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Old 06-19-2011, 04:05 PM
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Spring height vs. Spring rate question

Hello all,
If I wanted to go from my 12 inch coil over to a 10 inch coil over would I have to change the spring rate? These are the rear coil overs in a back halved Chevelle. The coil overs presently in the car are QA1 12 inch with CE 150lb. 12 inch springs. I wanted to go to a full QA1 10 inch coil over.

The reason being is I ran out of adjustabitlity in the rear. The coil overs are set the lowest they will go and the adjustment brackets are also on the lowest setting.

Thank you in advance to all!

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Old 06-19-2011, 10:29 PM
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I strongly suggest you first weigh the rear of the car. You need to establish what you have. Before you remove your coil-overs measure the compressed length. If there is a way to determine at what range of compression the springs are working with-in this too will be very helpful.
The new 10" springs (free standing) at #150 working with-in old spring working range will be stiffer.
There are several web-sites that can help with spring rates.
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Old 06-19-2011, 10:44 PM
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I take it that you wish to lower the car more..the spring rate does not need to change..the shorter coil overs will allow the car to have a lower stance in the rear..

Sam
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Old 06-22-2011, 09:35 PM
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OneMoreTime yes you are correct I would like the car to sit lower. The only thing I am worried about is coil bind with a shorter coil over.

4Chuck- I do have the measurements and the car was weighed.


My main concern is if the shorter coil overs will have enough travel and not coil bind esp. on the launch. Thanks again guys!
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Old 06-23-2011, 02:42 AM
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I think I'd build some new brackets.
Don't raise the bridge, lower the water.
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Old 06-25-2011, 03:01 PM
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Quote:
Originally Posted by oldschool101
OneMoreTime yes you are correct I would like the car to sit lower. The only thing I am worried about is coil bind with a shorter coil over.

4Chuck- I do have the measurements and the car was weighed.


My main concern is if the shorter coil overs will have enough travel and not coil bind esp. on the launch. Thanks again guys!
Lowering the rear will put more weight on the shocks, so if the 12" shocks get close to bottoming out on launch now..the 10" will for sure. Measure how much the current shocks are compressing on launch and go from there. Take a zip-tie and put it on the shock shafts where they come out of the shock body and go launch it hard a couple times. Then look where the ziptie has moved to. If they have 3" or more of shock shaft showing on launch.. you'll be fine.
Basically... going to a shorter shock with the same springrate will only lower the car... though you will have 2" less shock travel at the new ride height (and possibly a bit less due to the weight transfer of the lower rear).
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Old 06-26-2011, 09:40 AM
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I have some 10" QA1 single adjustable shocks on my current project, and I want to replace them with 12" units. Would you be interested in a trade?

As to your spring rate, you don't have to change it, simply adjust your mounts and the spring seat to get the ride height you want. You will of course have less bump travel due to the shorter shock, but you can adjust for it.

As to what Motochris states, lowering the rear will not add weight to the rear of the car. The only way to add more rear weight is to physically add weight to the rear of the car.

To find out the stack (coil bind) height of the springs, call QA1 and they will tell you for the particular spring you are interested in. Unless you run a lot of pre-load you will be fine.

Andy
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Old 06-26-2011, 12:12 PM
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I'm guessing that your rear shocks are set at some angle other then 90*. If so that angle will reduce the effective rate of that spring. Let's just take some angle - 15*. A 150 lb.in spring would then have a .93 multiplier and would make that in reality a 139 pound spring. I don't know what the back of your car weighs, pounds on rear wheels, but I suspect that it might be more then 1600 pounds. That would then tell me that you would need a 170 lb in spring at a minimum (corrected to 158 @ 15*), but more likely even some more. AFCO and Speedway have the coil overs and the correction charts on their websites.

As a note, my '31 roadster has 10 inch, 200lb in springs in the rear, which corrected are 188 lb in and are just about right
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Old 06-27-2011, 08:50 AM
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Quote:
Originally Posted by techinspector1
I think I'd build some new brackets.
Don't raise the bridge, lower the water.

I agree with this and there is a little more you need to look at then just going to a shorter coil over. If you have any interest in maintaining a descent ride quality then you will need a minimum of 3" of compression travel and an inch of extension travel on a full size car. If you are setting this up lower to the point of your axle only being a couple inshed away from the bumpstop you are going to have to scrifice ride quality and go with a stiffer shock rate or spring rate.
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Old 06-27-2011, 11:45 AM
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Quote:
Originally Posted by aosborn

As to what Motochris states, lowering the rear will not add weight to the rear of the car. The only way to add more rear weight is to physically add weight to the rear of the car.
If the entire vehicle is lowered the same amount, the weight percentage front to rear will stay the same. If you lower one end... you get some additional weight transfer to that end. It won't be ALOT, but it's measurable on scales.
If you and I are carrying a dresser into the house level, we are holding the same amount of weight. If I start to go up stairs with it... you end up holding more of the weight as it has transfered to the lower end.
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Old 06-27-2011, 06:51 PM
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Quote:
Originally Posted by Motochris
If the entire vehicle is lowered the same amount, the weight percentage front to rear will stay the same. If you lower one end... you get some additional weight transfer to that end. It won't be ALOT, but it's measurable on scales.
If you and I are carrying a dresser into the house level, we are holding the same amount of weight. If I start to go up stairs with it... you end up holding more of the weight as it has transfered to the lower end.
To lower the car an inch or even two in the rear is only maybe a degree in rake (depending on the wheelbase). It won't affect your spring load enough to warrant any changes in spring rate. Especially if you are talking typical spring rate increments of 50lbs. To change the spring rates in smaller than 50lb increments is usually not noticeable on a street car with rubber body mounts and suspension bushings and with higher aspect ration tires.

What you say is true regarding the dresser, but pitch angles like you describe are only seen in a motor vehicle if it is a wheelstander.

Andy
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Old 06-28-2011, 10:52 AM
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Quote:
Originally Posted by aosborn
To lower the car an inch or even two in the rear is only maybe a degree in rake (depending on the wheelbase). It won't affect your spring load enough to warrant any changes in spring rate. Especially if you are talking typical spring rate increments of 50lbs. To change the spring rates in smaller than 50lb increments is usually not noticeable on a street car with rubber body mounts and suspension bushings and with higher aspect ration tires.

What you say is true regarding the dresser, but pitch angles like you describe are only seen in a motor vehicle if it is a wheelstander.

Andy
I wasn't saying anything about him having to change spring rates. What I said was "Lowering the rear will put more weight on the shocks, so if the 12" shocks get close to bottoming out on launch now..the 10" will for sure."
So as you confirmed.. lowering the rear 2 inches will put more weight on the rear at ride height. More weight on the rear at ride hieght will make for additional weight transfer upon launch. If his 12" shocks were close to bottoming out on launch before...the 10" will for sure.
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Old 06-28-2011, 10:06 PM
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Quote:
Originally Posted by Motochris
So as you confirmed.. lowering the rear 2 inches will put more weight on the rear at ride height. More weight on the rear at ride hieght will make for additional weight transfer upon launch.
No, as I stated, what he is planning to do will have no noticeable effect other than less bump travel due to the shorter shocks.

Another factor you didn't take into consideration is what affect lowering the rear suspension will have on the percentage of anti-squat. It is possible the car at launch will transfer more load to the rear tires and squat down less so as to not use up suspension travel.

Andy
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