Do springs unseat when they reach their free length?

[TJim]

I am just trying to figure some stuff out, about my next shocks. I know the right method is to cycle the axles in order to determine the correct shock length. Until I find the time to do that, I am just "playing" with some measurements I have.

*Please correct my thinking since I am might be missing something.*

For example, my front shocks right now (RS55128) have 5.7" up travel and 2.28" down travel. That must be changed ASAP.

RS55128 has a compressed length of 13.05". So, ride height should be: 13.05" + 5.7" = 18.75"
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ABOUT springs unbinding. (I AM TOTALLY IGNORANT IN THIS TOPIC, SO PLEASE HELP)

I am running OME 932 springs, with a free length of 18.90". My front springs now are sitting at roughly 15".

Question: Does that mean than any shock that with down travel greater than 3.90" will make them unbind?

Example:
RS55239 has rougly 5.1" down travel at 18.75" ride height.

That means that my springs at full extension will be 15" + 5.1" = 21.1" ? Does that make sense?

So, my springs have a smaller (18.90") free length, than the number above (21.1"). Does that mean that my spring will unbind in full extension?


Sorry about the numbers? I am just trying to understanding the logic behind this stuff. (Of course the numbers are not 100%, but roughly estimates)

 

[SvtLdr]

Yes, you're thinking is correct. Your current springs will allow about 4" of downtravel before they are loose. It seems like the shocks you have now are a little short. A longer shock would get you closer to 4" up / 4" downtravel, the ideal setup for that spring height.

 

[TJim]

Yes, you're thinking is correct. Your current springs will allow about 4" of downtravel before they are loose. It seems like the shocks you have now are a little short. A longer shock would get you closer to 4" up / 4" downtravel, the ideal setup for that spring height.

You mean 4" on top of the free length of the spring, or on top of the ride height?

 

[TJim]

Yes, you're thinking is correct. Your current springs will allow about 4" of downtravel before they are loose. It seems like the shocks you have now are a little short. A longer shock would get you closer to 4" up / 4" downtravel, the ideal setup for that spring height.

I am between Rancho RS55239 and Skyjacker Black Max B8525, that are going to give me a ratio close to 50/50, by combining a 10mm OME spacer (0.4")

My main concern is whether their extended length will cause the springs to unbind

 

[SvtLdr]

You mean 4" on top of the free length of the spring, or on top of the ride height?

4" past the ride height, according to your numbers.

 

[jjvw]

Starting at ride height, if the remaining shock travel is greater than the remaining spring travel, then the spring will unseat.

For the sake of terminology, the spring unbinds the moment there is space between the windings after coil bind. That is a much different measurement than when it unseats from the spring seats and is knocking around.

 

[TJim]

4" past the ride height, according to your numbers.

My front spring ride height is 14.95". It's free length is 18.9"

That means 14.95" + 4" = 18.95"

So that means that since RS55239 will give me an estimated length of 14.95" + 5.1 = 20.05 will be too long?

 

[SvtLdr]

I am between Rancho RS55239 and Skyjacker Black Max B8525, that are going to give me a ratio close to 50/50, by combining a 10mm OME spacer (0.4")

My main concern is whether their extended length will cause the springs to unbind

You should take a measurement of when the springs get loose. Remove the shock, droop the axle to where the spring loosens up. At that point, measure the distance between your shock mounts. Look for a shock that's close to that max. length. It's not going to be prefect, if your spring loosens up a little it's not the end of the world. The bump stops will keep them from exiting the vehicle.

 

[SvtLdr]

My front spring ride height is 14.95". It's free length is 18.75"

That means 14.95" + 4" = 18.95"

So that means that since RS55239 will give me an estimated length of 14.95" + 5.1 = 20.05 will be too long?

The spring free length isn't the same as the max shock length because they're mounted in different positions. Measure as I described above to figure your max shock length.

 

[TJim]

Starting at ride height, if the remaining shock travel is greater than the remaining spring travel, then the spring will unseat.

For the sake of terminology, the spring unbinds the moment there is space between the windings after coil bind. That is a much different measurement than when it unseats from the spring seats and is knocking around.

Thanks for the terminology. I was completely off here. I meant when it will unseat.

 

[jjvw]

The problem with a loose front spring is that the bottom tail can slip out of the pocket that is formed into the lower seat. When that happens, the ride height changes a bit.

We all know this happens to a whole bunch of rigs with owners chasing down travel and they will say everything is fine. It only really matters when you are trying to fit parts together well.

 

[TJim]

The spring free length isn't the same as the max shock length because they're mounted in different positions. Measure as I described above to figure your max shock length.

Yeap I know!

Shock length at ride height: 18.75"
Spring length at ride height: 14.95"

Total shock (RS55239) length: 23.85"
Spring free length: 18.90"

23,85 - 18.75 = 5.1" (Shock down travel)
14.95 + 5.1 = 20.05" (Theoretical Spring length at full extension of shock)

Since the 20.05" is greater than the 18.9 (free length) that means that the spring will unseat. Correct?

