Dual Rate coil springs
Dual Rate coil springs
(OP)
I've just fitted a pair of new aftermarket dual-rate coil springs to the front of my 60s Triumph. It's a double wishbone, coil-over-damper setup and the original spring rate was 220lb/in. The new springs are rated 330/530 lb/in and are considerably shorter than the OEM ones. The length isn't a big problem as the dampers have adjustable seats, so I've been able to maintain the previous ride height.
However, once fitted, I find that even before driving the car (just rolled back and forth to settle the suspension), the relatively short, close coiled 330lb section is fully compressed, with the coils in full contact. To me this means that the spring is already in "hard" 530lb mode at rest. I would have expected to see some gap between the close coils at rest with the higher rate only coming into to play when the suspension is working reasonably hard? The supplier insists they are correct.
Am I misunderstanding the way these are meant to work or is the supplier fobbing me off? What I can find via google etc (not a great deal) seems to be saying I'm correct..... but confirmation either way from someone who knows would be nice!
Thanks
Nick
However, once fitted, I find that even before driving the car (just rolled back and forth to settle the suspension), the relatively short, close coiled 330lb section is fully compressed, with the coils in full contact. To me this means that the spring is already in "hard" 530lb mode at rest. I would have expected to see some gap between the close coils at rest with the higher rate only coming into to play when the suspension is working reasonably hard? The supplier insists they are correct.
Am I misunderstanding the way these are meant to work or is the supplier fobbing me off? What I can find via google etc (not a great deal) seems to be saying I'm correct..... but confirmation either way from someone who knows would be nice!
Thanks
Nick





RE: Dual Rate coil springs
My own experience with aftermarket dual-rates was on the rear of a big ol' Impala, when Impalas were big cars. Those springs were quite a bit longer than the OEM springs, and I think (without knowing the actual numbers) that their initial rate was lower than the OEMs' rate.
I'm wondering if what you bought was intended for a different model that's lighter in OEM trim, or has been lightened for racing, or normally has supplemental springs fitted.
... or if they're really intended to limit rebound deflection associated with aerodynamic or cornering forces.
Mike Halloran
Pembroke Pines, FL, USA
RE: Dual Rate coil springs
Cheers
Greg Locock
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RE: Dual Rate coil springs
Thanks. I can confirm that the coils are in full contact - no feeler gauge going in there - I tried that!
The car is a Triumph Vitesse, with a heavy 6 cylinder engine. There is a 4 cylinder version of it (which is the Herald) so I suspect you are correct about the spring being for the lighter version - though the supplier denies this. The GT6 (6 cylinder) and Spitfire (4 cylinder) are the sports car versions and very similar mechanically.
Greg,
This is sort of what the supplier is now saying, that they should be compressed at rest. Myself, I was expecting (from his own description)the 330lb rate at normal running with the 530lb coming in only on bump and spirited cornering. This would be an advantage as these cars don't have much suspension travel, especially when already lowered a bit. However, with 530lb all the time on a separate-chassis convertible, I think most suspension will be by chassis-flex!
I suppose that calculating the correct number and particularly, the spacing of coils on a dual rate spring is relatively tricky?
Will drive the car (once the snow has gone) and see how it is....
Thanks again
Nick
RE: Dual Rate coil springs
Norm
RE: Dual Rate coil springs
A design priority on ride would perhaps have put the transition well into bump as you suggest - or been truly progressive with continuously variable coil spacing at the closely spaced end and no step change in rate. In any event, I'd expect softer to much softer rates if this was the intent.
Norm
RE: Dual Rate coil springs
530lb will be far too much for this car in road use, which is mostly where it gets used. Hence the dual rate choice and the hope that the higher rate would come in only fairly close to full bump. The dampers are adjustable but your observation about the transition point is well made. Your observation about putting the transition point somewhere in the rebound area (which is where mine will be) is also interesting. Rather negates the point of dual rate though I would have thought.
Would have liked proper progressives - not available for this car (unsurprising, it's 45 years old) but what becomes obvious is that the engineering of dual or progressive rate springs is a little more complex and getting the transition point/zone in the right place is important.
Have a feeling I'll be going back to stock 220lb/in or maybe fixed rate 330lb/in and trying a longer, more compliant bump-stop.
Thanks
Nick
RE: Dual Rate coil springs
Regards
Pat
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RE: Dual Rate coil springs
Incidentally your two rates look suspiciously like 275/550, which means that only the coil spacing varies, not the wire size or coil OD. The spring rate equation is easily found on wiki for example if you want to check.
Cheers
Greg Locock
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RE: Dual Rate coil springs
This makes no sense. The shock still has rebound resistance - unless it is blown out
Unless I don't understand your comment.
Jim
RE: Dual Rate coil springs
Norm