Vespa front suspension

Vespa_PX200_01By Kevin Ash


Pictures: Kevin Ash, Piaggio Press



Go back 60 years from the Vespa 300GTS and one of the many design consistencies across the decades is the unique front suspension. The fact that it’s single-sided is rare in itself, as only the Gilera CX125 in recent history also had a single-sided front. The Gilera’s was a telescopic leg above the wheel, and like the Vespa’s was based on aircraft design. Vespa suspensionNo real design change in the last 60 years
The creator of the original 1946 Vespa was Corradino D’Ascanio, an aircraft designer during WW2, and the front suspension on this (the same as today’s Vespas in principle) took its cues from aircraft practice. Where most axle and linkage designs use a leading link (such as Earles forks or the leading link on Honda’s C90/Cub series), the Vespa has a trailing link. The fork leg extends down and forward from the steering head to a point in front of and above the axle. An arm pivots on the end of this and trails back to the stub axle.

The suspension duties are taken care of by a standard shock absorber mounted at the bottom directly on top of the axle (which gives it very good control of the axle movement), and at the top to a point just under the steering head where it meets the top of the fork leg.

At first sight though this doesn’t look like a very good idea as you’d imagine under any sort of weight transfer to the front wheel, ie under braking, the suspension would compress substantially. But the front brake caliper is mounted at the bottom of the shock absorber, and when the brake is applied, like any brake caliper it tries to rotate forwards around the wheel. Vespa_PX200_03The suspension does swop sides with different models - this is the PX200, last of the manual gear two-strokesIn doing this it pushed the bottom of the shock absorber forwards, and that has the effect of forcing the trailing link downwards, countering the weight transfer pushing it upwards. By altering the angle of the trailing link you can dial in varying degrees of natural anti-dive in the system, to the point where it will rise under brakiing rather than dip.

The advantage of this system over a leading link design is that the wheel moves backwards at the same time as it moves up when it hits a bump. This gives a more compliant ride and is easier on the suspension components as well as the rider, especially on post-war Italian roads. Or modern British ones...

On a low powered machine like a Vespa the twisting forces of the asymmetric layout don’t affect the handling badly, and when punctures were frequent, the facility to remove the front wheel quickly and easily was very useful.

Expect Vespa to continue using this for the next 60 years at least...


shuggiemac
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Joined: 23/11/2008

So what is the down side of the design on machines of this style and power? I only ask as it seems to be mainly plusses and we must then ask why the other manufacturer's have not followed suit?

kevash
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Joined: 05/10/2008

Rigidity's poor, which doesn't matter too much when speeds aren't that high, and when it does flex the wheel doesn't stay in the same plane as it does with twin forks, which compounds the problem. But it's also a case of this being closely associated with Vespa, which is another reason others don't copy it, and why Vespa continues using it.
On modern Vespas the wheel change facility is not so important anyway as punctures are rarer these days and Vespas no longer carry spare wheels either.

pittsy
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Joined: 06/08/2011

On the face of it, the basic idea of trailing link seems to offer some virtues. Anti dive directly related to braking forces plus compliance to bumps. If it were a double sided arrangement instead of single sided, what are the reasons we don't see it on larger machines I wonder?

Also, when the brakes are applied is the suspension still reactive or will it stiffen up?

Markyboyzx6r
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Joined: 30/11/2009

I have a Vespa LX125. Ride 50 miles per day in and out of London. My front wheel isn't straight. It tilts to about 10 degrees off upright. It's never been crashed or pumped so I don't know why it's doing it, but I bimble along quite happily. It can't corner for toffee but then again, it's a Vespa.

100% reliable, though.