 

[TJim]

So, just to state it more clearly without numbers.

Will the spring unseat when the free length is reached/exceeded when fully extending the shock?

 

[jjvw]

So, just to state it more clearly without numbers.

Will the spring unseat when the free length is reached/exceeded when fully extending the shock?

If the remaining shock travel is greater than the remaining spring travel, then the answer is yes.

 

[TJim]

If the remaining shock travel is greater than the remaining spring travel, then the answer is yes.

Ouch. That makes my life so much more difficult.

The RS55239 is now a no go.

I just realised that for someone with a very light TJ, that a spring will net him more lift than a heavy one is more difficult to find the appropriate springs.

In my case, I netted a lift close to 3", with a 18.90 free length (OME 932) 2" spring. That leaves me with no options at all for a near 50 50 shock.

 

[JMT]

Ouch. That makes my life so much more difficult.

The RS55239 is now a no go.

I just realised that for someone with a very light TJ, that a spring will net him more lift than a heavy one is more difficult to find the appropriate springs.

In my case, I netted a lift close to 3", with a 18.90 free length (OME 932) 2" spring. That leaves me with no options at all for a near 50 50 shock.

Welcome to the club that figured that out. 3” is hard to find a good shock for, which is one reason we end up at 4”.

 

[SSTJ]

So, just to state it more clearly without numbers.

Will the spring unseat when the free length is reached/exceeded when fully extending the shock?

I'm still pretty new at this, but thought I'd try to sum up what I'm hearing so far from the helpful replies above.

First, I agree that it probably doesn't make sense to do too much math in advance, because the shock and spring are mounted in different locations, and at different angles. This means that a 1" change in spring length does not equal a 1" change in shock length. So, although some estimation may help in the choice of shocks, it would be very difficult to do the math perfectly. Instead, it sounds like it's worth the time to remove the shock, droop the axle to the point that the spring is about to un-seat (not 'un-bind'), and then measure the distance between the upper and lower shock mounts. That will tell you the max length you want for a shock. (But, I don't know if the lengths offered online for shocks are meant to refer to the distance between the shocks mounting points, or if they are meant to refer to the length from some other points on the shock. Others probably know.)

Second, it sounds like if you end up with a shock that will allow the spring to extend a little bit past the spring's full extended length, you're probably ok because the spring will not exactly fall off. The bump stop will prevent it from falling off up top ('tipping over'). And on the bottom, the lower perch has that little clip that holds the bottom of the spring in the little seat/groove that carved into the lower perch.

Third, this is all just to ensure that the choice of shocks aren't too long, and yet give the maximum length possible. But I wonder, is maximum length always best? Does maximum length ensure that you'll have the desired ratio between up-travel and down-travel? Maybe so, but that's not obvious to me yet.

 

[SSTJ]

Ouch. That makes my life so much more difficult.

The RS55239 is now a no go.

I just realised that for someone with a very light TJ, that a spring will net him more lift than a heavy one is more difficult to find the appropriate springs.

In my case, I netted a lift close to 3", with a 18.90 free length (OME 932) 2" spring. That leaves me with no options at all for a near 50 50 shock.

This is true. I used it to my advantage when wanting a very minor lift in the front, to even out the factory 'rake' on my SE. I bought Moog replacement springs for a Rubicon model, and it gave me almost exactly the desired 0.75" of lift up front. But I had it easy in making that choice, because I found another forum user who had done the same, and got 0.5" of lift on a TJ that was heavier than mine.

But I thought you were sticking with the same springs and just changing your shocks? Why the worry about how springs length will vary with ride weight?

 

[TJim]

Welcome to the club that figured that out. 3” is hard to find a good shock for, which is one reason we end up at 4”.

Actually If I had a 3-3.5" lift with a spring with a free length around 20", RS55239 would be perfect.

My problem is that my TJ is so light that the free length of my springs is too small.

 

[TJim]

This means that a 1" change in spring length does not equal a 1" change in shock length.

On the front side this should be very close.

Third, this is all just to ensure that the choice of shocks aren't too long, and yet give the maximum length possible. But I wonder, is maximum length always best? Does maximum length ensure that you'll have the desired ratio between up-travel and down-travel? Maybe so, but that's not obvious to me yet.

More shock length is always desired. But of course you should aim for a near 50/50 ratio.

But I thought you were sticking with the same springs and just changing your shocks? Why the worry about how springs length will vary with ride weight?

I am sticking with the same springs. I would add only a 10mm (0.4") spacer.

My point is the following:

LIGHT TJ - 3" with 18.90" free length springs. That means a shock with greater than 18.90" cannot be used.

HEAVY TJ - 3" with >20" free length springs. That means there are more shocks available for the same "actual" lift for a heavy TJ than a light TJ, since a spring with a greater free length is used.


*In my case, having so much lift with a so small free length spring leaves me with ZERO options of shock with near 50/50 ratio, since a shock with less than 22.75" extended length should be used.