Suspension Question

"SteyrTMP" (SteyrTMP) 10/14/2013 at 19:54 • Filed to: Suspension

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Edit: No answers? Fine, I'll ask the evening crew.

Ok, this one is slightly above my suspension knowledge. The Seven has 450lb coolers in the front, worn-out, cut OEM Miata springs in the back. I was mocking you'll the rear fenders, and realize that the suspension will have to be limited go two inches of travel. It can have all the droop in the world, but after two inches of travel, it will contact the fenders. I know that the cheap and amateur way is to put thick neoprene bump stops so it'll run on the bump stops, but what's the correct way of fixing this? I'm looking out for a set of mildly used rear adjustable coilovers for the Miata.

BiTurbo228 - Dr Frankenstein of Spitfires > SteyrTMP 10/14/2013 at 20:13

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Maybe very progressive rate springs so they lock up and increase the stiffness of the spring. Sounds like it'd be difficult to do with just 2 inches of travel though.

I know it might be sacrilegious, but could you raise the ride height?

Bird > SteyrTMP 10/14/2013 at 20:20

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I'd go with bumpstops (or coilovers with them integrated). Try to find (or make) a progressive bumpstop.

My daily Miata is pretty low, not scrape the frame on the arch of the road low, but low. The bumpstops are a very integral part of my suspension! When I have the ride height dialed in just right, it basically works just like a progressive spring, only the progression is to a much stiffer bumpstop. If I have the car too high, I can feel it hit the bumpstop, and the progression isn't nearly as linear.

But setup correctly, I think a good bumpstop would solve your problem, and it wouldn't be a 'hack' way of doing it...

Ramblin Rover - The Vivisector of Solihull > BiTurbo228 - Dr Frankenstein of Spitfires 10/14/2013 at 20:20

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If it were even so much as 4" of travel, I'd second the progressive rate suggestion. Since he only has 2", though, it might almost make more sense to just go with a chopped coil out of a much heavier car and just ride like it's on rocks all the time.

Ramblin Rover - The Vivisector of Solihull > SteyrTMP 10/14/2013 at 20:23

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I'm taking it that the fenders can't ride the suspension? Really, really hard springs are about the only answer at only 2" other than your bump stop tomfoolery idea, I'm afraid, unless you want to change mount points to something deranged or otherwise make a more progressive rate response than anybody actually makes: progressive springs + progressive geometry.

BiTurbo228 - Dr Frankenstein of Spitfires > Ramblin Rover - The Vivisector of Solihull 10/14/2013 at 20:24

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Yeah, that's not a bad idea. Make it trickier to keep traction though. Softer springs tend to be better for that.

He could retrofit hydrolastic suspension. I'm fairly certain OG Minis had sod all suspension travel.

davedave1111 > SteyrTMP 10/14/2013 at 20:30

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I'm not an expert, but aren't you supposed to have stiff enough springs and damping that you don't use all your travel?

Also, do I understand you to say that you've lowered a Lotus/Caterham 7? What was it, did someone bet that you couldn't make a 7 less practical?

Ramblin Rover - The Vivisector of Solihull > BiTurbo228 - Dr Frankenstein of Spitfires 10/14/2013 at 20:30

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This is a genius idea.

I was thinking the only way he could get a progressive rate to "hard as nails" in 2" from anything sane would be... I dunno, fitting bellcrank/relay arm rear suspension with acute angle upper compression, but hydrolastic would actually work, if for nothing else than it has bump stopping built in and fluid buffered.

BiTurbo228 - Dr Frankenstein of Spitfires > Ramblin Rover - The Vivisector of Solihull 10/14/2013 at 20:36

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I was thinking something along the lines of a bellcrank with one arm 2x or more longer than the other. That way he could have 4" of spring travel at least for his 2" of wheel travel, and use a more normal progressive rate spring.

Ramblin Rover - The Vivisector of Solihull > BiTurbo228 - Dr Frankenstein of Spitfires 10/14/2013 at 20:44

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This is also true: a standard progressive rate spring rolled by a bellcrank with, say a 2" arm/4" arm and 2" travel would pass from ~110 degrees vs. the actuator rod to the wheel at rest to around 20 degrees at full compression: that's cutting mechanical advantage to around a third, thus the progression of the spring (pushed on the other side over an evenly split 90 degrees) would be tripled on average, despite being softer for the first few bits of exxtension.

SteyrTMP > davedave1111 10/14/2013 at 21:08

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It's a Locost, I made it. It rides low, about 4-5" in the back, maybe 4" in the front (2.75" from oilpan to ground), but the fenders I made don't have a lot of clearance. With the original Miata rear springs, the back sat almost 8" high. Maybe on a 80's Camaro, not on a Seven.

davedave1111 > SteyrTMP 10/14/2013 at 21:21

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Sounds awesome. If it's that low already, I guess you can't just raise the fenders and use the extra travel.

With only 2" to play with, I'd start wondering about alternatives to a conventional setup. By the time you've set it up hard enough to keep you off the bump stops, it'll barely move at all. For something as light as a 7, you might get better handling and traction as well as a more comfortable ride doing something like setting the back end up completely solid and running fat tyres. Or at least forget about springs, and shove in a couple of big chunks of elastomer.

Then again, I have no idea what I'm talking about right now.