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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

RE: Dual Rate coil springs

I think that dual-rate springs work as you have described. ... but to be sure that your assertion is correct, check that you can't get even a thin feeler gage between the coils at rest.

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

If the softer coils are already shorted out at kerb weight then you will get no benefit from them in normal driving. Now, that may be deliberate, sometimes manufacturers fit a soft tail to a hard rate spring so that it doesn't discombobulate at full rebound, but that still leaves you with the 530 rate instead of 330 in normal use



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Dual Rate coil springs

(OP)
Mike,

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

That 530 rate is nearly 2.5 times the rate of the OE springs, which rather strongly suggests that at least moderate competition is the intent. Under such conditions you might not want to be constantly switching between rather disparate rates during bump travel. Chances are that the dampers are valved for the stiff rate (and have no way of knowing when the transition between the rates happens anyway).


Norm

RE: Dual Rate coil springs

FWIW, it seems to be fairly common for aftermarket springs to be advertised as "progressive" when in fact they're dual-rate with the higher rate being the "working rate" over much/most of the suspension travel as yours are. The rate transition then happens at some rebound position and happens at the wheel that's momentarily carrying less load and should upset the chassis less.

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

(OP)
Norm,

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

As others have said you can't tune your shocks to dual rate. Once you compress the bump stop you pretty much have uncontrolled rebound. This makes for the handling characteristics of a Pogo Stick.

Regards
Pat
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RE: Dual Rate coil springs

It is tempting to go down the progressive rate path with coil springs, particularly if you have rear leaf springs (the truck I am working on does) that give you a progressive rate at the rear. But, for road use you end up with an overly damped ride unladen, and there are also noise problems from coil clash.

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


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Dual Rate coil springs

Once you compress the bump stop you pretty much have uncontrolled rebound.

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

The shock would put up a bit more resistance compared to when it's only damping the road spring, but the bump stop may be an order of magnitude stiffer. In terms of % critical damping, "blown out" might not be such a bad image after all.


Norm

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