Front Suspension Geometry

[Jim Cowden]

No problem Phil

Anyone.
I have another question I have looked through all my info but to no avail.
I have been on my cad plotting instant centres and roll centres from old to new arm lengths.
I have reduced it dramaticly but I cant find anything on what is an acceptable amount of movement?.

It is double a arm, upper is 75% length off the lower SAL 128".
I know a lot more info is required but I was just looking for rule of thumb to see if my senarios are what I should expect.

Jim

 

[Malcolm]

I would not agree with the latter part of this statement. In a corner without ackerman the tyres will be arcing on a different radius so no working directly against each other but certainly not working together. I cannot give a definitive answer but my experience was that I had lighter steering and the ackerman got the blame! Rightly or wrongly!

 

[Jim Cowden]

Ok I said I would come back with what I came up with when I was happy.
I looked at all my options did a lot of reading not just from one source and tossed it around for a while.

I will be going off memory on this for measurements as I had a disaster with my memory stick with all my info on it (it went through the washing machine)but all my cad drawings have been lost so I am currently redoing them of my hand sketches another lesson back up more frequent and print every drawing.

Enough dribble, the path I chose is not uncommon but I feel confident with my decision.

Firstly I went Frank Catt advice (also in the herb Addams suspension design) and chose a wheel off set.
It was a bit of a juggling act at first till you get that part confirmed.
What I ended up with is an 8” wheel 235 tyre 5” inset 3” outset.

I spoke to the wheel manufacturer and he was ok with making this offset.
I expect it to be a 17” the wheel I used as a dummy was an 18 but the tyre has the same rolling diameter as the 17 I will be using.

I played around with the upright design and came up with a tubular design.
I machined an outer and inner tube to size then made a jig to hold them in place with 6 tubes between not unlike an air frame or someone said a sub hull.
It’s caped either end or fully welded.
It has 6.5deg of KPI.

The wheel bearing is compact and a fairly robust unit it helped pull back 20mm from the ball joint holes to the wheel face.

 

[Jim Cowden]

With the suspension arms I deleted the plate work and went up in tube size on the bottom arm and strapped the top arms with small plates, I opted to use rose joints instead of ball joints top and bottom for adjustment purposes and the outside diameter is smaller than a ball joint and allowed me to rake back more room as I can get it closer to the disk.
With these changes I was able to make the suspension arms 75mm longer than STD.
The scrub radius is 13mm with the knock on or pin drives.

I found that with this added arm length my roll centre in the front is much more stable, with the rose joints I can adjust the upper arm up or down at the upper joint to adjust the roll centre height and effective arm length.

The lower rose joint I have left a spacer between it and the upright if I wish to lower the car further I can change the spacer to retain a level lower arm (basically if I change the ground clearance the geometry of the arms will remain the same).
I will start at 120mm ride height.

The longer arms will reduce the camber gain to a degree but that is one of my compromises , this will help under brakes or undulating road conditions but what I will do is use as much caster for high speed stability as necessary hopefully about 3-4deg and the 2.5deg or so left in the kpi will pull some neg only on turn in.

The Tie rod and steering arms in the photo is not the one staying it is 0 Ackerman I am getting some laser cut with 100% Ackerman if I wish to change that is no big deal to do more.
Bump steer is down to about .003”- .004” over the working range.
The sway bar tube is a dummy tube.
I machine some saddles to hold nylon bushes the arms are alloy only the centre section gets replaced.

Spring rates have not been decided I will probably wait till I scale it.

That’s about it I think obviously the proof will be in the pudding when it hits the road and track.

Jim

 

[Russ Noble]

Jim,

That's a different approach to building an upright, you've obviously put a lot of thought into it. Simple, effective, and light, I would think. And relatively easy to build!

Out of interest, how have you done your caliper mounts?

Keep up the good work.

Cheers,

 

[Jim Cowden]

Russ

They are welded on to the tube about 40% the way around the tube and tie in on the lower ball joint.

They are 4mm thick x 2 as they sandwich the caliper.

