Pushrod suspension setup

[Altair]

While setting up my puhsrod. I find that while I keep the length of the pushrod and the damper equal on the two sides, the length of the spring has changed.
that is the preload on the right and left is very different.

Is it a problem? If yes then how does one eradicate it?

[Tim.Wright]

Are you talking about a real car, or a model?

If its a real car, then there is probably some difference due to manufacturing tolerances. This means that if your springsdampers are set to the right length, the ride height will be wrong. Or if you set the ride height correct the damper length will be wrong.

This is normal for hand built prototypes. You need to prioritise one of the measurements over the other. Basically I'd aim to have the control arm angles in front view correct and live with the different damper lengths. You will also want to have the same static spring length to ensure the corner weights are as equal as possible.

Hopefully I've understood your problem correctly..

[Warpspeed]

Springs can vary quite a lot.
While the spring rate will be pretty consistent, depending on wire size, coil diameter, and number of coils, the free length and the loaded height can vary slightly from spring to spring.

Don't worry too much about small differences the final loaded spring hight, that is why the dampers have an adjustable spring perch for final adjustment.

The correct way to do it is to disconnect the antiroll bars, and measure the corner weights to get them as even as possible side to side at each end.

[Altair]

Originally posted by Tim.Wright:
Are you talking about a real car, or a model?

If its a real car, then there is probably some difference due to manufacturing tolerances. This means that if your springsdampers are set to the right length, the ride height will be wrong. Or if you set the ride height correct the damper length will be wrong.

This is normal for hand built prototypes. You need to prioritise one of the measurements over the other. Basically I'd aim to have the control arm angles in front view correct and live with the different damper lengths. You will also want to have the same static spring length to ensure the corner weights are as equal as possible.

Hopefully I've understood your problem correctly..

Yes, I can see some manufacturing defects in the car so that might be one of the reasons.

[Altair]

What I have done now is that i have kept the static preload of the spring and the damper length the same and I just need to vary the push rod length a little (less than an inch difference in the two).

Is that okay? Also, should be measuring the corner static weights with the springs on or should I replace them with a solid member?

[Buckingham]

This is one method. Some might disagree, but think through the process and decide for yourself.

(Assuming coil-over shocks)

1. Replace springs with solid members of length equal to design suspension travel (if shock has 2" travel and you want 1.5" jounce and 0.5" rebound, how long is the shock at static height?.)
2. Set pushrods (and all adjustments) to Design Length.
3. Set corner weights by adjusting pushrod lengths.
4. Replace solid members with shocks/springs and recheck corner weights, adjusting spring preloads only.

[Z]

I would add to Buckingham's list above:

Step 0. Put car on FLAT, HORIZONTAL ground.

(If ground not flat, then all else is a waste of time...)

Z

[Tim.Wright]

That's not a bad method if you have the time. However, I would put a bit more focus on the ride height not defined to the ground but the vertical delta between a point on the hub (wheel cen) and the chassis (eg lower arm front pickup pt). This is important to make sure the static roll centre heights are correct. I would give this equal priority to getting the corner weights right.

If getting the corner weight right means the chassis is rolled by 5mm on one of the axles, then your geometric load transfer could be asymmetrical meaning the car will respond different going left compared to right. The same way as it would if the corner weights are out.

Basically you would want them both to be correct, but if its not possible then you need to be aware that a compromise between them should be made, not just set the corner weights and finish.

For sure the final corner weights should be set with springs installed.

EDIT: Yea what Zep said, I forgot to add it. if your floor is pissed by 3mm and your wheel rate is (I don't know say) 3kg/mm then your corner weights will be out by 9kg with the car supposedly "level"

[Warpspeed]

The primitive way concrete slabs are poured, you will never find a truly flat and level surface to work with in a typical workshop/garage/factory. Its not uncommon to see puddles over an inch deep form during initial building construction before the roof goes on.

However, with a long spirit level and some squares cut from suitable material you can very quickly shim up four wheel pads to be sufficiently level with each other, just about anywhere.

That, plus some tubular spacers cut to give correct ride height (to replace the springs) will give you an excellent starting point.

[Altair]

Thank you guys,

I was able to save the problem with some work and a lot of help from you guys http://fsae.com/groupee_common/emoticons/icon_smile.gif