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Old 16th April 2017, 15:32
ozi jim ozi jim is offline
Join Date: Apr 2015
Location: Sydney Aust
Posts: 72
ozi jim is on a distinguished road

I had a bit of a look through the posts and I have built suspension from scratch.

This will give you some rules of thumb that may give you a guide or confuse you more.

My approach.

Start with the wheel offset that you want for the car that you are building.
The upright ,disk and caliper has to fit inside the chosen offset.
Once you place this on the ground you then know how long the arms need to be.

Scrub radius.
A line through the top ball joint passing through the lower ball makes contact at a point on the road.
This is part of an angle called KPI
With the wheel fitted it will land somewhere on the inside of the Tyre patch, this is the scrub radius ,zero SR is the centre of the tire this will make the car very vague at high speed, front wheel drives generally have a low SR as it cancels out torque steer but not good on RWD, I would not go any less than about 60mm inside the CL off the tire.
If you have to much ,lets say it lands not under the tire contact patch but inside that it will give heavy steering and high feed back on the wheel if you get ruts on pot holes on the road.
SR helps with self centring which is very handy if the car steps out you can just let go of the st wheel and the wheels will self centre then you just regrab the wheel and you are back in control.

The lower arm height location is all about the upright lower ball joint height of the road.
You need to have the lower arm running slightly uphill to the chassis pick up point.
This will help control the roll centre movement across the car and also in the vertical position when the car dives and turns in.
A car that has poor RC control is unpredictable, EG All the weight above the RC applies a load onto the tyre, the higher the RC the less load applied.
The lower the RC the more load because you have more weight above the RC.
If a car if cornering and it has X RC then you get on it harder and the RC changes dramatically then the attitude of the car can change sometimes for the worse.
The upper arm needs to run down hill from the upright to the chassis, changing the angle will change the RC height I have found on rear eng cars it likes to be lower say at about 60-30mm of the ground.
Reason being you have less weight over the front (no eng) so you use RC to get tire loading.
Steering arm.
I like a bolt on steering arm as it gives you a better chance of getting the bump steer correct.
Steering rack location.
If it’s a front steer you need to be careful that you can get a reasonable ackerman.
On rear eng cars I always run anti ackerman meaning it has neg toe out on turn because no weight in the front the front inside tire goes into slip very early so having less to out on turn will give more grip on entry but makes no difference after apexing.
Also having the rack forward of the steering arms on a front steer will give the same effect.
What ever you choose you need to know the KPI if it has 6 deg that is about the caster you will run as a starting point.
Knowing this will give you a point to start on upper arm length and setback for the upper ball joint.
Rear roll centre
Very important on a rear eng car as they generally have to much rear grip because of the weight on the back tire.
This will generally cause under steer issues because the rear just keeps overpowering the front tires.
To sort this you raise the rear RC and reduce the grip and it balances the car and allows you to get on the power early and get it to sit on the back tire at point of entry.
Hope this helps you make decisions on set up or what components to pick.
Bit hard to change RC on struts from memory ,probably easier to adjust the lower inner up or down.

One thing I have seen done and it was smart was a strut leg was cut down and a fitting welded in that had a 5/8 thread in it to accept a rose joint that was fitted in the upper arm, so the leg was turned into an upright.

That arm in your pics made to take a r/joint as a ball joint is fine but you cant do that on the loaded arm ( the one with the spring/shock attached) say the lower you cant use that idea it will break eventually, I know this because I have done it and it did break at 23.000km road and track miles.

Any r/joint if in single shear (upper lower b/joint, outer tie rod on steering arm) should have a washer larger than the joint so as it is still captive if the joint fails.


Last edited by ozi jim; 18th April 2017 at 08:59..
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