Many of you follow my build https://grassrootsmotorsports.com/forum/grm/scca-emod-mgb-gt-v12-build/26822/page1/ and I have run into a problem that I haven't been able to sort out. I do not have enough suspension travel in the back of the car so I want to change from the pushrod suspension with bike shocks to a conventional coilover shock mounted between axle and frame. I am looking at getting QA1 double adjustable shocks (Part #DD501) http://www.qa1.net/suspension/street-performance-racing-shocks-struts-and-pro-coil-systems/custom-mount-shocks/proma-star-double-adjustable and need to figure out what coil springs to get. I called QA1 tech line and gave them all my info and they figure I need a 450 lb/in spring but also advised that with just 1017 lb total weight on the back of the car, it might not compress much when I set the car on it. I need more droop (currently have 2", looking to get 3") and he suggested maybe I should get shorter springs (10" vs 12") so the car sat lower. So, I know there are some knowledgeable people on this forum and would like some help. Here are some numbers to help figure it out.
Current spring rate: 575 lb/in
Current spring length (extended): 7.125"
Current spring length (at rest): 6.375"
Wheel rate: Dunno but shock moves about .750" for every 1" wheel moves
Track width:48"
Width of axle shock mounts:35"
Weight on axle (total): 1017 lbs
Unsprung weight: 300 lbs
Angle of shock if I switch to conventional coilovers: 25 degrees from vertical
Don't forget a 400 lb. spring At uncompressed Height is an 800 lb. spring when compressed 2" and a 1200 lb. spring when compressd 3". Our late model race car only used a 200 to 225 lb. Spring on the rear of a 3000 Lb.car that was around 1200lb.on the rear.What you need to determine is How much travel you want,If just 1" you want a pair of 250's if 2" is required you want a pair of 125's
GTXVette wrote:
Don't forget a 400 lb. spring At uncompressed Height is an 800 lb. spring when compressed 2" and a 1200 lb. spring when compressd 3". Our late model race car only used a 200 to 225 lb. Spring on the rear of a 3000 Lb.car that was around 1200lb.on the rear.What you need to determine is How much travel you want,If just 1" you want a pair of 250's if 2" is required you want a pair of 125's
Im trying to wrap my head around this...
Springrate is pounds per inch.
The springrate doesn't change no matter how far it is compressed.
However, it does take 800 lbs of force to compress a 400 lb spring 2 inches. It will take an additional 400 lbs to further compress it another inch.
In other words, once the weight settles on the springs, no matter how far the spring is compressed statically, it will always take 400 lbs to compress 1 inch. This is true if the spring starts off compressed 1 inch or 6 inches.
Does the current pushrod suspension connect to the rear axle in the same place you're planning on mounting the shocks (35" apart)?
Sitting at free height there is no weight on it It must be compressed 1" to achieve he 400lb.rating, the second inch takes another 400 lbs. 4+4 =8 another 1" requires another 400lbs. 4+4+4=12. that is in a perfect world, there are very expensive springs that stick to the math dead on. what I can afford do move around a bit in a More or less liniear movement by a few to several pounds of extra weight but they are still close. the first 1" is the rating of 400lbs. to compress it 6" would take another 5 times the initial 400 pound weight.
In reply to APEowner:
Yes, same lower mounting location but the top will be new. The shocks I want have 5.2" of travel and I would like to use most of that. Now, keep in mind that 5" of shock travel works out to even more wheel travel.
like 6.25" total, you want it near the middle,but leaving room for more compresion,2.5" so a 125 to 150 lb. spring on each side will compress 2 inchs to 2.5. finding that spring in a tight wound size like 4 inch will be tough the larger the winding the more spring rates available
In reply to GTXVette:
The spring for that shock is 12" long and 2.5" inside diameter and QA1 has spring rates from 80 lb/in to 600 lb/in. They also have rising rate springs.
Your current wheel rate is about 323lbs/inch according to your data of 3/4" shock = 1" wheel movement. For a mod car on slicks, that is really low.
For example, my EP VW Rabbit has basically the same front weight as your car's rear weight and I have a wheel rate of 450lbs/inch with no sway bar. I have added low rate helper springs as there's over an inch of shock travel after the springs fully unload.
Your unsprung weight means pretty much nothing for spring rate, the shocks control that, not the springs.
By my math, your wheel rate with the shocks at 25° off vertical should be about 550lbs/in if using 450lb/in springs. Consider adding tender springs too. This rate may be a bit too high.
If you choose to go with a rate low enough to keep the springs loaded over the full travel, be prepared for the back end to flop around. Real race cars on slicks need very stiff springs to work well, they are not like normal cars.
Note that with the shocks angled, wheel travel will be less than shock travel which is why wheel rate is higher than the spring rate.
In reply to GreenVWs:
Thanks for that info. I also feel that my wheel rate is too soft because the car leans too much. I may have to change the front shocks as well because stiffer springs for my shocks are hard to come by (2"x7").
In reply to loosecannon:
Have you disabled the mc shocks preload like No Cones mod creation? I was under the impression that this is why one would choose mc shocks for autocross.
In reply to BarryNorman:
Yes, I did that at last years Nationals and it increased my droop, but not enough
+1 for tender or helper springs, that's a good way to get more droop travel. Tender springs have a meaningful amount of stiffness while helper springs don't - they basically just keep the spring from unseating, you can squeeze them to bind height with your bare hands. There are advantages and disadvantages to both. I run helper springs on the back of my Corolla.
GTXvette, a linear-rate coil spring has practically the same stiffness throughout its entire travel.
GreenVW, I think you made a -1 multiplication mistake somewhere. When you tilt the coilover away from the path of suspension travel it becomes effectively softer and you need a harder spring:
https://www.qa1.net/technical-support/street-performance-racing-spring-rate-tech
In reply to GameboyRMH: I knew that and they are the more expensive springs I spoke of.
Just a simi Literate Hillbilly here, used the right word in the wrong place. and also I was using my experiance on a race car that ran a HIGH banked Oval and we wanted travel in the suspension, Your needs may vary, I try to word things so folks can see it in their Head. I only Hung Around at that North ave. Trade School I am Not an Enginere,Or english Major, But I did graduate the Jethro Bodine School of Cypherin and can do my Ought's an Ought's AND Goesinto's.
In reply to GameboyRMH:
Huh, I guess you are correct there. I was thinking that with the shock at an angle, the shock would travel further than the axle for any given displacement, causing a multiplication of the spring rate. I defer to actual suspension people.
This makes the recommended 450lbs/in spring 495lbs/in at the axle. That sounds reasonable for the car.
I measured how much my shocks move for each inch of suspension travel then plugged the numbers into a spreadsheet that was given to me and it came out to 302 lb/in wheel rate, or 174 cpm front and 166 cpm rear frequency. These are both lower than I would like and I have it on good authority that the guy who wins every year has a 360 lb/in wheel rate on the front of his car. So, I ordered 650 lb/in springs and these will give me a 365 lb/in wheel rate, or 192 cpm front and 182 cpm rear frequency. As a guide to what to aim for the following ranges have been extracted from the Competition Car Suspension book by Allan Staniforth: 60 – 80 cpm Comfortable road car, 80 – 100 cpm Sports road car, 100 – 125 cpm Racing cars without wings or ground effects, 200 – 350 cpm Current racing cars with downforce, > 500 cpm Ground effect era cars. I hope these do the trick, if not I still have the option of changing the motion ratio, which will also give me more suspension travel.
