Leaf Springs

[DannytheRadomski]

I know you're all recoiling in disgust at the title of this thread, but I digress.

So I've been called out before for my pie in the sky thinking on these forums a few times. My opinions on how a FSAE car should be built range from the extreme Keep It Simple, Stupid to the opposite extremely labor intensive and expensive, usually to problems that don't actually end up existing. This is on the extreme KISS side.

My idea is instead of using shocks and springs, what about pushrod actuated leaf springs. I'm not trying to come off as a complete idiot, but Corvettes use leaf springs, and so do the blade runners in the Paralympics. Since a lot of teams have their suspension mounted inboard, sort of near the centerline of the car; why not have carbon fiber blades in there, weighing less and taking up less space? If you had a football shape or an elliptical shape mounted longitudinally, it would take up almost no room and weigh nothing.

I'm sure there are a lot of kinematics I don't understand that will affect all of this, but leaf springs worked well for Ben-Hurr and Bullitt. Also, while it would be less tunable,the tuning would be built in, yet they would not be hard to manufacture, and with how simple I'm imagining this setup, replacing the blades for tuning purposes wouldn't be a huge loss.

And to all those guys who love the brown cart ideas, what's simpler than leaf springs?

[Big Bird]

Well I am not recoiling. Nice pun, by the way.
A leaf spring combines springing medium with wheel constraint in one direction - one part, two functions. Nice principle.
I'm not so sure about pushrod actuation. Why complicate things?
Keep on imagining, keep on creating, keep up the good work!
Cheers,
Geoff

[DannytheRadomski]

I just said pushrod for packaging reasons. Unless you did it with pushrods or like a Corvette, it would look weird have them over the wishbones.

Also, I wasn't even thinking of the pun.

[mech5496]

I believe that UWA's lower flexures as used on 2008-2009 (don't really remember the year) were the best way to combine springing (except in roll-mode) with wheel constraint. Think of a transverse composite leafspring replacing springs and lower a-arms... ;)

[Zac C]

I believe that UWA's lower flexures as used on 2008-2009 (don't really remember the year) were the best way to combine springing (except in roll-mode) with wheel constraint. Think of a transverse composite leafspring replacing springs and lower a-arms... ;)

GM ran this setup for decades on the rear suspension of the Corvette.

[Drew Price]

GM ran this setup for decades on the rear suspension of the Corvette.


Bill Cobb may chime in about this, but I was partially led to believe that this was to help keep the trunk space large enough to be useful, like, bag of golf clubs useful.

It definitely does have potential in an FSAE car though, it could probably be implemented very well.

[mrdben]

Google "Smokey Yunick Capsule Car". It was a car that Smokey built for Indy in the sixties. It used a transverse leafspring as the front upper A-arms.

[Zac C]

Bill Cobb may chime in about this, but I was partially led to believe that this was to help keep the trunk space large enough to be useful, like, bag of golf clubs useful.

It definitely does have potential in an FSAE car though, it could probably be implemented very well.

Corvettes didn't really have a trunk between 1963 and 1997. There's just some space behind the seats where you can stash things. Using a leaf spring vs a traditional lower control arm doesn't free up that much more real estate. It makes packaging a full size spare a bit easier, but I would think you could the same thing with dual a-arms (see 1984 corvette).

[DougMilliken]

Corvettes ...
The original Corvette independent rear suspension uses the halfshafts (fixed length on left and right sides) as the upper control arms. The transverse leaf spring was connected to the hubs through tension links on each end of the spring. I believe the wikipedia article is correct, http://en.wikipedia.org/wiki/Corvette_leaf_spring
> In the C2 and subsequent generations, a leaf spring is mounted transversely in the chassis and used in conjunction with several independent suspension designs. Common to these post-C1 Corvettes, the leaf acts only as a spring, and not a suspension arm or a link. Because it is not required to stabilize the wheels, the leaf functions in much the same manner as a coil spring.

There are suspensions where a transverse leaf spring is used as a control arm, check the photo in the middle of this article,
http://www.velocetoday.com/mike-hawthorn-part-2-by-denise-mccluggage/
Caption under the photo is, "Again at Charterhall in Scotland, Mike wins the Formula Libre event with his Cooper-Bristol ahead of Denis Poore's Alfa Romeo."

[mech5496]

I like the idea of using a leafspring in place of the lower A-Arms both front and rear, as long as there are no camber compliance issues. It seems much more integrated, using one part for two jobs, both locating the wheels and for springing. If I were to do it though, I would do it pretty much like UWA's lower 1-piece cf "flexures"....

[Z]

Danny,

Yes, transverse leafsprings are definitely a feasible and potentially good idea for FSAE (ie. lighter, more integrated...).

As noted above UWA used them for quite a few years integrated into their lower wishbones. The more radical UWA-2012 car had 2 x composite "W" leafsprings acting as both springing and location of the chassis to the unsprung undertray/beams/suspension.

