McPherson Suspension: The Devil You Know

"Boss2452stolemylunchmoney" (boss2452stolemylunchmoney) 04/20/2014 at 10:37 • Filed to: None

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This is what the stock tires on a Boss 302 look like after after two seasons of autocross. Many of you may think that this tire looks pretty good and could easily be used for another two seasons. Well these PZero's are getting replaced by some Dirreza ZII's later this week. In the mean time, it does give me time to consider the 8 inches (200 mm for your European Jalops)l of good rubber still on these tires. Many of you are very happy that Ford has included a well designed IRS into the S550 platform. I, however, am very disappointed that Ford did not rid us of the real bane on our tires, the McPherson front suspension.

You see, with a McPherson suspension, the only camber gain you get is a a result of caster angles. When the body rolls onto the outside tire during a hard turn, the tire actually loses camber; the top side of the tire leans outward and, thus, the only part of the tire that does any work is the very outside edge (see the left side of the photo above). For normal daily driving, this is not a problem at all. It's less costly for the OEM and takes up less space under the car. But, when you take the car onto the track (you know, like you saw in all those Boss 302 marketing spots on YouTube??), you end up with tires that overheat very quickly and wear out almost as quickly.

The easy fix is usually to add caster/camber adjustment plates, but this comes with it's downsides as well. This, especially in combination with other modifications such as heavier springs, dampers, and raising the roll center (ball joints) is pretty effective at making the front outside tire much happier. However, all that happiness comes with consequences to the inside tire. Since we've added so much negative camber and the inside tire tends to gain even more negative camber, the very inside edge of the inside tire tends to be all that remains on the ground. So, more static camber isn't a good thing either. Here's a photo.

This is a Griggs Racing Cobra that still has the McPhereson front suspension. Look at all that space under the inside tire. Now below is a Griggs Racing Mustang with their SLA suspension. Take note of how much more inside front tire is on the ground.

With the number of companies out there offering aftermarket solutions for Mustang suspension, often costing less than $5000 for the entire kit, that takes it's on-track performance from snor inducing to world-class; it's a little hard to believe that Ford still hasn't taken the hint. Most of these systems don't even require bone crunching levels of spring rate to keep them happy on the track either, so it's not like they'd have to alienate the majority of Mustang customers who just drive their Mustang back and forth to the Hair Dressers. And, in case you were still wondering about the wagon axle, my rear tires still have about 50% tread life remaining across the entire tire. While the rear axle can use for some modifications, those modifications involve decreasing brake dive, lowering the rear roll center, and reducing pinion angle deflection. All of which can be done for less than $500.

So, in conclusion....I really hate McPherson suspension.

YSI-what can brown do for you > Boss2452stolemylunchmoney 04/20/2014 at 10:47

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Yeah, for my car at least a good amount of camber up front for track and autox set up is nearly -3 degrees of camber, with street tires obviously. While the rear(which is double wishbone) is -1 to -1.5. I haven't added anything yet. Although I should, the edges of the tires are wearing much faster because of this. The rears aren't as bad, but camber adjustment in the rear costs 500 bucks vs 16 for the front.

Opposite Locksmith > Boss2452stolemylunchmoney 04/20/2014 at 12:39

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very cool! Nice write up. I rarely learn anything here anymore

The Transporter > Boss2452stolemylunchmoney 04/20/2014 at 12:51

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It makes me wonder if an MN-12 T-bird's front double wishbones could be installed on a SN-95 or S-197 front end. Anything can be done with enough money, but I wonder how complicated it would be.

Boss2452stolemylunchmoney > Opposite Locksmith 04/20/2014 at 16:20

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Thanks!

Boss2452stolemylunchmoney > The Transporter 04/20/2014 at 16:22

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There are a number of companies that offer SLA bolt on suspensions for the Mustang. No need to cobble on an MN12.

phillipmp > Boss2452stolemylunchmoney 04/20/2014 at 20:48

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I have the strong impression you're an internet engineer. Your complaints aren't meaningful.

