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Coilover Spring Rates

12K views 29 replies 8 participants last post by  Jus Cruisin 
#1 ·
My 32 fenderless coupe has a Pete & Jake frame with a 4 link rear. The coilovers are mounted at about a 45 degree angle parallel with the rear axle. The car rides lkke it is a "hardtail". For those of you with my set up, what spring rate do you use?
 
#4 ·
You can also weigh just the front half and back half of the car on a common agricultural truck/trailer scale or a truck stop scale.

Estimated with 48% of weight on rear axle and allowing for the 45° angle you'll want a rear spring in the 200-250 lb range if using a 12" tall spring.

45° is a bunch of shock angle, I personally don't like tilting them more than about 15° max, as it really plays havoc trying to match spring rate and shock rate with highly angled mounts. Angled is not a good way to go IMO.
 
#7 ·
The shock shaft speeds are gonna be pretty slow at 45'. Slow means not very effective at spring dampening thus the reason for the rough ride.
If at static ride height the car is sitting at the right attitude then I'd say your spring are the right rate or close enough and the shock inability to control the spring is.
 
#8 ·
Spring/shock in the pic doesn't look like the spring is preloaded very much, which tells me it is on the stiff side.

Combine that with what Johnsongrass1 posted about angled shock mounting above is a compound problem.

Going to a softer spring rate and just preloading it on the shock adjuster farther to hold the ride height will help, but may not be enough to your liking if the shock damping rate is just too far off due to the mounting angle.

Is the current spring marked for rate with a tag or anything?? Or do you have the paperwork for the shock/spring kit??
 
#10 ·
Doesn't look like 45 degrees to me. Looks closer to 30 degrees. I'd put a angle finder on the spring to determine exact angle, rather than guessing, as angle has so much to do with spring rates.
I would guess when you check weight, that the rear will be closer to 45% of total weight on a fiberglass car. Once you have the weight, and check shock angle, you will be able to determine spring rate easily.
 
#12 ·
That I doubt is 45*:eek: My guess is less then 25*

I suggest that you use an angle gauge and find out what it really is on that chassis then use the charts Staleg provided a link for. I have a TCI chassis and those coilovers are at 13/14 degrees front and rear. The car weighs about 2400# and the springs are: Rear springs, AFCO CR23200 (200# rated 188#@13*) and the front springs AFCO CR23325 (325# rated 300#@13*)

Dave W
If you click on his posted shock photo, it takes you to his PhotoBucket page, where there is also a direct view from straight behind the car and you can see both shocks....the angle appears to be 40-45°, or within a few degrees of that.
 
#11 ·
That I doubt is 45*:eek: My guess is less then 25*

I suggest that you use an angle gauge and find out what it really is on that chassis then use the charts Staleg provided a link for. I have a TCI chassis and those coilovers are at 13/14 degrees front and rear. The car weighs about 2400# and the springs are: Rear springs, AFCO CR23200 (200# rated 188#@13*) and the front springs AFCO CR23325 (325# rated 300#@13*)

Dave W
 
#17 ·
Any spring angle in any axis will add to the problem. Not just linear to the axle center line. Adding spring angle to the front(tilted front and inward) add another level of complexity to the spring table and handling. If you need a good shock guy let me know as I have delt with several over the years.
 
#18 ·
Spring angle does add to figuring out the rate, but it is also a good thing to have some spring angle. Totally vertical shocks don't work as well as the body rolls in cornering, as that works the shock and spring at the wrong angle. A 15-20 degree angle is better, as the body roll will act directly on the shock, and not tilt the shock as the body rolls.
 
#19 ·
I don't disagree but there is one thing to note. A shock will be be mechanically most effective when the the shaft travel is linear to wheel travel. Vertical or other wise. Sometimes you can move things around and sometimes you can't due to packaging concerns.
 
#20 ·
Yes, and a spring needs to be the lightest possible if it's perfectly vertical. But the majority of cars (with or without coilover shocks) are mounted at an angle, and that's not by chance. Most manufacturers mount shocks at the optimum working angle, which isn't straight up.
 
#21 ·
My perspective comes from a a racing career as a wheel man and working with national known shock designers and engineers. No, I don't mean rebuilders either, I mean design engineers, guys who make a living making race cars go faster. None, I can say with confidence, would agree with you on any type of shock or spring.
We don't have to agree...that's what makes the world so cool to live in.
 
#22 ·
Ok, I finally checked out the spring rate on the car. 300lbs Pete & Jake branded springs. Looking at available springs from Pete & Jake, they are about the stiffest available and they start at 130lbs and work up to 350lbs. I'm thinking I have way too much spring for my lightweight car.
 
#26 ·
Just got off the phone from Pete & Jakes and they think 300 lb is too much for what I have (no fenders, small fuel cell). They think 220 lb springs might be better. They did mention one thing for me to check. That would be the bolts that mount the 4 bars. They ran into a customer that was having the same issue as me. The customer bought lighter springs and it didn't help it. Come to find out, the bolts on the 4 bar were so tight they pinched the brackets to the point the rear end was basically solid. I think I will check that out on mine.
 
#28 ·
Going to revive this thread for an update. I attended the Nationals in Louisville in August. While at the show Ridetech was weighing cars. I took advantage of opportunity. Here are the results:

Total weight - 2, 597 lbs.
Left Front - 636 lbs.
Right Front - 611 lbs.
Left Rear - 697 lbs.
Right Rear - 653 lbs.

I decided to order their front and rear shock package this week. The springs for the coil overs recommended were 200 lb. They should arrive by the first of the week. I sure hope it helps. After attending the Nationals, I went up to the Woodward Dream Cruise. The car rode and drove terribly on the pothole, frost heaved roads in the metro Detroit area. The rear felt like a hardtail and the front was bottoming out the shock. I put a ziptie on the fronts to see what was happening and it confirmed the shock was completely compressed going over the bumps.

I will report on my seat of the pants review once I swap everything out.
 
#29 ·
Front coil over adjustments

Having just installed front coil overs on a '38 car (Fatman, TCI, MII, QA1), the recommended position is, with the car loaded with fuel and passengers etc, there should be 60% of shock travel available for upward movement, when hitting a bump, and 40% available for the tires dipping into a pot hole. I found the cars front end height is determined by where the shock needs to be adjusted to in order to provide the recommended shock travel. Ours is 100% better with proper spring strength and adjusted shocks, proper travel and dampening.
 
#30 ·
I just had time for a short ride. I'll be taking a couple longer rides over to Daytona this weekend (knock off Dream Cruise). I could immediately tell the rear will have a softer ride. The front, not sure. My issue will be the short travel. I have only 3.5" of clearance under the front axle (6" drop). Whatever the ride is, it is. I don't want to raise the front end. I like the look.
 
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