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Front Coil Over Shocks on a Straight Axel!

21K views 22 replies 5 participants last post by  jaw22w 
#1 ·
It's not a topic with a lot of answers on this site, but I appreciate those who did give me feed back. I finished removing the single leaf and upgraded my Model A with front coil overs. I have added many details of the project to my photo journal and would be happy to expand the discussion if there are any specific questions.
 

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#3 · (Edited)
I have also eliminated the buggy spring on my '27 T. It makes the old roadster handle pretty well with 4 corner coil overs with 4 bar suspension front and rear. I think I see a panhard bar on your front end? What rate springs did you use? I used 225# springs. It may be just a little stiff, but handles well. My car weighs 1050# on the front What does yours weigh on the front? The front suspension was modeled after a pavement sprint car.
 

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#4 ·
Front Coil Over Shocks on a Straight Axle!

The passenger axle mount shown painted red has a pan mount welded to it with two adjustment holes. The drop down mount on the driver's side is attached to the frame and the pan bar sits level right behind the front axle. This allows the bat wings to be free of any pressure and effectively attaches the pan bar to the front axle. Quarter inch angle iron with a quarter gusset and 3/16 tabs.
 

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#8 ·
The passenger axle mount shown painted red has a pan mount welded to it with two adjustment holes. The drop down mount on the driver's side is attached to the frame and the pan bar sits level right behind the front axle. This allows the bat wings to be free of any pressure and effectively attaches the pan bar to the front axle. Quarter inch angle iron with a quarter gusset and 3/16 tabs.
It is difficult to tell from the photo, but it appeatrs that the frame mount is simply butt welded to the bottom of the rail. If it is, you might want to consider gussets that come up the inside of the frame rail for strength. There is quite a bit of side load on a Panhard bar.

Regards,

Andy
 
#6 ·
Thanks, but the engineering came from the sprint car. I just adapted it. Your car is only 60# heavier than mine on the front axle. Have you not tried it with the 175# springs yet? I am anxious to hear how it works on the street. I have been playing with the idea going lighter on my springs. I have 225# front and 150# rear. It is firm but not like a buckboard. Being from upstate NY, you are in the same boat as me as far as driving your hot rod until it warms up and the snow goes away
 
#7 ·
I am running #300 springs in the rear with a weight of 1680. They work great. The rear springs are shorter, 10" as compared to the 12" spring in the front (in the prior thread I stated 14" but that was wrong. 14" is the total length of the shock and spring). Just didn't have the room in the back for a longer shock (the back shocks sit 12" total) The longer the spring the less spring weight that is needed because more coils get involved. If I were using a 10" in the front I would move up to #185-200. You may have a stiffer spring than is needed at #225. I can't tell you how this rides yet......till I see some pavement Haha! (Upstate NY). Later, I'll put an update in my Journal on how the ride and handling goes. In the mean time....

Here is an online spring rate calculator you can play with. This should help. Spring Rate Calculator « Ridetech News and Information
 
#9 · (Edited)
My understanding of coil springs is that (as an example) a 300# spring will compress 1" with 300# on it, 2" with 600# on it, 3" with 900# on it, etc. regardless of the length of the spring.
I see from the calculator that going to a longer spring requires a lighter weight spring. I don't understand.
 
#10 ·
jaw22w

The more coils involved in a longer spring and shock the more vertical movement with the same impact. 10" spring moves 1" but a 12" will move 1 1/8" You use lessor weight spring but the additional movement results in the same weight after you multiply. 1 *300 or 1.125 * 275 roughly the same. I think that's how it works!!!!
 
#11 ·
It is a channel weld up one inch on both sides of the frame, BUT since the mount already has holes, and there is still a buggy spring pad on the cross frame, I could easily bolt a rigid diagonal brace from the middle of the pan mount up to the middle of the frame. Thanks for the feedback. Super Idea
 
#17 ·
On a street car, you would normally want the shock in the center of travel at ride height. With a 12'' spring, you have 6'' of shock stroke (3'' to center of shock stroke). A 10'' spring will have 4'' of shock stroke(2'' to center of shock stroke).
Using the same front end weight with both springs, the 12'' spring will need less spring rate so it can compress the additional 1''.
 
#18 ·
I have always used 2/3 travel in bump (compression) and 1/3 travel in rebound (extension) as a basic rule for designing mounts and specing out shocks. The length of the spring won't affect the rate necessary. The same load is applied to a 10" or a 12" coil-over with everything being equal other than shock length. The only real difference we are talking about is more travel with a longer shock which is what I would recommend.

With that being said, If the rate I am after falls between two available choices, I go with the softer spring and just add pre-load to make up the ride height. By going this route, having the longer coil-over and spring is good so you will have plenty of travel without the spring binding up.
 
#19 · (Edited)
I always use 60% bump and 40% rebound. Yeah, that calculator is a little awkard and is for an a-arm suspension front end . I have a straight axle. For the motion ratio, I jacked up one front wheel 4". When I did that the shock moves only 3". So I used a .75 motion ratio. Not sure if this is completely accurate. I based my original spring choice off of the sprint car. We used 250 to 300# springs on the front of the sprint. Of course the sprint only has about a 300# corner weight in front. The hot rod has a 525# front corner weight. So I decided on a 225# spring. It is not uncomfortable to drive and handles well, but could be a little softer. I was just looking for some confirmation before I spend another $200 or so on new springs.
 
#20 ·
I don't usually factor a motion ratio on a live axle rear or beam axle front. Just use 1-1. Typically they have more unsprung weight which can throw you off if you don't factor that in. Sounds like you are on the right track though.

Sprint car huh, I would love to take one of those for a few laps sometime. I had an opportunity to drive a midget for a friend of mine, but he got out of racing before we could make it happen.

Regards,

Andy
 
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