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SIIA 109 Springs


David Sparkes
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Series 2A, 109" Diesel.

Standard front springs have 11 leaves.

Standard rears have 8 +2 leaves.

HD rears have 8 leaves (presumably each leaf is thicker).

Using the Paddocks Spares site as reference, other series motors have 9 and 7 leaves, depending on model and application.

However, there seems to be a trend that front springs have more leaves than the equivalent rears, the 88" petrol has 9, for example.

So, what is the reason for 'a lot' of leaves at the front?

It can't 'just' be weight, as the HD rears get less leaves to carry a potentially greater poundage.

Just because it's Series doesn't mean it's Simple :-))

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Generally, the more leaves a spring has for a given rate, the more flexible the spring is, due to lower surface tension of thin leaves vs thick ones. A multi leaf unit would also have more inherent damping due to interleaf friction when compared to a spring with fewer leaves. landeys with standard leaf springs can be driven without shock absorbers even over some quite rough terrain due to the damping effect of interleaf friction, whereas an arrangement with fewer leaves, parabolics for example are very dependant on good shock absorbers in a similar way that coil sprung vehicles are.

Bill.

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"Generally, the more leaves a spring has for a given rate, the more flexible the spring is, due to lower surface tension of thin leaves vs thick ones. A multi leaf unit would also have more inherent damping due to interleaf friction when compared to a spring with fewer leaves."

That's pretty much where I got stuck, because it seemed like a contradiction.

Thinner leaves equals more flexibility.

More leaves equals more interleaf friction, equals more inherent damping, equals less flexibility.

If we postulate that the increase in flexibility outweighs the increase in stiffness caused by the greater inherent damping, then why is the increased flexibility required? Is it just to reduce the vibration / shockwaves travelling up the steering column? (And perhaps contributing to increased wear on the various ball joints?).

Cheers.

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I see your point but feel insuffiently qualified to answer your question. I always presumed that interleaf friction did not alter the springs rate. but as you point out it must do, at least initially when hitting a bump, which means the initial damping reduces flexibility.

One recommendation I might make when playing around with front springs is to try to select a spring with a thick main leaf. in the days when I had leaf springs I had some 3/8'' thick main leaves made, and built up the rest of the spring with thin leaves. This had the effect of controlling front axle tramp under power or brakes much better than thin main leaves did. It wasn't as good as links or radius arms of course, but the improvent in front axle traction/performance was noticable.

Bill.

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