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Old 08-05-2010, 05:58 PM
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Roll Center Height/Designing Control Arms

I got the Performance Trends Circle Track Analyzer program to help design the front control arms for the 41 Chrysler I'm working on.

I've been entering different dimensions for pivot hgts and distances from center. Of coarse the roll center height changes every time I enter different figures. The dynamic caster also changes. I've been checking it at 10* roll to check the changes.

Questions: Does anyone here have experience with this program and if so:
Is it essential that the roll center be very low?

How about the dynamic caster? I'm sure the car will handle better if the caster doesn't change too much thru the roll. Is it more important to have a lower roll center rather than trying to control caster gain?

Upper control arm length: What are drawbacks of having the upper arm too short or long?

I took a few pics of some different numbers entered into the program and attached them below.

All comments appreciated.

Thanks,
Pugsy
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Old 08-05-2010, 09:33 PM
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H'mmm have to do a bit of study...longer control arms will help with camber change..

Sam
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Old 08-06-2010, 12:34 AM
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Luckly i use a program similar to this when setting up race cars


Questions: Does anyone here have experience with this program and if so:
Is it essential that the roll center be very low?
Having a roll center that is low to the ground will better balance the car in turns, letting the suspension roll freely, with a high roll center it is trying to roll at a much higher point which is further away from your center of mass. in simple terms lower roll center is better, a good static for a street car is 4-6", then add roll and steering input in a perfect world it should move under the tire taking the most load (left turn load on RF, right turn load on LF) 1-3" off ground.

How about the dynamic caster? I'm sure the car will handle better if the caster doesn't change too much thru the roll. Is it more important to have a lower roll center rather than trying to control caster gain?

In a street car i would put NO more that 2 degrees and thats alot. caster gain coensides with BUMB STEER and ackerman. start with your bumb steer then ackerman and with those set the caster gain shouldnt be to drastic.

Upper control arm length: What are drawbacks of having the upper arm too short or long? if your control arms arent preportioned correctly your bumb steer will be drasticly off along with your caster gain.

I took a few pics of some different numbers entered into the program and attached them below.

All comments appreciated.
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Old 08-06-2010, 12:37 AM
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I looked at your pictures

you should try adding dive and steering input when checking the roll center, it will give you a better reading.
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Old 08-06-2010, 03:56 AM
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Quote:
Originally Posted by AndersonRacing44
I looked at your pictures

you should try adding dive and steering input when checking the roll center, it will give you a better reading.

A couple of pics with 4" dive and 5 and 10 degrees roll.


I've read that you don't want the roll center moving around alot thru dive and roll. The question is how do you know what's best?

I guess some other info would help:
This is just a street car with a BBC and Cobra IRS. I am designing it to try to get optimal handling. (I don't like slowing down for curves and corners.)

The third pic is of one of the program example cars in dive and roll. I believe its a circle car set up.
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Old 08-06-2010, 08:08 AM
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Personally, I think you are thinking about this way too much. If you don't fully understand all this, why design your own when there are tons of suspensions out there that you KNOW work and could be copied for your use?

Brian
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Old 08-06-2010, 10:30 AM
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i would suggest that you get a copy of Carroll Smith "Tune to Win" Yes we want a low roll center..long locating arms..little or no camber change in cornering. and the biggy is no bump steer..at leat that is my take..the roll center will move around a bit in roll but no biggy if it stays in an acceptalbe range. then there is the small matter of packaging the suspension so it fits in the car and is doable for the fabricator..Hopefully in the end you have a better mousetrap and it is your own and not some off the shelf "dunno what it is" deal..

Sam
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Old 08-06-2010, 10:41 AM
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Quote:
Originally Posted by MARTINSR
Personally, I think you are thinking about this way too much. If you don't fully understand all this, why design your own when there are tons of suspensions out there that you KNOW work and could be copied for your use?

Brian

Hey Brian.

I like to think and do things outside of the box. Its in my nature.

You can see this by checking my project journal.

Thanks,
Pugsy
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Old 08-06-2010, 11:15 AM
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Quote:
Originally Posted by 123pugsy
Hey Brian.

I like to think and do things outside of the box. Its in my nature.

You can see this by checking my project journal.

Thanks,
Pugsy
I'm with you, I built a mid engine 48 Chevy pickup, I KNOW "out of the box". But honestly, there are SO MANY other factors like the weight and height of the vehicle, the rear suspension, the center of gravity, the location of the motor, the tires, there is SO much more that will effect what ever suspension you put in there it isn't funny.

Brian
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Old 08-06-2010, 02:40 PM
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Quote:
Originally Posted by OneMoreTime
i would suggest that you get a copy of Carroll Smith "Tune to Win" Yes we want a low roll center..long locating arms..little or no camber change in cornering. and the biggy is no bump steer..at leat that is my take..the roll center will move around a bit in roll but no biggy if it stays in an acceptalbe range. then there is the small matter of packaging the suspension so it fits in the car and is doable for the fabricator..Hopefully in the end you have a better mousetrap and it is your own and not some off the shelf "dunno what it is" deal..

Sam

I just ordered the "Tune To Win" book.
I've already got "How To Make Your Car Handle" by Fred Puhn and "Chassis Engineering" by Herb Adams.

Can't go wrong with another book.

As for bumpsteer, with the spindles I'm using the lower control arm pivot must be in the exact same spot as the inner tie rod pivots on the rack. I'm pretty sure I've got that under control.
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Old 08-07-2010, 02:44 PM
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Quote:
Originally Posted by 123pugsy
Is it essential that the roll center be very low?
Pugsy
The upper limit is when you go "over center" and the rear of the car jumps up like a jack rabbit. This was a problem with the old VWs and Corvairs.

I'm pretty certain Puhn discusses this.
http://www.racetec.cc/shope
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Old 08-08-2010, 12:42 PM
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Everyone has there own opinions on roll center, you can ask any nascar crew cheif and get a million different answers...PERSONALLY i prefer my rollcenter to mive from center to almost under the tire with the most load in a corner. Some people like there roll center to do the opposite to get less body roll.

if you do it my way, theres a little more body roll but more bite in the front.

the other way less body roll (mabye you might like it) but less front bite.
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Old 08-09-2010, 02:20 PM
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Thanks for the replies guys.

Less bite in the corners does not sound like a good thing.

I'll see if I can dial it in the other way.
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Old 09-16-2010, 12:33 PM
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Attached is probably the best I can get using the spindles I have.

Comments?
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Old 09-16-2010, 05:07 PM
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Quote:
Originally Posted by 123pugsy
Attached is probably the best I can get using the spindles I have.

Comments?
that looks pretty good, that should handle pretty good!
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