First gen Ackerman question

[baz67]

Here you go Mr. Pozzi or Mean69 or anyone else that may know.

This is what I have: DSE power box, correct pitman and correct idler arm for my 67. What I do not have is the correct short steering arms. I have the longest ones. I will get the shorter ones when the price is sane for them.

What effect will the longer steering arms have on the Ackerman steering angle? Will it increase, decrease or stay the same? In my mind I can make an argument for all ways. Is the change a good thing or bad?

Brian

[Salt Racer]

https://static1.pt-content.com/images/noimg.gif

See, I was right! hehe.

Longer Pitman/Idler will increase Ackermann angle, plus they'll make overall ratio quicker.

According to David P's measurements, stock sub with G-mod has near parallel steering. It could use some Ackermann for better turn-in.

[EDIT] My statement above is incorrect. Longer pitman/idler decreases Ackermann angle.

[MrQuick]

I told you! he he so whats the answer?

[David Pozzi]

I think the shorter arms would increase Ackerman a bit. But the first gen's do have a good amount of Ackerman and it works in the "right" way, it turns the inside wheel more than the outside. I haven't done much ackerman testing on my car except for years and years ago. I don't have data for long vs short arms, but if someone sent me some long arms I could compare them.

Some front steer cars seem to turn the outside wheel more than the inside, that's "reverse ackerman" what's good? This'll kill ya, "it all depends on your tires" that's what I was told. Just send your tires to Calspan and have them tested on their tire machine, and you'll know what to do! :) You'll have a graph for everything you can think of.

[Salt Racer]

...But the first gen's do have a good amount of Ackerman...

Really? Do you remember the pickup point measurements you sent me a while back? Which pitman/idler & steering arm combo did you use for that measurement? WinGeo shows almost parallel steering with those dimensions.

[chicane67]

Short arm = more ackerman, and better low speed turn in, that's for sure.

[Salt Racer]

I was wrong. Moving the inner tie rod pivots away from axle centerline (ie, shorter Pitman/idler in this case) increases Ackermann on rear steer. Sorry!

[baz67]

Ok, Chicane says more and Salt Racer says less. That makes sense, not. I guess what I am after is is the relative change of the Ackerman angle with the longer arms good or bad. Like most things too much Ackerman is not a good thing.

Brian

[chicane67]

Moving the inner tie rod pivots away from axle centerline (ie, shorter Pitman/idler in this case) increases Ackermann on rear steer.

But.....but......he did say that, specifically about a rear steer chassis.

The long arms turn faster per rate of input angle. At the top of the input angle, it will also induce ackerman at a faster rate per given input.

In conclusion: Long is good, and that's whats she says.......

The short arms will increase steering effort and build more heat in the system as well. To counter that buy itself, you would need to change the bypass valve rating in the box itself. As we all know, even ask Carl, steering heat is bad....... uhm'k.

[David Pozzi]

I bent one set of arms out 1/2" for reduced Ackerman. I didn't notice any improvement.
I DID notice my tie-rod ends hit my rims. :(

Salt Racer, I think there is an error in my steering linkage measurements, I know the outer tie rod positions are not correct. I measured the suspension but didnt have toe in set . All the other steering linkage points are OK.

I just wasn't focused on steering when I input the measurements, but once I entered part of the linkage, I had to fake some of it or I would get error popups when I ran it for camber curves.

[Salt Racer]

Makes sense, David.

Brian,
Chicane and I are basically saying the same thing. Shorter pitman = more Ackermann on this platform (rear steer w/ pitman pointing front). Longer pitman will reduce Ackermann, but if there's enough Ackermann built into the steering arms on knuckles, the benefits overweigh its short coming.


...At the top of the input angle, it will also induce ackerman at a faster rate per given input....

I read this in Carroll Smith's book too, but I have a hard time believing it. Moving inner pivots back&forth does have effects on Ackermann, but I haven't seen any noticeable change in the rate on most suspension that I've analyzed. There are probably other factors involved (my guess is outer tie rod location).

As for the effort, I assume you talking about the effort (or stress) in the system. Longer pitman (quicker ratio) seems to require more effort at the steering wheel, but less stress transmitted from road wheels within the box.

[David Pozzi]

in the 70's, It used to be thought very little Ackerman is best, now a fiew articles say more Ackerman is better. By "more" I mean closer to 100% Ackerman correction.
I think 100% for a Camaro at full lock is 4.5 deg more angle of the inner wheel compared to the outer.

A camaro at full lock would have a static or very low speed turn center point in line with the rear axle, the "inside" rear wheel would track 24" farther inward toward the center of the turn circle than the inside ft wheel. But some oversteer would kick the rear wheels out and move the dynamic turn center forward more in line with the center of the car.