Bill Verburg

ERP is not showing a monoball for the leading A-arm bush

TheOtherEric

From what I've read in other posts, that leading a-arm bushing is already basically a monoball. It certainly doesn't look that way to me, but I've never removed one to inspect.

chris walrod

It essentially is a monoball. I have a few in my garage now that I can snap some images of if someone would like them. The rubber dust boots already cut off for access.

vincer77

Interesting discussion. So does this mean that if all 8 monoballs are installed, that the kinematic toe adjustment no longer occurs? Or is there enough deflection in the link arm?

TheOtherEric

Right, if you install the monoball in the rear of the lower a-arms, then the kinematic effect no longer occurs.

Bill Verburg

So Chris, you are saying that the 993 rear A-arm leading leg already has a monoball on it?

VNTGSPD

BUT, isn't one of the keys here that when you replace the rear suspension with ALL monoballs, you are virtually eliminating bushing deflection and therefore most if not all of the rear wheel toe deflection under load that the KT was designed to compensate for?

So basically, when changing all rear bushings to monoballs, eliminating the KT function results in little or no negative dynamic results.

But, if you were to change the KT bushing to a monoball only, then you would be defeating the KT function yet the rest of the suspension would still deflect, allowing for increased toe-out under load. Not really a good thing.

Bill Verburg

i just dug out my original A arms, I assure you that both bushes have rubber in them, I can post pics if you want. the leading one is quite stiff and cannot be moved by hand even w. some leverage, the rear is extremely flexible and can by moved w/ almost no effort at all.

On the RS arms the rear bush definitely has a green spot indicating harder rubber, unfortunately I don't have any good pics of the leading arm bush and can't see a green dot in any of my pics(Mike, do you have any?)

jdistefa

Correct me if I'm wrong, but doesn't the kinematic link control dynamic toe to some degree independent of the 'soft' bushing on the trailing link of the lower control arm?

Bill Verburg

I agree w/ Eric, KT is facilitated by the elastic(and boy is it ever soft, I have an arm on my desk now).

You still need to set it, on mine I used the race KT tool that hangs from the rear caliper and set it for the minimum that could be obtained on both side equally. The handling has been outstanding. I've also been in monoball equipped 993s and could not detect any better dynamic behavior. I'm sure it's there but way beyond where I was.

Bill Verburg

KT changes the axis of rotation for the rear wheels, go back to post #10 to look at the diagram showing the axis. the KT arm pushes the top of the axis laterally out and forward, or pulls it back laterally in and backwards. This changes the bump steer characteristic by increasing/decreasing toe change due to suspension travel.

I need to model this more to fully understand it, but that is the basics

TheOtherEric

Also, setting KT is essentially setting rear castor. Which is kind of cools since most cars can't do that. The autometrics KT gauge, for example, is just a rear castor gauge.

That said, the dynamic toe change under loading (i.e. kinematic toe) has nothing to do with castor. It's simply a permissible bushing deflection that causes toe-in.

Confused yet?

jdistefa

Bill, I get that

Hence why I was asking re. kinematic control arm 'controlling' toe to some degree through sweep of the suspension.

The amount of possible dynamic toe of course depends on your ride height re. where you are on the sweep curve and spring rate re. how much range you allow.

I'm just still uncertain about the role of the flexible bushing in an otherwise moderately to high spring rate car with monoballs and low KT setting (mine is at 2).

Well, yeah

I get the castor component of the kinematic, but the arm angle isn't purely in the castor plane, but rather is a combination castor/camber/toe axis.

Bill Verburg

Like I said I need to model this for a better understanding, on mine I set KT to 2(least I could get equally on both sides using the race gauge) but have the harder RS bush's, solid sides and monoball toe arm. Handling is fantastic. I do need more front grip though

jdistefa

^ Ha, yeah we all need more front grip!

That's what the increased front track in the RSR setup is for