Lost a lot of photos but found this pic with the bearing attached.

Weight wise it came in at exactly the same as the STD unit.
My upright wheel bearing and steering arm.
Cortina stub axle billet hub to fit the floating disks .

both came in at 7 KG It supprised me I thought I would have come in under.

Jim

 

[Russ Noble]

Jim,

Looks like you've got your bases fairly well covered there. You didn't find caliper alignment was difficult mounting in double shear?

There is a good opportunity with that setup to feed air through the middle of the upright into the inside of the disc. Are you planning on doing that?

I think you have an excellent concept here. Looks like you've got a lot stiffer upright than standard with no weight penalty and lots of other advantages.

Well done.

What is the wall thickness of the tube you used?

Cheers

 

[Jac Mac]

Jim,
The wheel bearing assy your using, is that from the later Holden Commodore models?

Jac Mac

 

[Jim Cowden]

Russ

The wall thickness if 2.5 mm I am playing it safe but I do think you could go to 2mm with no problem.
You start out with an idea then when you get there you go I could have gone lighter but not to be.

Yes I looked at passing air for brakes I worked out the area of the holes that I could put in and they still only came out at about 31mm from memory as you can only get 4 holes in the outer tube near the disk.
The ball joint spigots steal the spot for 2 more

I have designed a brake duct that sits ontop of the steering arm an travels all the way around to the caliper it will pass more air almost like a spiral ramp it also fits very well in the space left

I did drill holes in the inner tube(look on the top pic last posted next to the left off lower ball joint spigot) and on the wheel bearing face 3 holes(6 in total) on the lower pic.
Air from the brake duct will pass throught the upright and out the outer face to curculate air to the back of the wheel bearing assembly.

Yes the alignment was tricky for the first caliper but the second was easy as I used the first to make the jig.
I made a plate that the upright bolted down onto through the wheel bearing bolt holes.
2 spacers came up off the plate the first bracket went on the 2 more spacers the thickness of the caliper ears then the other bracket then 2 bolts through everything into
the plate.

Yes Jac Mac it is from VT I used one STD and one abs as I will run the speedo of the abs or traction control if I wish.
Jim

 

[Malcolm]

Jim

I was looking at your anti roll bar adjustment system. I had thought of a simialar set but became worried over the bolt holding steady the drop link in the slot you have machined in the blade when under load conditions. Never seen this system in the field, so wondered if you could say why you went for the slot over the more common row of holes. My concern is over weakening the blade when the bar is set on a softer setting.

 

[Jim Cowden]

Malcolm

It is an experiment I will be making the slots longer towards the front to give more stiffness.
I have to machine some T slots like on the mill bed clamp kits to go in the back a thick washer on the front under the rose joint I am hoping this will stop it opening up.
If not I will have to go with holes.

The flange were the blades mount to I also used 2 dowl bolts out off the 4 that counter bore into the flange to stop it flogging the holes.

Jim

 

[Malcolm]

You really do like fabrication and design!!!!!!! Just be careful, a failure in the anti roll bar mid corner may chuck you off the road or track with consequences. I am sure you already knew that....

 

[Trevor Booth]

Jim,
machine a "C" section MS piece to fit over and under the ARB arm, hole in the middle for rod end. This will stop the ARB arm spreading at the slot. I hope you did not lose the X rays on your memory stick -)

 

[Jim Cowden]

Malcolm and trevor

I keep looking at it and you are making me concerned as I can see the ill effects of a failure so I am going to make 2 arms with holes.
I wanted them shorter anyway so it will fix 2 problems.

Thanks for the advice Jim

 

[Malcolm]

You could test its strength by jacking the car up by the suspension on one side only a little to see what flexes? Not the same as a bump loading but a start?

 

[Jim Cowden]

New ones are made and on the car.

Jim

 

[Malcolm]

I hope your car is as quick as your fabrication!!!!!

 

[Eric]

Jim, what are you considering 100% ackerman geometry?

and are you using any anti-dive?