Leaf Springs

[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 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 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.