Building your own SLA front suspension takes a lot of skill. Measuring, cutting and welding are all needed, but they must follow the planning and calculation. One problem I have is that suspension geometry involves a lot of math, particularly trigonometry. Most of my attention in trigonometry class was given to the rather buxom blond next to whom I sat. Her curves were far more captivating than the angles being taught.

Thankfully, double A-arm suspensions are very common in racing and there are resources available to racers that do all that uninteresting math. Plus, I have another aid – Dan Durusky. Dan has graciously volunteered to be the official geek of the Mongrel suspension project and run the numbers for me. Dan is an engineer who is thinking about turning his hobby into a career. With his professional training and drive, I think he can do it and do it well. The V8Mongrel suspension project will hopefully give Dan some experience.

I was also fortunate enough to have Jon Rathbone and Brad Thompson spend some more time in the garage helping me line things up. While the final position will be determined by the combination of careful measurement and calculations, I find that I have a much better time understanding what needs to be done if I can see the problems. Moving a full wheel end assembly isn’t a one man job, so thanks again to Jon and Brad for putting in some sweat on the project.

Dan and I will be selecting the software to use shortly, and will share the output here. In the meantime, Dan has hit me up with a bunch of questions that I will attempt to answer. Hopefully this will set the backdrop for readers to know exactly where we are and where we intend to go. Dan asked:

What do you have already?

Here is a picture of most of what I have.
Mocked up wheel end assembly

That is hub, spindle, lower control arm steering arm and upper control arm. On the white stand, you can see part of the upper control arm mount.

Where are our variables?

Variables are as follows.

  1. Small changes in lower control arm inner mount horizontal and vertical mounting point, approximately ± 2″.
  2. Location of upper control arm mount with only restriction being the engine and existing frame horns.
  3. Choice of short or long upper control arms, length variation to be reported. More available inexpensively if needed.
  4. Two settings for the lower control arm balljoint on the spindle, high and low.
  5. Complete control over length and chassis mounting location of the strut rod.

One of the main challenges will be fitting the wheels. The plan is to run a zero offset 10″ wide, 15″ diameter steel wheel due to their low cost, availability and capacity for large tires.

View of wheel from the backside.
Using the existing lower control arm mount and eyeballing everything else, here is a view of the wheel end assembly, a rotor and wheel. Plenty of clearance for travel.

The view from outside
Unfortunately, the clearance is less encouraging when looking from the outside of the car. This will likely be a balancing act in terms of moving everything further inboard and maintaining decent steering angle for a small turn radius.

Up next, more questions from Dan and some better measurements so we can really get started.

5 Responses to “Planning – Front Suspension – Geometry and Design (Part I)”

  1. #1 Ian says:

    That wheel in the last picture is mounted on the wrong side of the car is it not? The directional arrow should point in the direction of the wheels rotation like a directional tire, no?

  2. #2 dirtydog says:

    Ian, thanks for the question. The arrow isn’t a direction arrow, it is a marker for the air valve. If it were a direction arrow, I’d be in big trouble as all the wheels I have have the arrow pointing the same way! As far as I know, there aren’t any directional wheels like there are directional tires.

    Hope you enjoy the site, and thanks again.


  3. #3 steve says:

    I was wondering how you converted to 5 lug pattern. I want to do the same in my 81 rx7.

  4. #4 dirtydog says:

    Steve, thanks for the question. The conversion to 5 lug hubs was done by fitting and entirely new hub and spindle assembly from a NASCAR-style stock car. I believe there is a way that you can use second generation RX-7 parts (FC3S) to get the five lug components, but I am not entirely sure. The way I am doing it is significantly more involved.

  5. #5 Philip says:

    HI ,I am trying to build a copy of the T REX 3 wheel trike / car and need some help in desighning the suspension .I am using Opel Corsa hubs and spindles up front. Is there any body out there that can tell me where to get help? Ihave done some of it but am not sure if it is going to be ok i have numerous photo. I will really appreciate some help .

    Thank you,

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