GT40NZ spring rates

[Kevin Box]

Guys

Can you give me and idea of spring rates you are running for road cars.
The suggested spring rates for GT40NZ chasiss (Similar to GTD ) is 350 front 450 rear.
These settings are for 302 block and I will be running 351W block which is about 70 lbs heavier.
I am about order spring so would appreciate comments.
My preference is to go on the light side to avoid skipping on bumps etc.
The other thought is to go with progressive rate springs as in the QA1 listings which would give good ride quailty but firm up when increased load is on

I'd appreciate any comments.

thanks

KB

 

[Andrew Robertson]

KB,

AFAIK the GT40 suspension set-up is virtually identical to what I have in the McL canam replica. My chassis came from Autotune with springs that they had found to work well in racing, and they indicated that they were on the soft side for a track set-up - i.e. a good place to start - i.e. might go up a bit in rate later on depending on driver confidence/ability and track etc. They are 450 rear, 400 front, non-progressive, run in conjunction with adjustable front and back bars, vehicle weight probably around 800kg dry. This was what they ran on their own car which they campaigned for a number of seasons running SBC and hewland.

I am not running yet so can't give you first hand experience but your own numbers are close to these.

Cheers, Andrew

 

[Neal]

I'm running the 350/450 springs and Koni shocks on my car. Car is 2425lbs w/ fuel. Ride quality is firm but not harsh on the street. I run 15" tires that are 60/50 series aspect F/R. It's a good all around setup.

 

[Kevin Box]

Thanks Neal

I will be running 17s (315/35 rear 255/40 front) which have a reasonable profile so should not be too bad.

I looked at the progressive rate springs on the QA1 site but the information does not allow you to do a good comparison so i will probably run with 350 /450

Since you have the same frame can you do a couple of checks for me.

1 Check the length of the shock at rest if possible can you tell me extended length and compressed lenhth of the shock

2 Can you check the uppper and lower locating rod lengths (trailing link) as installed. The ones supplied in my kit of bits look like the rod ends are not engaged enough when the wheel is in the correct place relative to the body. Probably the best way to measure this would be from the centre of the bolt where they attach to the frame behind the driver to the front of the upright casting That way there is no allocation needed for different clevis or rod lengths

Thanks in anticipation

Kevin B

 

[Howard Jones]

GTD (aprox 2500 lbs) with, QA1 shocks single adjustable 14" extended, 350F 450R springs, 245X40X17 front, 315X35X17 rear on 8s and 10s wheels, standard 5/8 GTD front bar with 3 inches of torsion arm adjustment, set to full firm, rear anti roll bar 5/8 with 6 inches of torsion arm adjustment set to 50% of adjustment range.

Very good compromise street and track setup. My guess would be that the 450s on the rear even with another 50 lbs for the 351 should be just right for a road car. My car has a 302 with alum heads and is just a bit firm on a bad road. Fronts, 350s, are near perfect on the road.

I run the shocks at about setting 12F 10R track and 9F 8R street. Clicks up from full soft. Max clicks is 14. The car has a bit of push at high speed >90mph. neutral at low speeds. Very controllable power over-steer, with big foot and Quaife TBD.

Unless you track it on slicks you will not need to run stiffer suspension setting. On the street very soft 225F 250R isn't fun and feels like a Cadillac with the mother in law in the back seat and 4 sets of golf clubs in the trunk.

By the way I can't get 255X40X17 on the front of my car. Rubs and there isn't any material left to cut off. I couldn't even fit 245X45's as they are too tall. The 245X40's are just right on a 8 inch wheel.

 

[Kevin Box]

Thanks Howard

Sound like 350 / 450 is the way to go

 

[Woody]

Hi KB
I asked Peter Ransom of GT40 Australia the same question and this is what his reply was

" For road/Cometittion use we have found that 250lb front and 400 lb rear springs are a good compromise. We generally fit 150/100 AVO damper units (15" extended and 10" compressed- less the bump stop)
I tend to run my GT40 very low ( 65mm front and 100mm rear chassis clearance for competition) and use 14" / 9.5" double adjustable QA1 damper units and for interstate travelling adjust ride height to 105mm and 145mm and do not get any bottoming out "

I've gone with this set up but after reading this thread maybe its a bit light. He has built a lot of 40's tho:huh:

 

[Neal]

Will do this evening Kevin. I had issues with front tire fitment as I used bolt-on knock off adapters which pushed the offset out a bit. I think your tire diameters are similar to my 15's. The lower valence is asymmetrical on my car. The passenger side (right) clearance is tighter than the drivers yet the bodywork aligns.

 

[Kevin Box]

Neal

I think I have every thing in the correct place now.
I have made the top trailing link slightly longer to get 5 - 7 degrees of castor into the back to take care of any bumpsteer issues. (original rod did not have enough thread engagement.)
Easy solution almost done now - two new pieces of 1" free machining aluminium rod with left and right hand 5/8 UNF thread.
Bottom trailing link appears to have enough thread engagement so have left it as was.
Eyeometer suggests this has overcome most of the bump steer but will check further when every thing finallised

I probabaly need to look at the bottom trailing link mount point on the chassis as I think the other BumpSteer fix is to drop this mounting point. - maybe I will replace existing top and bottom mount brackets with a plate with a row of holes in it ???

My shock length at rest is approx 14.5 inch with 120mm rear ground clearance
I will check max compressed length and max droop length tonight so I can finally decide on shocks. (just ran out of day)
I am thinking QA1 DDR5855 or maybe DDR7855 depending on the compressed and droop lengths.

I have tracked down some Eiback 10" 350s that were trial fitted and changed out and waiting for confirmation of pricing on the 10" 450s. Will need to confirm shocks first !!

regards

KB

 

[Neal]

Sorry, been on the road. I'll check the dimensions and PM you.

 

[Kevin Box]

Thanks Neal

I am progressing to the front so I can order front & rear Shocks together.
I understand what its like towork away. You have my sympathy

regards KB

 

[Neal]

Hi Kevin,

Finally grounded! 15" extended and 10.5" compressed. 4.25" ride height. 350/450lb Front / Rear. Nominal at rest is about 14" Hope that helps.