Auto-cross is not part of the S197's design objectives, particularly in stock form. In race trim the S197 chassis has done very well, particularly in NASA AI, AIX or ST classes. I'd like to point out that the 2013 AI championship was won in all three classes by MacPherson strut cars. They weren't griggs cars either.

You bought a muscle car and took it to autocross, and now you're disappointed by tire wear when moving around 3600+ pounds of nose heavy mass. It was a street/muscle car first, and all Ford did was add higher spring rates and more rear bar. To make your street car into the nimble race cars you've pictured will very likely compromise the street ability of it. I'd like to point out that Ford does sell what you're asking for; it's called the FR500-Boss302S .

The Macpherson front suspension design is used by BMW, including many of their chassis known for being the driver's car. The S197 front suspension borrows from that, and the S550 borrows even more.

You also touched on the relationship between Ford OEM cars and the aftermarket offerings. I don't need a crystal ball to show me the status quo has been ongoing for 5 straight decades. It makes sense when you consider the broad demographics Ford markets anyone of their cars to- the one exception would be the Ford GT. Note that the Ford is interested in profiting.

You have about a gazillion options, which boil down to keeping it stock or modifying it, or getting a totally different car that's more aligned with what you want to do. Note that the bowtie offerings are heavier.

I'll leave these here:
- /Drive on the '13 GT
- An option from Maximum Motorsports

A little disclosure:
I am a mustang fan. I have a '03, which started off much more poorly designed than your car. It's decked out with the MM catalogue, and I beat on it at autocross and track days.

Boss2452stolemylunchmoney > phillipmp 04/20/2014 at 22:58

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I'm an actual engineer with actual steering and suspension design experience. I'm not sure what you're on about. Are you defending McPherson strut on performance cars? Because my article points out why it's bad on a technical standpoint and your response merely rattles of talking points about cars racing successfully on McPherson. I don't want to race successfully on McPherson, I want to have fun in my car without paying a ton for underutilized tires.

phillipmp > Boss2452stolemylunchmoney 04/21/2014 at 00:55

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I am saying your original complaint makes really no sense , and it seems you should've picked a different car in the first place. You've generally mis-informed every next reader that comes here. Did the engineer in you not recognize what you were buying into?

Calling the McPherson setup bad is wayyy over-simplifying. The success I referenced show how McPherson works very well when the rubber hits the road. The only distinct advantage of SLA over McPherson is higher dynamic camber gain due to caster, at the expense of more weight and a few more joints. You might also point to fitting larger wheels, but in either case that's an exercise in geometry.

So yes I am defending McPherson suspension as a viable suspension for high performance motorsports or aggressive driving. The way Ford packaged it for the masses isn't enviable, but workable, hence the massive aftermarket support.

Anticipate spending money on tires, regardless of suspension design; they're consumables.

Bottom line is you got two seasons of autocross on those tires, which are generally expected to last 20-25k miles. If your two seasons includes driving on those tires the rest of the year, you did par for the course .

Boss2452stolemylunchmoney > phillipmp 04/21/2014 at 08:18

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You actually don't seem to know anything about suspension design. Therefore you're the one misinforming.

SLA has many, many advantages over McPherson, of which increased camber gain due to caster isn't one. That actually is the only way that McPherson does manage to generate low speed mechanical grip.

McPherson was originally developed as a way to package front suspension with front wheel drive. But, it also happens to be very cheap and for passenger cars the disadvantages are not great enough to cause concern. Camber loss is mitigated at low speeds by caster and stiction can be mitigated by fancy bushings.

But, given many front drive cars these days no longer use it, why does Ford, BMW, Porsche, and GM continue to use it on their performance cars? Asside from the fact that most customers of these high volume cars only drive them on the highway, i can't understand why they wouldn't seek to differentiate themselves with a better design that makes the car work better on the track. To me it is a baffling problem.