Need Help- Setting Caster on 80-90’s F1 Type Suspension

[Rhiggins13]

Hey yall, looking for some help here. A good friend of ours has a 1965 Porsche 911 that he had built into a full tube chassis race car with suspension that is based off a 80’s-90’s formula suspension. He owned and raced the car for about 30 years before cutting it up, but that’s not the point of this post. We are having issues trying to set REAR caster on the rear of the car. We are hoping to find someone who has some ideas on how to check and set this. We have a couple ideas, but we’d like to hear other outside suggestions.

Thank you for any help!

[marchsv]

Can you please post a few pictures of the shock attachment points?

Thank you

[David Ferguson]

I think it's adjusted by changing the lengths of the legs in the lower a-arm. But really that's would only be adjustable by a very small amount.

I don't think that would be very important since there should be very little toe change with heave motion assuming the upper links are close in length.

[Rhiggins13]

Can you please post a few pictures of the shock attachment points?

Thank you

I will! I leave for a two day trip tomorrow and I’ll take more when I get back. Thank you.

[Rhiggins13]

I think it's adjusted by changing the lengths of the legs in the lower a-arm. But really that's would only be adjustable by a very small amount.

I don't think that would be very important since there should be very little toe change with heave motion assuming the upper links are close in length.

Adjusting is not our current problem- it’s how to measure where we are right now and how much caster it needs to have. Possibly considering setting it close to a newer 911, but trying to figure out an accurate way to check it and see where we are now. Thank you for the response!

[Fred Michael]

Bump steer guages

[Rick Kirchner]

Depending on how accurate it needs to be, Compress the suspension to ride ht using a jack, and then project the axis to the ground using a straight edge and drop a vertical with a plumb bob, and then just do some geometry to get the angle?

Carroll Smith book Tune to Win have a method?

[Lee Johnson]

Hey yall, looking for some help here. A good friend of ours has a 1965 Porsche 911 that he had built into a full tube chassis race car with suspension that is based off a 80’s-90’s formula suspension. He owned and raced the car for about 30 years before cutting it up, but that’s not the point of this post. We are having issues trying to set REAR caster on the rear of the car. We are hoping to find someone who has some ideas on how to check and set this. We have a couple ideas, but we’d like to hear other outside suggestions.

Thank you for any help!

Following Carroll Smith's methodology, I would focus on bump steer as the parameter to measure and optimize at the rear of the car. Typically one would try to setup a road racing car with the same bump steer behavior on the right and left sides (at either front or rear). Although Smith mostly wrote about (and worked on, to my knowledge) formula & sports racing cars. I would think that similar principles should apply to a 911.

For a modest power-to-weight formula car a starting point for rear suspension alignment would be 0 degrees of toe-in change at the rear (through the full range of droop to bump travel). Toe-in at ride height might be set at 0 degrees to 0.5 degrees (or more if typical practice for racing 911s). A full rear-engine car like the 911, typically has a tendency to over-steer (hope this isn't an over-generalization!!). A modest amount of toe-in increase (+ 0.5 to 1 degree, perhaps) on bump at the rear would tend to counter this.

It will be interesting to hear what approach the original poster decides to follow & what the on-track results are.

Lee

[Lee Johnson]

Ryan --

In the last 1/3 (or so) of following (2021) thread on Ackerman there are some comments from Steve Lathrop & Greg Rice about rear-end toe (toe-in versus toe-out) settings relating to handling balance--

https://www.apexspeed.com/forums/sho...erman-question

Although they discuss static (that is, @ nominal ride height) rear-end toe settings, try to visualize similar "steering" effects related to the rear outside tire toe change during cornering-roll induced bump.

Let us know what you try out & results.

Lee

[Steve Davis]

..he had built into a full tube chassis race car with suspension that is based off a 80’s-90’s formula suspension
Thank you for any help!

Seems like a few good suggestions here, but I would think that WHOMEVER actually BUILT (see above snip) this chassis, would have to have had some ideas about just WHERE to put things .. and therefore HOW to set it up. Is he/she NOT still around? Sure seems like that would be the best source to get your answers. Decently 'built' cars (and this one certainly LOOKS well built) aren't just thrown together without a LOT of knowledge... even to get started on something like this would take a MINTFULL of knowledge background.
Steve, FV80 (knows NOTHING about anything with fenders <VBG>)