Pirate4x4.Com : 4x4 and Off-Road Forum - View Single Post - Suspension geometry for GETIN IT!
View Single Post
Old 11-16-2012, 11:01 AM   #179 (permalink)
Mieser
Registered User
 
Join Date: Jun 2001
Member # 5326
Location: Southwest Colorado
Posts: 11,068
I have a new design project going. I have read this thread a few times now and seed some more info.

This vehicle is going to be an all around thing. Front mounted engine, 12" travel shocks, 6up/6down, 40" tires, etc. Front suspension is a 3-link + panhard as illustrated in the following screen captures....

Static ride height.



I tried to attain some rearward 'wheel recession' from ride height. Nice flat control arms helped that. This chassis sits pretty dang low overall for 40s with about 18-19" at ride height. Front axle roll axis is tuned for slight understeer. Anti-Dive was tuned to be fairly neutral at close to 50% ( I had to start somewhere?). That should be able to be tuned with a few additional frame side upper mounting points.....going up or down is pretty possible.

This chassis will be running a conventional steering box, so I am stuck with a panhard I think. I can't really see any way to raise the roll center without going WAY into a negative roll axis. The roll center is only about 13" away from the spring CoG ( which is a pretty big guess at this point with the CAD models )

Now for the fun stuff.......what the heck happens when things start moving around??!?!??!

Here is the suspension at full bump. I didn't go through the headache of keeping the link length constant just yet. I just dropped everything to full bump. I hope the calculator, in the 3-link version, can automate this. With the arms being approx 30" long I didn't see the need to get that precise yet.



So....

-The roll axis goes WAY negative....just about 10 degrees. So as the suspension compresses it is going to ??????. My guess would be that it is going to understeer much more. The axle is going to almost anti-steer right? At full bump and trying to corner it is going to take a lot more steering input right?

-anti-dive goes NEGATIVE? Basically this means that at static height approx 50% of the CoG brake transfer force is going into the springs/shocks. As it approaches full bump the spring/shock is going to see more and more of the transfer force due to braking. When it goes negative the suspension is basically going to see a small multiplication of that weight transfer braking force to the front suspension?

-I need someone to explain anti-lift to me more? And how having a negative number at full compression will affect things? My understanding is that the suspension might fight itself slightly when getting on the power at full bump....like adding rebound damping?

-Roll center. At static there is 12" between it and the roll center. At full bump this decreases to approx 10". The roll center basically drops below the axle tube I think? The panhard is envisioned to do this...it is forward mounted similar to the older Tri-County Gear design. This is basically a packaging consideration. The frame end of the track bar shouldn't be lower than the pumkin at full bump. Having a normal steering box is a total headache.

I guess most of my questions revolve around static and full bump....

Here is the suspension at full droop.....



-The roll axis slope goes positive to just over 6 degrees. I am worried that this will make the chassis a little twitchy when the tires are first contacting the ground? Roll oversteer I guess is the term?

-The roll center goes up, but the separation between the CoG and the roll center also goes to 14" approx from 12" at static height.

-Anti-Dive goes up to 93%. I am wondering how this will effect the suspension if I where to have to brake right as the vehicle is coming back onto the ground? Basically, I worry that the suspension might get a little 'stuck' at this point?

I am working on the rear suspension now, but wanted to get the questions flowing.....

Thanks for any help.
__________________
42 MB, 225, T18, big kid go cart with clown tires
92 Dodge W250 daily driver on 39s
Mieser is offline   Reply With Quote