Camber

[anasasisxenophontis]

I've been drawing up some stuff for my sand rail and came across a problem. My spindles have got 15° of KPI. Now in order to maintain proper camber through the suspension cycle, you'd have to have top and bottom a-arms of the exact same length right? Or would it be negligible enough to be able to use a-arms where the bottom is longer than the top? During suspension compression, the wheels would move towards positive camber. If it's too much, that looks like it could cause some serious stability issues while cornering. One remedy I can think of is to mount the bottom inside a-arm on the 15° line outside of the top point so I can keep the same length top and bottom a-arms. Ideas?

[yoshi]

Originally Posted by anasasisxenophontis

I've been drawing up some stuff for my sand rail and came across a problem. My spindles have got 15° of KPI. Now in order to maintain proper camber through the suspension cycle, you'd have to have top and bottom a-arms of the exact same length right? Or would it be negligible enough to be able to use a-arms where the bottom is longer than the top? During suspension compression, the wheels would move towards positive camber. If it's too much, that looks like it could cause some serious stability issues while cornering. One remedy I can think of is to mount the bottom inside a-arm on the 15° line outside of the top point so I can keep the same length top and bottom a-arms. Ideas?

I can't think of any reason you would want equal length a-arms, the top should always be shorter, you want the wheels to tilt in at the top under compression, you also need to have proper kingpin inclination, which is almost impossible unless you have a shorter upper arm...

[Artie on Edge]

Hi mate, what Yoshi is refering to is known as Camber Gain. Its what allows the outside loaded wheel to still be upright to the road surface while you are cornering hard. If you had equal length arms you would be presenting the outter edge of the tyre to the road when cornering hard. Less contact patch less grip = understeer or push.

Drop into Rortys site (advert in this site = Rorty Racing) there are some descriptions and pics as to why you need Camber gain through suspension travel. Some pics of cars with no or not enough camber gain and you can clearly see why it isnt "healthy".

To confuse things a little (why not?), KPI when combined with the camber angle is known as the Included Angle (ie: KPI -1.5 Deg and camber -0.5 deg = -2 deg Included Angle).

Good luck

Rob

[anasasisxenophontis]

I get that. So what is good for a 24" a-arm? Obviously theres going to be a difference in gain between a 12" arm and a 24" arm while using the 15° KPI spindles I have, so is there some kind of sweet spot or formula to figure that out?

With a theoretical amount of travel on my spindles with 24" a-arms, I calculate 2.14° of negative camber gain at 10" bump, and 3.54° of negative camber gain at 10" droop. How does that sound?

[bdkw1]

Originally Posted by anasasisxenophontis

With a theoretical amount of travel on my spindles with 24" a-arms, I calculate 2.14° of negative camber gain at 10" bump, and 3.54° of negative camber gain at 10" droop. How does that sound?

Sounds like you need to drop the inner upper arm pivot a little and make it a little shorter.

[anasasisxenophontis]

You mean the top pivot point where it connects to the frame? That calculation was based on perfectly parallel 6" apart top and bottom a-arms.

[Artie on Edge]

G'day again mate. What you are asking for is very difficult to answer on a forum. Im assuming you are using some form of electronic modeling software???

Camber gain is used for two things, most importantly, maintain tyre road surface contact angle and less important to reduce bump steer (this can be designed out in other ways but can be reduced by fiddling with camber change)

What you need to do.... (sounds soooo simple when put like that..it aint!).

Model your anticipated body roll under cornering, at this point have the wheel slightly negative (camber)to the road surface then design you way BACK through the arms and mounts to achieve only slightly neg camber at static ride (loaded) height.

This might be simplest to do with a cardboard mock up rather than electronically... well, would be for me.

What bdkw is suggesting will start the upper arms arc of movement slightly before the lowers. Thus inducing earlier, and slightly more, camber change.

The better handling cars I have seen usually dont have symetrical control arm mounting points, in facts some are way out there, but seem to work really well (generalising here, some are crap). Having said that,I have seen some cars doing things really well WITH symetrically mounted arms. But none of these cars had equal length arms, absolutely none.

Hope this helps mate, (damn you ask hard questions!)

Just adding one last point camber angle change isnt so critical if the body is coming down at a flat attitude (equal left to right like off a normal jump), what is critical is that both side of the car change EQUALLY otherwise you goinna have a car that likes to turn hard when coming off a jump...that would not be fun!

[bdkw1]

Originally Posted by anasasisxenophontis

You mean the top pivot point where it connects to the frame? Yes


That calculation was based on perfectly parallel 6" apart top and bottom a-arms. You don't need or want them to be parallel.

A good set-up will gain about 4-5* in bump and gain about 2* at full droop.

Next you need to figure out your castor curve......

[Artie on Edge]

Oh bdkw, now you really gonna confuse things...lol

[Artie on Edge]

just changed wording slightly in my previous post, i think it wasnt too clear . better read again