Front Suspension

[Rallyace 3]

I would like to, within certain limits, optimise the front suspension geometry on my Targa/Road rally 205. 

I am making my own adjustable wishbones, so will be able to adjust the camber. I have fixed platform Bilstein struts and have no plans to change to coil over struts at the moment. I realise that this limits the amount of caster that I can achieve, but would like to get as much as possible. I’m sure that I’ve seen a modification to the rear mount on the subframe, which would move the bottom ball joint forward slightly. Does anyone have any info on this modification? Is it possible to move the top of the strut back when using  the fixed platform struts and standard top mounts? TIA

[Tom Fenton 1,542]

Allan at Bridgecraft makes a kit of adaptor washers to move the rear bush outwards to add castor. I’ve done it to my road rally 205.

[petert 588]

Cut off the rear mount entirely and weld a new one on at 45º. I'll post a pic later.

[Evo Smith 0]

7 hours ago, petert said:

Cut off the rear mount entirely and weld a new one on at 45º. I'll post a pic later.

Please do post up the pictures of this mod.

I'm interested in gaining some more castor and I to run the fixed platform Bilstein's.

[petert 588]

Like this.

[Rallyace 3]

I’m only modifying my wishbones with a spherical joint to replace the ball joint. I’m using Baker BM replacement rubber bushes on both attachments to the subframe. Does this rule out your modification? I’m guessing that this mod would require spherical bearings at both of the points where the wishbone attaches to the subframe. 

[petert 588]

Sorry, this mod requires all new wishbones.

[Rallyace 3]

I may want to revert to a historic compliant specification at some point in the future, so here is my current thinking.

I will keep the subframe completely unmodified, to give me the easiest route back to standard specification. All proposed (significant) modifications will be to the bottom arm. 

I will calculate the amount to necessary for the caster change required and remove a wedge from the front edge of the bottom arm, as shown in the attached photograph. The gap will be closed up and strengthened appropriately. 

I am expecting to have to compromise somewhere along the way, as we all know how tight things are in there and I’m conscious of the potential for contact with chassis/bodywork. I will reassemble the suspension without a spring and simulate as many “bump and steering” inputs as I can. I’m already envisioning the need for a thin wheel spacer. I’ll update with progress and let you know if it proves to be effective - or not. Thoughts welcome.

[welshpug 1,657]

I'm  no engineer but I wouldn't  like to cut a cast steel arm in the middle like that, why not cut the slot for the threaded insert in the end at an angle?

[welshpug 1,657]

Also subframes are easy to get hold of, even a base model woild  do the trick, saves cutting off the rear mount as its not even there.

[Tom Fenton 1,542]

A couple of the caster washers before welding. Not too hard to get hold of a spare subframe. I don’t like the idea of cutting the arm at all!

[Tom Fenton 1,542]

I’ve made my own arms, much the same as your idea but I hold the arms on a jig and machine with a cutter so the threaded boss is a nice fit in the arm, less gap better fit and a nicer welded joint, I TIG the insert in.

[Rallyace 3]

Thanks for your thoughts. I’d be lying if Isaid that I hadn’t had the same doubts. 

Back in the day, I used to lenghten bottom arms on my mini to get some negative camber in and never had a problem. 

My background is in mechanical engineering and I’m really fortunate to have a couple of friends that will be assisting me with the project. One spent most of his career working in the nuclear power industry, but also had a spell working on suspension design and development at Nissan and also worked for a company making vehicles for the MOD. The other is a coded welder and has access to a wide range of rods etc. I will do the initial tacking up and leave the final welding to him.  On paper, having done some “rough and ready” calcs, we’re more concerned with the risk of failure of the rod end, as a result of stresses in the threaded portion. I’m taking comfort from the fact that rod ends have been used in this way for years. We’ve got a couple of ideas about how to mitigate against this problem. I’ve already gone up to 20mm rod ends, instead of the more common 16mm. 

Tom, thanks for the photographs showing your macining jig. I’ve done something similar. I’m also incorporating a piece that will give me the initial length to match a standard arm when postioning the threaded boss. It may not be obvious from my photograph, but to accomadate the larger diameter rod end, the diameter of the boss is also larger. I’ve machined flats onto the sides to mate up with the sides oof the cut out. Having listened to your and Welshpug’s comments, I’ll develop it further to provide accurate  alignment of the “cut and shut” joint. 

While I’m asking questions - how do you macine the top surface of the pinch lugs on the hub? I want to get it flat to ensure the best possible fit and biggest surface area for the head of the 16mm drop pin. TIA

[petert 588]

Strip the hub. Turn the head of the mill to 11 degrees. Mount the hub to table.

[petert 588]

Does the head of your drop pin look like this? It needs to have a flat and a chamfer. The radius of your milling cutter needs to match the chamfer.

Edited December 2, 2022 by petert

[Rallyace 3]

I haven’t made the drop links yet. I’m going to ensure that the hole through the pinch lugs are perfect (not oval) when I strip them down. If not I’ll adjust (drill/ream) the size and then make bespoke pins to match. I want to machine the lugs to achieve as close to flat and square as possible. I want to get as much surface area in contact between the head of the pins and the lugs as possible. That, together with the “perfect” contact of the pin in the pinch lugs, should give me the best possible strength. 

I like the idea of the chamfer on the pin, as, I presume it gives a nice transition between the two faces on the pinch lugs. 

Thanks for your help.