Double A vs McPherson

[Badhand]

Please be gentle. I'm a newb and an Englishman. But I'm eager to learn.



I'm just starting my first real build with a recently imported, barely started, '69 Mustang project. I live in the UK and I plan on building a road legal track car to tackle some of Europe's finest circuits: Spa and Nürburgring etc. and as the car is virtually a 'blank canvas' am looking to gather as much input before I start building. I've just bought a take-off IRS assembly from a 2015 Mustang for the back end and will be trying to figure our how to get that in when it arrives next month. So question is, what to do with the front?

I'm building and sharing costs with a mate and one of us favour's McPherson struts for the project. As the S550 Mustang back end is paired with McPherson struts up front, a lot of European (BMW, PORSCHE etc) performance / touring cars use that setup, it's simplicity seems to make sense. BUT there is an awful lot of talk and 'evidence' for SLA or Double A arm suspension being more effective. More complicated, more costly yes, but these are not major considerations for me. I want the best solution for hard track and road use.

Any thoughts, suggestions and opinions welcome...

For reference, this is the 'blank canvas'. Don't worry, there's a Coyote going in there.

[Twentyover]

My position is that for a track car, a SLA (double wishbone) has greater potential than a Mac Strut car. Mac strut cars are a case of development over fundamental good design.

You make a Mac Strut 'work', you need to dial in huge negative static camber, which reduces contact patch for braking. Why the big big static negative camber?The nature of the design is that the upper A arm is infinitely short (meaning it doesn't exist,) so every degree of body roll results an a degree of positive camber being added to the outboard tire, reducing grip. To get there, you need to snub the front end down so it doesn't roll, or dial in lots of negative camber.

On a SLA, the upper arm also rolls outboard when the car rolls, but the inboard pivot also moves down pulling the top o the tire in, making the positive camber gain on the outboard tire less than one degree per degree of body roll.

So how do the high dollar Germans do it? Get good handling w/ a Mac Strut? First, I don't know they are true Mac Struts- They do some interesting stuff with virtual pivot points. Never really studied them so I can't speak in detail on why I think they work. The second way is to tune grip out of the rear. Achieve balance by giving up enough rear grip to match the front grip you've given up with the Mac Strut.

[Badhand]

Thanks Greg. You're confirming our thinking. Still, a lot of stuff to get my head round but I'm getting the basics.

I'm assuming though, that there is no merit in the argument that the IRS we're using is designed to work with Mac struts, so we should go that way?

[Twentyover]

Do not misunderstand.A cleverly design Mac Strut will perform better than a poorly designed SLA. It looks like the suspension shown in you pic is a Mustang II- hardly a stellar version of a SLA suspension.

I'm not familiar with the rear IRS you are using, but it does have an upper and lower arm, so has the potential to work better than a Chapman strut (rwd rear suspension variant of a Mac Strut.) The argument that your car needs to be a Mac Strut front because the car the rear suspension came out of is full of flaws. (For the record-2015 is a modified mac Strut. A true Mac strut uses the sway bar for track arm longitudinal location.)

But I would be remiss if I did reiterate poor detail design can turn a good idea into crap.

[Badhand]

Good point re design. And don't worry about the existing setup, we're prepared to replace everything if neccessary. So the real question is, what's the best option here?! There are kits of all kinds and I have some very good connections with engineers to fabricate mounts and arms etc. But I just need reassurance that the most important part of the build is the best it can be...

The plan before doubts set in was to get rid of all of the suspension that is currently attached to the chassis, and replace with bespoke made lower wishbones (and mounts) that were double-adjustable with rose-joints on the inner mounts. This all hooked up to 2015+ Mustang steering knuckles and hubs paired with the best adjustable shocks we can get our hands on. Fab some top mounts and hey presto, fully adjustable Mac Strut front suspension.

Now that can all be adjusted for ride height, camber and caster etc, but is that 'clever' enough?

[Twentyover]

Depends on what you're satisfied with. As an example of my thinking, my Ford Capri avatar came with Mac Struts. I'm sure you've seen the stuff from Burton's etc- adjustable spring perches, adjustable track arms, compression or tension struts-sounds alot like your original plan.

Me? I'm replacing with a SLA. IMO you can't overcome the inherent failings of the lack of an upper A arm and it's abiliity to improve negative camber gain in roll

[Badhand]

Depends on what you're satisfied with.

I think that's the point. Add what I need and what I'd appreciate and benefit from into the equation, and the decision is becoming easier. A good friend of mine who's a very talented, experienced engineer summed it up the other day when he said frankly "You're not a pro race driver. You're not competing. I doubt you could tell the difference between the two systems if the Mac Strut was designed well and set up correctly." It felt a bit harsh, but he's probably right. His pragmatism has rubbed off on me over the years and I'm starting to come round to the idea of not what's 'best' in theory, but what's best for me in practice. Real World time.

I'm going to be driving this car to my limits not its limits, so for that reason I'm going to use that dreadful phrase from the business world - ROI / Return On Investment. And that's a simple, lightweight, adjustable McPherson setup with some top quality coilovers. I can make that happen easier and cheaper than going SLA.

But thanks so much for your input. Really appreciated Greg! I''l let you know how I get on!