Many production cars from the 1930s to about 1970s used them, mainly for front suspension in Europe, but at rear first, then all round (and still?) on the Corvette in USA. (My late-1940s Tatraplans have double-transverse-leafsprings (acting as wishbones) +R&P at the front.) Doug's Wiki link covers the pros and cons quite well, including Carroll Smith's positive endorsement.

For kinematic calculations, a cantilever leafspring (ie. clamped to chassis at one end) that is "L" long has its outer end travelling in an arc of radius about 3/4 x L (ie. it acts like a slightly shorter wishbone).

For material you could use Glass or Carbon Fibre Reinforced Plastic. Glass FRP has significantly better strain energy capability than good quality spring steel (ie. lighter for given load-deflection, or energy-storage, capability). Does anyone have the numbers? Have a look in Archery stores for inspiration, or watch "Arrow" on TV.

Glass FRP "pultrusions" (fibres dipped in resin then pulled through a heated die) are widely available in many different cross-sectional sizes. These often used for things like stiffening-battens for yacht sails. These would have to be machined to a taper for less weight and more even stress. Or you could lay-up your own from "rovings" + epoxy resin...

For tuning of spring-stiffness effects on handling (eg. LLTD) you could make the main transverse-leafspring/wishbone so that it is at the softer spring-rate end of the expected range, then add extra leaves on top of the main leaf to increase the rate (ie. like a conventional stack of leaves...). Or many other ways...

A Swing-Arm (http://www.fsae.com/forums/showthread.php?8950-Suspension-Design&p=45363&viewfull=1#post45363) suspension with transverse leafsprings would be very simple, low parts count, low CG, low cost, rugged +++...

Z

[dynatune]

The (lateral) leafspring is actually quite an intelligent solution and although the concept has it origins a 100 years ago it has ever since been investigated by many OEM's including recent years. Surely we will see some "new" applications on the old theme on production cars soon again. One other very interesting characteristic of a (single) lateral leaf spring that is mounted on the chassis with 2 points (like 2 lower links) is that there is a different "deformation" of the spring in parallel jounce movement, a U-type deformation and in roll motion an S type deformation (between the 2 chassis joints) with consequently different rates in heave or in roll. Now there is a new playground ... by moving the 2 chassis joints or limiting the vertical movement of the spring between those joints a whole tuning array becomes available .... it just needs to be worked out to the details .....

Cheers,
dynatune, www.dynatune-xl.com

[dynatune]

Using a leaf spring for controlling toe- and camber settings is a bad idea since it cannot life up to the current standards and with 99% probability interferes with the very sophisticated elastokinematic compliance behaviors of modern suspension Using a (single) leaf spring as a load carrying device is however a very good idea.

Cheers,
dynatune, www.dynatune-xl.com

[Z]

Further to my above post (and as noted by Dynatune), transverse-leafsprings were used on the 1907-1920s Model-T Ford, which was a reasonably successful car (sold about 19+ million! :)). The Tatra-Twin of early 1920s, possibly the first production car with independent rear suspension, also used transverse-leafsprings. Also common pre-1930 were "quarter elliptic leafsprings" (ie. cantilevered from chassis) for control of beam-axles (eg. used by Bugatti for both its racing and road cars, and others).

The current revival of transverse-leafsprings may be partly due to a perceived "advantage" when using them with two, wide-apart, chassis mounting points. Here the axle-roll-mode gives an "S-shaped" deformation of the spring, which is stiffer than the axle-heave-mode "U-shaped" deformation. So this acts like adding an ARB, but is cheaper because only one part needed (ie. only 1 x leafspring for two wheels, versus 2 x coils + 1 x ARB).

However, I would NOT advise doing this because while it stiffens the whole-car Roll-mode, it ALSO stiffens the whole-car Twist-mode. This is BAD! (One day the auto-industry might understand this... maybe...)

Better is to use the transverse-leafspring with a central-pivot to chassis (eg. a simple rubber-bush), so that it acts like a lateral Z-bar. These fitted front and rear will control whole-car Heave and Pitch-modes. Whole-car Roll and Twist modes can then be controlled separately, preferably with stiffer Roll-mode, and very soft Twist-mode. This covered at length elsewhere...

Finally, the compound Archery bows (http://en.wikipedia.org/wiki/Compound_bow) sold nowadays (these are the ones with pulleys and lots of string...) have a rigid handle-centre-section, and shortish, bolt-on, composite (GFRP and/or CFRP?) leaves at each end. These leaves are about the right size (load and deflection) for an FSAE suspension, given the right MR is used, and possibly with doubling-up of the leaves. These would give neat, lightweight springing, available off-the-shelf, at ??Chinese?? prices, capable of replacing a suspension link, requiring only a single attachment bolt at the chassis-end (for quick swap), and with ready made clevis for a BJ at the wheel-end. Worth looking into...

Z

[rjwoods77]

Z,

I sent a message to your personal email about this topic since your FSAE.com mailbox says it is too full to receive messages. Also here is a nice writeup with diagrams about what you referred to with the Corvette...

http://en.wikipedia.org/wiki/Corvette_leaf_spring

...there are patent references at the bottom for using the spring as a lower control arm as well.