Leaf Spring Suspension 4

[WolfHR]

A friend and me are toying with ideas for a small, underpowered city car, and I'd try to do a cheap low-tech suspension (but hopefuly with adequate performances)... Seeing there are quite a few very knowledgeable members here, I'd be grateful for your opinions on the concept.

I was thinking of using a strut suspension, but seeing how coil springs are often not coaxially mounted on the strut, I thought of relieving struts of the task of providing the springing force (only guidance and damping) and replacing control arms with a leaf spring supporting left and right wheels. Should it need restrainig, I could use MacPherson type of restraint (with ARB linkage). Are there any obvious or inherent faults with this design that I'm missing?

I'm also a bit concerned that lateral loads on wheels might contribute, even significantly, to the deflection of the spring (seeing how spring will serve as control arm and hence support almost all lateral loads), and that it should be carefully modelled when designing the suspension (not that I know how, most of the stuff I've seen deals only with vertical loads*)... Am I correct in assuming so?

Thanks in advance.

* I should do some digging at my Uni, they're bound to have some good books on the subject of leaf springs there... but considering the manner in which we parted ways, I'd best use a proxy for that task

 

[Tmoose]

Austin Healey Sprites used quarter elliptic leaf springs to support and locate the rear axle

http://www.spridgetguru.com/Autocar052958.html

Peugeot used a transverse leaf spring clamped in the center as a locating member for the IFS on 403.

AC/Cobra did the same for the front and rear suspensions for some models

 

[WolfHR]

Thanks Tmoose- actually, my idea came as a hommage to my current tiny car (Fiat 126p) which, as I recall like its predecessor Fiat 500, used leaf spring as lower control arm in SLA suspension... So I thought to go one step simpler/cheaper...

 

[GregLocock]

It is a perfectly doable suspension, in fact I raced such a beast 4 years ago.

http://www.worldsolarchallenge.org/files/58_wsc_christine_2007.pdf

the lower member there is a leaf spring.

Yes I think for a city car it should work. Packaging is the issue, your leaf spring is running through the diff gearbox or sump, I suspect.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BrianPetersen]

The design that you are proposing - basically using a leaf spring as both the spring and the lower control arm - was used in many small production Fiat cars for decades. The original Fiat 500, 600, 126 all used this type of front suspension. Those were rear engine cars, so the front suspension didn't have to deal with clearance around the drivetrain.

http://126bis.de/en/imgwin.html?../sw/bis-44-15.gif

http://126bis.de/en/imgwin.html?../sw/bis-44-06.gif

 

[gt6racer2]

Wolf,
Apart from the spring packaging that Greg mentions, a second concern with your idea to use a strut with such a spring will be the conical compliance of the spring end bushing. In the SLA the upper arm prevents undesireable rotation of the knuckle - but with the strut you'll lose that constraint.

 

[WolfHR]

Thanks for comments, you're being most helpful... (As an aside- great pics, Brian, and Greg- Christine and her suspension look great)

Yes, the packaging will be even more of an issue (the idea was mentioned about possibility of at least two drivetrain configurations- either individual drive by 2 PM flat pancake motors, or one motor, or mechanical KERS, driving both wheels through the diff).

GT6Racer2- actually, my initial idea was to bolt the ball joint to the spring rather than to use 'eyes' and bushings. In end-effect it wouldn't matter because the concern you express would be translated to torsion/twist of the spring itself... Speaking off top of my head, maybe both concerns could (perchance!) be aleviated by using two springs in paralel- to straddle the drivetrain/motors and provide good and stiff LCA.

 

[gt6racer2]

Sorry - was automatically thinking rear susp Vs front - natural tendency for me as the GT6 has the leaf in the back!

 

[BrianPetersen]

The Fiats dealt with this using a suspension upright that did not turn with the steering (the two-piece upper control arm locates it and prevents it from rotating in that direction, and it's bolted at the bottom through the eye on the spring) and a separate king-pin that did the steering. If you look at the exploded view that I posted before, it's clear how they did this, and it's rather simple. Using a ball joint on the bottom sounds like a recipe for too much flex, because the ball joint pivot cannot coincide with the end of the spring.

You could use a king-pin design with MacPherson also, but the spring on its own probably won't give good enough anti-rotation of the strut. A strategically positioned radius rod could serve that purpose. Or, if you arrange it so that the pivot axis of the king-pin passes directly through the eye of the leaf spring dead center, anti-rotation of the strut won't matter so much because the various forces at the contact patch won't impose any moment arm that would cause it to rotate, and if it does rotate, it's on the same axis as the steering anyway, so it won't matter.

 

[loki3000]

there are some city cars with scooter engines available in europe, that don't require a driving permit (for cars), only for scooters. check them out, how they are made...

they are exactly what you mention, a "small, underpowered city car"

 

[calixtocapi]

Hello there !

I'm new in this forum: my name is Calixto and I'm from Spain.

I race competition sopaboxes and we use a double leaf spring system that combines A-Arm and spring functions. It's simple, light and reliable, considering that our maximum total weight is 444lbs, max speeds around 80 mph and max lateral acceleration around 1,3G

This winter I'm working on the damping system, inspired on FSAE cars

You have more info ( in spanish ) at:

http://www.zonagravedad.com

I hope it helps.

 

[Tmoose]

handsome little car

1.3 gs? on those tires?
banked turns?

 

[calixtocapi]

Yes, but 1.3G is max peak. Sustained lateral acceleration are usually around 1.0 - 1.1 with good asphalt on standard roads.
We use these 12,5 x 2,25 on slow tracks with lots of hairpins ( lower moment of inertia for slightly faster acceleration). In fast tracks we use 16 x 4 tires ( as shown in this picture of myself in Spanish Championship )

Our Center of Gravity is lower than motorized single seaters. Suspension travel is just around 1 - 1,5 inches.