire would lean outward as the body rolls and be much more likly to come off the rim. Traction at best would be poor.

[Duden89]

Whats a good number to shoot of degrees of camber for a small buggy? It would be seeing mostly dirt and grass. I've heard 12 degrees but I'd like hear what the experts have to say.

[Gene]

Duden89,
You might be confusing camber with king pin inclination. REcently in the Suspension section Nutz made a post pointing to a Radio Control site that explains suspension terminology. It is a Sticky so check it out.

Camber might be zero at full droop and then get progresseively negative as the wheel moves up through it's arc to full compression. At full compression camber could be -3, maybe more.

[evan]

IMO...I strongly suggest that anybody serious about designing / building a buggy or kart buy a chassis dynamics book. The $18 you spend will be well worth the headache saved. Most include diagrams and explanations well above what anyone can post on here. I strongly recommend devoting a weekend to reading any book by Carroll Smith. I always find myself referring back to "engineer to win." Also "tune to win" explains chassis dynamics which is the topic here. They refer to formula style cars so camber curves and other value are not helpful to us, but the understanding of these concepts allow you to deside what compromises you feel comfortable with.