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Cobra suspension - eliminating lift/squat
Those who have been following my build will know that I recently had work done on the front suspension to allow proper adjustment of camber and caster, and to eliminate rubbing on the front guards. Now that the geometry is fixed I'm trying to sort out the spring rates and damping.
I drove the Cobra on the weekend for the first time since Winton, and with the QA1 shocks set to 2 for compression and 5 for rebound. Cruising along Melbourne's roads, this feels sporty but comfortable - about the same as my little Volkswagen GTI. I'm pretty happy with everything in that regard. The problem is when I accelerate hard the rear end squats and the nose lifts - my mate who was in the passenger seat likened to to being in a speed boat. I can eliminate a lot of this by increasing the compression setting on the rear shocks but I lose compliance and the ride becomes bone jarring - as it was at Winton. What advice do people have for striking a balance between comfort and "body control"? I have McPherson struts at the front and Nissan IRS at the rear. Increase the rebound settings at the front and beefier springs at the rear? Different valving in the shocks? Is there any way to approach this other than trial and error? |
Pinion angle have anything to do with this? Not sure, just asking those who may know.
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Your car is doing exactly as you want it too!
Weight transfer is what you want, extra weight on the rears makes them stick. You could stiffen it up but then it won't hook up. I set mine to transfer as much weight as possible for launch, it hasn't hurt it's performance ;) |
Yee Ha
http://i1081.photobucket.com/albums/...psc3c178b4.jpg As long as your front wheels still have good contact with the road a little squat in the rear end is good for getting the power down. |
Gav is right. Weight transfer is your friend. As long as it's cornering OK just drive it. Suspension setup is a compromise which ever way you set it.
Big HP in a short wheelbase car will make it move around a bit. Think of your car as a lever with the pivot at the rear wheels under acceleration and at the front wheels under braking. The shorter wheelbase means the motor more leverage to lift the front and you have a lot less weight up there than a conventional car. You also have 50% or more of the cars weight on the back wheels. You could try adding more rebound damping on the front and force the car to try and lift the weight of the wheels as well. Or just jam a tennis ball in your air intake. That should fix it. |
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That picture just doesn't look right. We have a few members car's that can really do that though.
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I might make a few more adjustments to the damping. Those QA1s seem to be designed for a heavier car - every click makes a noticeable difference. If I experiment with higher spring rates on the rear, where would I go from 250lb? Maybe 350? |
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Thanks deano, good advice.
I had to preload the back coilovers by about 40mm to get the desired height so I wonder what the effect of increased spring rate will be, assuming the same amount of preload? I found a good explanation of coilover spring preload, will come back to it when I have a coffee in hand. |
Is this the type of lift you want????
Sambo
Perhaps this is lift you need?? 200kgd-lift_zps975f5390.mp4 Video by slowy59 | Photobucket A good portable model for tyre changing at least. My youngest bloke at a cancer charity support mini competition last year. I've just got into a new program at the gym which includes deadlifts. Did 75 kg this morning and expect to get more but not another 125 kg - DoH Enjoy....the back pain - ha |
Just some payback for that useful advice of yours on jag suspension mate - lol
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Sambo, angle of the rear arms(front chassis mount point higher, rear chassis mount point lower) determines the amount of anti-squat with some other factors. Just have a look at your rear end and see if it has any built into it, it will be obvious. I wonder if your cradle is mounted at the same angle as in the donor car. Is there any off-the-shelf mods available, maybe make some spacers for that cradle? If you haven't done so, maybe some searching of appropriate forum might give an answer.
We have built a fair degree of anti-squat into the PACE 427 as it has a slightly better than 50:50 weight distribution to the rear. With the weight(and strength) of those Nissan rear ends, and your light GM engine, your Cobra should be slightly rear heavy as well. We had to do it by moving the engine, far, far to the rear. |
Thanks Slowy but I'm not sure about driving around with your son in the boot of my Cobra. :D
Hi Craig. Interesting info, thanks. Can't get my head around it right now, does that cause a degree of binding in the control arm bushes or because the arms no longer move in a plane that is perpendicular to the ground? |
A couple of background questions: does your car have sway bars? How is the handling now: over/understeer?
If you increase the rear springs you will increase oversteer, that you could then adjust out with stiffer front sway bar and/or stiffer front springs. If your handling is where you want it, increasing both the front AND rear springs by the same percentage should stiffen the ride but leave your handling under/oversteer unchanged. I haven't played with it a ton, but certainly form my lmited experience you can raise OVERALL spring rate quite a bit without changing ride harshness as much as you might think. If you are just adding spring to one end, you would have to do that in much smaller chunks. For example: My cobra was VERY soft, I raised springs 50% both front and rear: handling balance stayed as it was prior to the change, ride stiffness went well up, but not nearly as much as I thought. For example, front springs were 350 lbs/inch, new ones are 525. I like the idea of trying to get a bit more anti-squat if you can. Not sure what other geometry issues that might cause (i.e. toe in/toe out change under compression etc). |
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You can adjust how the rear end reacts by using 'Pineapples-(well that's what they call them -maybe cause they're yellow :confused:) and they go either in the front or the back of the rear cross member depending on how you want the vehicle to launch or turn in etc. I'll try and find some more info for you tonight. Rog |
Hi Mark, the car seemed pretty neutral to me at Winton, compared to my late model GTI which tends to understeer when grip is exceeded (by design). On the two occasions I spun the Cobra, I was going into a corner too fast and touching the brakes - I had too much rear brake bias which caused a momentary lockup that spun the car around. Would you call that oversteer?
Body roll isn't too bad in the Cobra but I don't have sway bars to compensate as you have suggested. I suspect the Nissan IRS will toe out under squat but on the 2-3 occasions where I've given it heaps from a standing start, axle tramp is pretty bad. Not enough damping perhaps? This really makes me appreciate the R&D that goes into a well sorted car. I'm hell bent on getting my Cobra to handle like that. Time is on my side. :D |
yes, the R+D is what makes this fun! Mine was a nightmare when I got it: basically hung a hard right when I hammered it, and bounced along like a pogo stick- its getting good now, but I think it can be really good, just one thing at a time as time/$$/brainpower allows!
Compared to many car related things, springs are relatively cheap and easy to swap out. If yours is balanced now under steady throttle, braking doesnt count :) if you add springs by equal percentages for front and rear, you can end up with ones that you can swap from front to back, making it even cheaper. For example, I'm going to try a bit stiffer ones- I will move my fronts to the rear, and just buy new fronts. Currently my fronts are 24% stiffer than the backs, so I will buy new fronts that are 24% stiffer. |
I wish mine had equal length springs Mark, unfortunately 10" front and 12" rear. Also springs are rather heavy and freight to Australia isn't that cheap any more (post GFC). But as you say, they are still relatively cheap compared to other components.
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