Ok this particular case is OT but the information should be usable to all . I'm wondering how much rebound " suspension drop from ride height to tire raised just off the ground would be normal . My 35 coupe gets very squirley on the back end when encountering rough road and sometimes sound like bottoming . After jacking it up and doing some measuring I have about 2 1/2 " of rebound or drop . I don't believe it's bottoming out more likely the shock hitting full travel . These are coilover shocks and leaf spring suspension and I don't want to change the current ride height any more than necessary but the ride is also a little stiffer than necessary . I know the shock on one side is leaking compounding the issue but before I source new shocks I would like to make sure that ( 1 they don't need to be longer ) ( 2 can I possibly get away with gas shocks and eliminate the coilovers without changing my ride height allot ) .

 

Gas shocks have a reputation of riding even stiffer than oil filled shocks so if the current ride is "stiffer than necessary", I would not go with gas shocks. Generally speaking, three inches is the minimum desired amount of travel. You are very close to that and if you can get a replacement shock another inch or so longer than what you have on there now you should be okay. I recently went through something like this on my '29 Ford PU. It was bottoming out in the rear and I replaced the coil overs with new shocks with a lighter spring rate. Seems to have solved the stiff rear end but I still need softer and possibly shorter shocks in the front since they are operating in the top 1/4 of the shock's travel. I think slightly shorter shocks will get the travel more in the middle.

 

Don't know if I am understanding you correctly. But you have coilovers on the front and leaf springs/telescopic shocks on the back? If you have 2.5" of rebound then you likely have 1.5" of useable bump which could explain the problems with ride. Short answer would be to raise your ride height an inch. If you don't want to do that then shorter coilovers are likely to make it worse because you will also have less travel. Ideally you want to extend the position of the mounts to keep the travel you have and be 2/3 bump and 1/3 rebound

 

The rear has leaf springs and coilover shocks ( the PO towed a boler trailer ) and I currently have 2 1/2 " of rebound and what seems to be more that adequate bump ( 3" - 4 " estamated ) . The 2/3 1/3 definitely helps . I am quite sure my thumping is related to the suspension at full extension not at bottom , which in turn could be solely caused by the bad shock . With the formula I can make sure the shocks are actually the proper length then I think I will need to remove the shocks to see how much " boost " the coilovers are giving . If the ride height doesn't change too much I think that trying a gas shock without the coilover should soften the ride and still compensate for the coils somewhat . And hopefully by making the suspension softer " more compliant " it will help the squirley attitude over bigger bumps . Thanks for your replies guys this is why I love this site .

 

Take a look at this page - It is complicated enough to be good information
You will have to turn every thing upside down cause its Aussy

 

I get what you mean. It is an interesting set up but not one you see on factory designs. Good idea to remove the shock and check open and closed length. It would take a very long shock to give that amount of travel - maybe 20 inches. Also these things usually have stops built in so it is possible it is bottoming on the shock only. One other thing I wondered and I have asked a spring specialist this previously - if your leafs springs have a rate of 200lb/in and you add 100lb/in coils do you have a total of 300 lb/in or something else? The guy did not know. I suspect it's not that simple. I do know in rally raid cars they often use coil springs in series and the equation to calculate it is more similar to electrical resistance in parallel circuits. So in the example above you would have a total spring rate of 67 lb/in which is less than either individually. With your setup I don't know whether that would be classified as in series or parallel or somewhere between the two. Removing the coils is the best way to find out and I'd sure be interested to hear what the result is.

 

If the shocks aren't leaking too badly you might try temporarily running the old shocks without the coils first just to see what you have and then try the gas shocks. I am serious about gas shocks being stiffer. Another thing you can do is lower air pressure in the tires. That will also soften the ride. I run 22 pounds in the rear and 26 up front but my 29 Ford probably has a lot less weight on the rear than your 35.

 

Yes the gas shocks will be stiffer than a normal shock but I'm thinking not as stiff as the coilovers . The extended length of the shock right now attached with the wheels off the ground is about 15.5" ( it could be more when unhooked ) so if I knew how much travel the shock had I could at least determine whether the shock is the right length . This car previously had a corvette rear and rode far better and I would like to get as close to that as I can . My hope is that the shop that installed the diff did everything correctly and the PO installed or had them install the coilovers because of the trailer causing the stiffness and handling issues .

 

In this case the springs would be in parallel and the spring constants are just additive. If they were in series, then the spring constant would be calculated like resistance in a parallel circuit.

 

That is what it looks like on a freebody diagram but remember that the leaf springs have no pre-load on them so 200 lbs on a 200 lb/in spring gives you 1 inch of travel. But 200 lb on a 200 lb/in coil spring may give you no travel at all if you have 1 inch of pre-load on it. The point I am making is that results with mixed designs may be unpredictable.

 

So I had a little time while waiting for primer to dry . I took the shock off and measured the resting height 1/4" lower so the coilovers have little affect on ride height , the shock is 14.5" extended and 9.5" fully collapsed so 5" of travel . I put the hitch in and jumped on it and couldn't bottom the shock out with allot of movement so again I don't believe that my " thumping " is from bottoming under compression but from the shot shock bottoming at full extension . Using the 2/3-1/3 formula with a standard shock there would be 2 3/4 " of rebound and only 2 1/4 of compression ( with coilover 50/50 ) . That being said without removing the springs there is no real way to know how much compression is available for tire clearance although measuring the distance from the top of the tire resting with no shock it looks like at least 3" before the fender starts to curve . My problem's now are did the PO install the coilovers because of the trailer , were they installed to prevent bottoming without the trailer , can I get a shock that will allow a little more compression and a little less rebound ? At this time my brain is starting to hurt but I welcome any ideas or insight .

 

Do you have separate bump stops!? Only easy way of increasing bump travel is to raise ride height. If you have axle under spring you can do that with lift blocks. If axle over springs then adjustable hangers or re-arch the leafsprings I guess.

 

The travel you have should not be a major problem though.

 

The travel is currently limited by the shock " no bumpstops " If my thinking is correct a shock 1/2 " shorter shock with the same travel will give me 3" bump and 2" rebound closer to proper . From my measurements there is allot more room for travel but right now my interest is in making the current setup work better not remount the shocks or rework the suspension .

 

So for the update . I had no luck finding anyplace to buy shocks by dimension but I did find a monroe site that showed the dimensions , all 84 pages . I found a shock that seemed really close and phoned Napa with the part no. They told me it was available and was off a 1963-1982 corvette . So here's my DUU moment . The PO " a very good friend now deceased " was known for being cheap at the oddest times . The car previously had a corvette diff which was removed because it didn't stand up to pulling the boler . So knowing Doug the most likely scenerio is that he purchased the coilovers for the vette diff and got the rod shop to use them when the diff was changed to the ford . Not that I'm a great fan of air shocks but I went with them for the simple reason of them allowing me to determine what amount if any of extra support needed . I still need to fine tune them but the initial drive made it hard to believe it's the same car . Me very happy . Thank's everyone for your ideas and help .