Yet more questions on 4-bar design...

[autocol]

Hi everyone. I just found this group and I think it's awesome. Hopefully I can answer a lot of questions in the Autodesk Inventor group, and ask a lot elsewhere!

Being a competent Inventor user I've decided to design a fourbar linkage for my EH Holden project car. I may or may not have the guts to actually install it, but I'd like to design it regardless. Searching on the web tends to only bring up sites trying to sell you a four bar, not help design one, until I found this site as a result...

Anyway, I'd like to create an unequal length four bar with a pan-hard bar for lateral stability, and I can model and test it all in Inventor (kinematically, not dynamically). I'm wondering, is there a good website with the major points of four bar design? If not, what should I look out for -

Use unequal length to keep pinion angle at a minimum?
Triangulate the upper bars to remove need for panhard bar?
What type of bushings/mounts should I use?

Are there any hidden pitfalls I should know about?

I do have access to FEA software if necessary, though my basic plan is to over-engineer it, as it's a "one-time-only" installation...

Thanks for any help you can provide.

Col.

 

[GregLocock]

Seriously, why not copy the geometry from a Falcon's live rear axle? It is just about the best there is. You can have a Panhard rod if you like, it won't be much worse than the Watt's link.

"Use unequal length to keep pinion angle at a minimum?:

It is more complex than that, but has to be tuned once you have the car built. Certainly as a start keep the pinion pointing at the back of the trans, as the axle moves vertically. You'll find the pinion nose can hit the floor under acceleration if you are not careful.

"Triangulate the upper bars to remove need for panhard bar?"

Not wildly keen on that but it is often done, successfully. Reason I don't like it is that you will tend to get more compliance oversteer, and Holden's already have enough oversteer. It also reduces the orthogonality of the suspension, which I regard as an important fundamental principle.

"What type of bushings/mounts should I use?"

Race car? rose joints. Street/race? polyurethane suspension bushes. Daily driver? rubber bushes. Cheers

Greg Locock

 

[patdaly]

Greg, putting Falcon parts in a Holden, isnt that sacrilege? ;-)
Seriously, you taught me something, I am working on an Impala, working towards more a HD Touring type effect. I dont know if you have ever seen the setup, but it is a 4 link with both sets of control arms triangulated inward, the top I would estimate at almost 45 degrees inward, the lowers perhaps 15 degrees. My question to you would be, with a set of stiffer, tubular control arms, and Polyurethane bushings, would that help appreciably negate Understeer tendancies, or for serious work, should I look to a parallel 4 link and a panhard or Watts bar? I am not terribly concerned about cushy ride, as the car will have stiffer police car springs and 1-1/4" front/1-1/2" rear sway bars.

 

[GregLocock]

Well, I wasn't expecting an accolade for that suggestion!

OK: compliance understeer

Imagine a lateral force on the contact patch. It pushes the suspension IN towards the centre of the circle. Suppose we fit a stiff panhard rod or watts link , behind the centreline of the axle. The lateral force will tend to steer the axle into the centre of the circle, ie , axle oversteer, and confusingly but importantly, vehicle understeer.

I have never managed to meet sensible understeer targets with a suspension that does not use stiff lateral links behind the wheel centre.

I really dislike angled arms on 4 bar suspensions. For your purposes fit a Panhard rod, if you only have a couple of inches of wheel travel into jounce, or a Watts link for a road car.

I'm not saying that it can't be done another way, but for tunability separating the lateral compliance function from the longitudinal compliances seems like a good idea.

Does that make sense?


Cheers

Greg Locock

 

[patdaly]

Greg, you have to understand, I'm more a straight line, 1000 Hp engine kinda guy, so the things that seep into my brain about suspension systems will suprise you. I think I will spend some time under the Impy this weekend looking to straighten the 4 link and fitting a Watts bar. Thanks again!

 

[yawpower]

Hey there autocol,

Is this a race only vehicle with rod ends? If so, adding a 5th link (4 link plus panahrd) will make for one ill handling car. What's the application?

PY

 

[pmwltd]

Of interest to anyone looking at designing a live axle rear end, should be an article in RACECAR ENGINEERING vol.3 no.5 from 1994, by Arthur Mallock.

He had some very interesting and well thought out theories on 4 link live axles. And proved them with a line of very successful sports racers in England.

His geometry addressed anti squat, torque cancelling, bump steer, and roll center control. If a copy of the article can be found it a very good read. It may still be available through the publisher.

Dave