Improving Handling with a Lowering Kit

[Pav's 85]

Hey folks,

I am currently completing my third year of mechanical engineering at Lake Superior State University and I am seeking some assistance for a project in my Vehicle Dynamics Class. I decided to choose a project topic that would benefit me during the restoration of my 1985 C-10. The focus of my project is to determine which static lowering components (drop spindles, drop springs, rear spring hangers and shackles, etc.) or combinations of will improve vehicle handling performance the most. The vehicle of choice for this particular experiment is a 1973 to 1987 model year, General Motors half-ton, two-wheel drive, shortbox fleetside pickup truck. Unfortunately, my truck will not be completed before this project is due so no real life testing will be conducted. Instead I will be utilizing CarSim software to dynamically model my truck performing rollover and double lane change maneuvers. I will be using the truck's stock configuration as a baseline for comparison purposes. I have gathered quite a bit of information from the GM Heritage Center's Vehicle Information Packages. However,there are a few outstanding parameters I need to gather before I can begin and I am seeking assistance from forum members. These outstanding parameters include:

- Center of Gravity Height
- Rear Axle Assembly Weight
- Front Coil Spring Stiffness
- Rear Leaf Spring Stiffness

Any insight or suggestions you can provide are greatly appreciated. Once the experiment is complete I plan on sharing the results.

Thanks,

Tyler

[SSC's76]

Honestly I dropped mine for that reason. Bad news is it did not improve anything made the ride worse still don't understand why it did this but it did. I used the bel tech 3 4 drop kit listed as sport truck kit made to improve ride quality and handling to figure.
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[Keith Seymore]

First of all, kudos on your education and career choice, and on your project. Stay in school, stay off drugs, etc, etc, etc.

Now: More importantly, are you saying Center of Gravity height was not contained in the Heritage Center info packet? Can you tell me which packets you checked (so that I do not duplicate your efforts).

I know we used to measure that because it was needed for MVSS208 brake simulation work. It was also reported so that the "window" of acceptable CG locations could be communicated to aftermarket body upfitters and for in-box campers.

Finding the CG height is an incredible test that involves tipping the truck at extreme angles and measuring the change in load on the subject wheels. We also used another technique that involved "swinging" the truck (like a giant porch swing with a truck inside it). I'm guessing you won't be able to use either of those techinques.

Let me ask the old brake guys if they still have that information, or if I have it filed away somewhere.

Is your truck still together? In a pinch you can estimate CG height by using the dimension from the ground to the centerline of camshaft at the front of the engine. Which engine do you have?

The other parameters you are asking for should be more readily available.

K

[Keith Seymore]

I should also mention my current truck is dropped about 3" all the way around through the use of cut coils (front) and relocated spring hangers (rear), with no degradation in ride quality or hauling performance.

We've done 7 or 8 this way with no issues. My current truck has been a daily driver for 26 years and just shy of 200,000 miles with no issues.

You might be interested in reading more; my build thread and career hi jinks are linked in my intro thread below.

(...but don't forsake your homework goofin around reading my stuff!).

K

[Pav's 85]

SSC's76 my truck will probably have a lowering kit installed in it before I put it back on the road again so I will not be able to feel the difference. Nonetheless, I hope there is an actual improvement. My intentions are to theoretically obtain an improvement in handling.

Thanks, Keith! I hope all this hard work results in a rewarding career. I searched high and low through the 1985 Chevrolet Truck Information Kit and could not seem to find a CG height anywhere. However, this may have been an oversight on my behalf. If my truck was complete and I had access to a set of vehicle scales I was thinking about taking a similar approach to measuring the CG height. This would involve weighing the truck on a level plane, then elevating the truck slightly and measuring the load shift. I will most likely measure the camshaft height for starters and modify this parameter if anything comes up. I'll be sure to check out your build thread.

[piecesparts]

I have a lowered 84 GMC SWB fleetside truck with 3.5" drop in the front of the truck and a 4" drop in the rear. I used 3" drop spindles from Bell Tech and a 1" drop spring, then used a spacer to bring it back up approximately a 1/2", due to it being too low for my driving needs. I then used Bell Tech shackles and spring hangers to lower the rear end 4". I also installed a set of helwig sway bars on the truck to assist in tightening the cornering abilities. The front shocks are Rancho R9000 adjustable shocks and the rear shocks are from Bilstein. I am running a lower profile tire than factory design which is approximately 28" in diameter on the rear tires and 27" diameter on the front tires. Front tires are 255/50/17 and the rears are 275/50/17 on Billet Specialties 8" rims front and 9.5" rims on the back. I have to say, the truck corners really well. It is still a brick by design, but it will lock up with many of the pony cars and stick on them just as good as any can. The truck is in the avatar for my sign on, the rims on it at the tme of the picture were Weld wheels and not the Billet Specialties.

