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Old 05-09-2007, 09:59 AM
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Variable Rate Springs vs Linear Rate

I have rear coilovers on our 1939 street rod with QA1 adjustable shocks and 12 inch 200 lb rate springs for a comfortable ride. With 1000 lbs on rear springs, they are compressed 2.5 inches and the shocks have about 3 inches of additional compression available.

I would like a little bit lower stance in the back and was thinking about going to a variable rate shock (175 at one inch compression to 350 at 5 inches) so I could lower the stance maybe .5 to .75 inches without risking bottoming out.

Does anyone have any idea how this would affect the ride quality cruising on the interstate as well as city streets with some uneven pavement? Noticeably stiffer?

John

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Old 05-10-2007, 06:35 PM
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39 chevy springs

john, did you ever find an answer to you rear suspension question? I have a similar situation wherein I have lowered the rear 2 inches, changed to Chassis Engineering leaf springs and the suspension bottoms out when there is more than 100 pounds in the back seat. I was thinking about QA1 as the solution, but apparently they were not the answer for you. What about air shocks? My car has no room for true air suspension.

David Eldridge (allthumbs)
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Old 05-10-2007, 09:06 PM
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David,

Nobody has responded on my question about the ride differences between variable rate and linear rate springs.

But, as far as the QA1 coil over shocks, they work fine -- the car rides well and doesn't bottom out because I kept the rear at pretty much the stock ride height for a 1939 car. The shock has a little over 3 inches of compression length, so I can put 400 lbs of people in the back seat and still have 2 inches of compression stroke left.

To lower the rear .5 to 1.0 inches I would need to use a stiffer spring because the shock wouldn't have as much room left to compress. I thought the variable rate spring might work to keep a soft ride but provide a lot of resistance the last 2 inches of compression to prevent bottoming out.

Sounds like you need stiffer leaf springs or shorter shocks (or lower shock mount). What is bottoming out -- does the shock have only a short stroke length before it is fully compressed, or is the axle hitting a bump stop on the frame?

John
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Old 05-11-2007, 06:41 AM
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39 chevy springs

Thanks for responding. After the 2 inch lowering blocks were installed, the drive shaft scraped against the emergency brake housing (the brake had been installed between the front bucket seats). That was cured by lowering the transmission support 1/2 inch. However with weight in the back seat, I think the axle hits the bump stop on minor bumps with a load in back. I need to check with my mechanic to be sure that statement is correct because he had the car on the lift while I was not not at his shop. I believe there is 3 inches of shock travel available, but I don't know how that compares to what it should be in my circumstance.
I am afraid that trying to pick the right coilover shock would be hit or miss, and hoped an adjustable air shock might solve the dilemma. Your inquiry as to linear versus adjustable coilovers seems the right question.
Did you figure the suspended weight of the rear of the car at 1,000 pounds with passengers when you ordered the QAI's?
--David
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Old 05-11-2007, 10:42 AM
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David,

First, let me say I am not a suspension expert. My comments are based on reading some articles and basically trying to understand how the springs and shocks work together.

From what I have read, if you have 3 inches of free travel with the weight of the car supported on the springs, you have plenty of margin for the suspension to move up and down without bottoming. As you add weight, the spring rate will determine how much further the shock will compress, i.e. the ride height with passengers.

I think the shocks play an important role in the comfort of the ride based on how much dampening they have, and the springs based on how stiff they have to be to keep the shock from fully compressing as road bumps are encountered.

My cars original suspension was coil springs and shocks. A shop converted it to a coil over system with drag shocks, I think because they though it was cool. I didn't know enough to question them. The coil over system does have the advantage of easily adjusting ride height.

Before putting the QA1 shocks on the car (Ultrarides 12 way adjustable, 17 in. extended, 11.6 in. compressed), the coil over drag shocks in the back made for a stiff ride -- didn't seem to give much when hitting a sharp bump. With the same 12 inch 200 lb. springs that are currently on the car, the springs were compressed 2.5 inches with no one in the car. This calculates to a suspended weight of 1000 lbs. on the rear springs. My wife and I in the car adds about 175 pounds to the rear which would compress the springs a little less than .5 inches more.

Keeping the same springs, I changed to the QA1 shocks and noticed a big improvement in ride comfort. At a low setting the car would tend to bounce a lot when going over road undulations. By setting the adjustment at 3 (out of 12) most of the bounciness was taken out. The slightly stiffer setting of 4 also works well.

So, I now have a good ride and the shocks are about in the middle of their travel with one or two people in the car. I would be happy except that the car sits a little high for a "street rod". I could lower it by going to shorter shocks and much stiffer springs, but I don't want to sacrifice ride. So, I am thinking variable rate springs would let me lower it a little and still use the same shocks.

Hope this all makes sense. You can PM me if you want to talk in even more detail.

John
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Old 05-17-2007, 08:32 PM
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TTT

On one has any experience with ride differences between variable rate and linear rate shocks?
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Old 09-13-2007, 07:10 PM
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FYI followup:

Despite the fact the local hot rod shop said they had never sold a set of variable rate springs for a rear coilover suspension, I odered the 175/350 varibable rate springs to replace the 200 lb springs on the rear (both 12 inch in length).

I can't tell any difference in the ride comfort and I was able to lower the rear about 1/2 inch more. So, they work fine for my application.

John
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