Sidecar Suspension by Design

It’s on old problem with sidecars—the traditional shock absorber can’t handle the extremes—it’s either too hard or too soft. The solution is a French-made and designed Fournales shock absorber.

THERE ARE several issues with riding sidecars all associated with the fact the sidecar is attached to one side. In Australia it’s on the left side.

Firstly, when you accelerate, the sidecar drags the bike to the ‘left’, so you have to countersteer to the ‘right’ to keep it travelling in a straight line; and when you brake, the sidecar tries to overtake the bike so you have to countersteer the other way. All very interesting.

But the biggest problems come with corning, especially if you’re pushing the envelope through the twisties. 

When you turn left, centrifugal force throws your weight outwards, to the right, taking all the weight off the sidecar wheel. Get it right and the wheel just skims the road surface; get it wrong, the wheel lifts and there’s the very real possibility the whole rig will tip over.

Conversely, when you turn right, all the bike’s weight, and yours, is thrown out onto the sidecar wheel, compressing the single shock well beyond its design limit; as the sidecar dips, the motorbike itself leans outward, away from the corner, making the steering extremely heavy. Add a passenger to the sidecar and the problem gets worse. This time, if you get it wrong, the rear wheel of the bike will lift and the sidecar nose will dig into the bitumen.

Oh well, you know what they say: you never know what’s too fast until you get there.

Installing a heavier spring into the shock absorber helps in right-hand corners but then the suspension is too harsh on left-handers (the whole sidecar bounces up out of bumps) and when you’re riding in a straight line. The problem is that the traditional shock absorber just can’t handle the extremes of the sidecar. It’s either too hard or too soft.

Talking to Bertrand at Fournales Australia about the problem, he suggested I draw up all the specs of the sidecar swingarm (length, angles, etc) and he’d send them off to Fournales in France and have them design a shock absorber especially for my application. Apparently they’d had a lot of experience designing shocks for sidecars. How could I refuse?

Six weeks later the new Fournales shock arrived. It’s a good looking unit, polished with a black anodised cover, and with no spring (it’s completely dependant on air pressure), it’s a lot thinner than the original shock.

Fitting was a breeze: it was the exact right length and the bolt yokes at either end were the exact right size; 15 minutes max.

Not sure what air-pressure it needed, I rang Bertrand for advice. He told me to try different pressures; I’d know it when I found the ‘sweet spot’.

I started with 15 bar (218 psi) but it was definitely too hard. I dropped it down to 13 then 12 bar (174 psi) before I found the ‘sweet spot’ he was talking about.

The Fournales shock was so ‘sweet’ at 12 bar that riding in a straight line you could easily forget you had a sidecar attached; it soaked up the bumps beautifully; the sidecar just floating along. 

On hard right-hand turns, it compressed less than half of the old shock and continued soaking up the bumps with or without a passenger in the sidecar. With the sidecar dipping less, the steering was considerably lighter and more precise.

On left-hand turns, with minimum weight on the sidecar wheel, the Fournales shock continued to soak up the bumps beautifully.

The reason an air-shock works so well is because when you reduce the volume of a cylinder by half, the pressure inside doubles and keeps on doubling exponentially. Progressively wound springs try to do the same thing but aren’t really in the same ball-park.

I can’t speak too highly about the Fournales shock absorber on the sidecar. It was designed to handle the extremes of the sidecar and does it with ease. No matter how hard you work it, it just works better. My only complaint is that it made the rest of the bike’s suspension feel like shite.

If you have a special project, it’d be worth considering having a Fournales suspension designed for your needs. They are expensive, and you have to beg, borrow or steal a special air pump (the service station air hose just won’t cut it), but you won’t be disappointed with their second-to-none performance.

One Comment

  1. Just wondering about the air shock for my sidecar the shock I have on there now is 260 mm centre to centre , it is much to stiff wondering if you can help me out with one of you air shocks ?

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