[Keith Seymore]

Originally Posted by Pav's 85

Thanks, Keith! I hope all this hard work results in a rewarding career. I searched high and low through the 1985 Chevrolet Truck Information Kit and could not seem to find a CG height anywhere. However, this may have been an oversight on my behalf. If my truck was complete and I had access to a set of vehicle scales I was thinking about taking a similar approach to measuring the CG height. This would involve weighing the truck on a level plane, then elevating the truck slightly and measuring the load shift. I will most likely measure the camshaft height for starters and modify this parameter if anything comes up. I'll be sure to check out your build thread.

Brake group design engineers report an estimated CG height of 27" (at curb weight).

If you decide to do the CG tip you should know the more you angle it the better your results will be. Just measure the change in weight on the subject wheels and calculate through trigonometry.

I would recommend you go with the estimated CG height based on camshaft centerline because (a) your truck is together and that is easily measured, and (b) I assume your simulation will be an A-B comparison, rather than relying on an absolute value.

K

[Pav's 85]

Thank you very much Keith! That is great that you managed to find an actual value for the CG height. I will most likely utilize this value since I do not have access to a set of vehicle scales. Yes you are correct, regardless of what CG height I choose to use for a baseline, the modified CG height will simply be the difference between the arbitrary CG height and amount of drop obtained from lowering components.

[Keith Seymore]

You are welcome.

I'm working on the rear axle weight currently. I remembered that I had a spare axle at home and went out to weigh it but was dissappointed to find it was "empty" (no shafts, drums or center section). I don't think that would provide a very good estimate.

I'll see what I can find here at work.

K

[Keith Seymore]

Please use 183 lbs for the rear axle weight.

K

[Ziegelsteinfaust]

Our trucks have barely any camber gain for cornering, and that makes for bland feel of the suspension. Not to mention the steering boxes were vague at best.

Besides lowering which can help this will do the trick no matter how much you intend to drop or not drop your truck.

Get a good AGR power steering box or rack, and pinion if budget affords it. Supposedly a late 80"s T-Bird works for our trucks. After I bring my truck down to the desired level I will give this a better try, but measurements I have taken seem to be good.

Moving the lower A-arm forward in relation to the upper 1 inch does the trick. My solution is to cut the rivets holding the upper A-arm bracket on, and slide it back an inch. You can just move the lower A-arm forward 3/4 of an inch, but I am going all the way.

[INSIDIOUS '86]

Not sure if this helps your project but some of these trucks did and didn't come with anti roll bars. It really makes the difference in driving a whet noodle that will just kill you on the highway or something that reacts and drives somewhat difference.
When you lower a truck you change the roll center relative to the center of gravity which effects body roll and suspension characteristics such as camber gain and bump steer from the change in tie rod angle
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[Pav's 85]

Thanks once again Keith! You have been very resourceful! Does that rear axle weight include the leaf springs, and drum brake assemblies or is it just the weight of the housing, axles and third member?

Ziegelsteinfaust good point you brought up, I may consider upgrading my steering box in the future.

Insidious '86 I am aware of these effects and am planning on seeing how they change during this project.

[muthstryker]

Dropped my truck 6/8 and with the 20s the truck handled very well and rode great.

[bigblock73]

My 73 has never has sway bars on it, but when I dropped it and rebuilt the front with polygraphite bushings, it handled like a dream, believe it or not. I may add a front bar now that I have a bbc up front, but need to get out and drive it in the spring and see how it handles first.
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[Keith Seymore]

Originally Posted by Pav's 85

Thanks once again Keith! You have been very resourceful! Does that rear axle weight include the leaf springs, and drum brake assemblies or is it just the weight of the housing, axles and third member?

No leafs; includes drum to drum and all the guts loaded.

Working on spring rates now, but not very optimistic.

K

[Keith Seymore]

Originally Posted by bigblock73

My 73 has never has sway bars on it, but when I dropped it and rebuilt the front with polygraphite bushings, it handled like a dream, believe it or not. I may add a front bar now that I have a bbc up front, but need to get out and drive it in the spring and see how it handles first.

Actually, I do believe it.

I've done quite a bit of high speed/limit handling on full size trucks (probably more than I can write about publicly) and have found them to be quite "tossable".

None with big blocks, though - so you may appreciate the addition of that front sway bar.

Also - we always ordered our personal cars and light trucks with the F59 and/or F60 suspension packages.

K

[Keith Seymore]

Originally Posted by Keith Seymore

Working on spring rates now, but not very optimistic.

K

For spring rates, try 400 lb/in front, 200 lb/in rear.

I'll let you know if I find values I like better after further searching.

K

[Pav's 85]

I found a stock replacement coil springs on summit and they listed the spring rate as 843 lb/in. http://www.summitracing.com/parts/su...olet/model/c10 I have also contacted several dropped coil spring manufacturers and they gave me valves around 1100 lb/in. Do these values seem accurate to you?

[Keith Seymore]

Originally Posted by Pav's 85

I found a stock replacement coil springs on summit and they listed the spring rate as 843 lb/in. http://www.summitracing.com/parts/su...olet/model/c10 I have also contacted several dropped coil spring manufacturers and they gave me valves around 1100 lb/in. Do these values seem accurate to you?

Those sound really high to me, but I trust their data more than I do my own.

I wouldn't know what to suggest for a rear rate if you were to use those higher front rates.

K