Where do the lift height numbers come from? Is it just marketing?

[BanjoJeep]

So this came up on the OME thread, and I've seen it for some other brands too, so now I'm curious if more experienced people know.

What is a 2.5" lift, really? Every place you go, you see them selling 2.5" lift kits or 4" lift kits. But in reality, they're almost always closer to <2" and ~3". Some people report even less. What gives? Obviously, there's variance for loading. We don't need to go through all that, I understand that heavy jeeps won't get the same lift from the same spring as a light jeep. But it seems to me that even with the lightest vehicle, there is no way you're actually getting 2.5" from what is being sold as a "2.5 inch lift kit".

Why not just sell them as 2" kits? Is the 1/2 inch really that big of a marketing pull? It's across the board too, all retailers and brands use these convention.

 

[jjvw]

A company with a TJ has a set of springs made to some set of specifications. The company installs these springs on the test TJ and measure the amount that the ride height increased from before. If the company is satisfied with the results, they offer these springs for sale with an advertised ride height based on what happened on the test TJ.

What resulted on the company TJ may be different than what happens on your TJ.

 

[sab]

In addition to what jjvw said, I can think of two other reasons why the lift you realize differs from the lift advertised:

1. Tolerances in spring manufacturing (spring rates from one spring to the next vary more than you'd think in production)
2. Jeeps are some of the most modified vehicles out there, and many of the mods add considerable weight, which makes predicting a lift number for your TJ nearly impossible

 

[BanjoJeep]

But how can a spring that is only 2" taller than stock be advertised as a 2.5" lift? I don't get it. Lots of variables, sure, but none of the variables are going to stretch the springs upward to attain the advertised lift regardless of how you adjust the jeep itself. How can a 2" longer spring add 2.5" of lift? I've gotta be missing something here.

 

[JEEPCJTJ]

the actual spring length by itself is irrelevant. When installed and with the Jeep on the ground, the original stock front spring should measure at 12" the 2.5" lift spring should measure 14.5".

I'm not sure if they're measured correctly or not but I wouldn't be surprised if the accountants and the marketing people are involved. Designers instead of engineers may play into it too.

 

[sab]

...I've gotta be missing something here.

You are - spring rate. Free length and spring rate determine the length of the spring when stationary based on the corner weight of the TJ. The spring equation is Corner Weight = Spring Rate x (Free length - Installed length)

 

[BanjoJeep]

I foolishly assumed aftermarket springs would have a lower spring rate. Thanks boys.

Are the oem spring rates published anywhere?

 

[jjvw]

I foolishly assumed aftermarket springs would have a lower spring rate. Thanks boys.

Some aftermarket springs have a high spring rate.

 

[sab]

I foolishly assumed aftermarket springs would have a lower spring rate. Thanks boys.

There's a spring rate chart in the TJ Resources forum, and if you look at it, you'll see that they are all over the place.

 

[jjvw]

...

Are the oem spring rates published anywhere?

Years ago, I made a spreadsheet listing a whole bunch of TJ springs with their rates and free lengths.

 

[Gilaguy23]

I bought a bone stock 99 save some 31's. I installed a 2" coil spacer in front to level it out and stop the tires from rubbing when flexed. IIRC that made for 4 3/8" from the tire to the edge of the flare. I yanked the spacers and installed BDS 2" lift coils. I now have 6" from the edge of the flare to the tire. So basically I gained about 4' in front from 2" coils. The rear I threw in a 1" coil spacer and not it sits at 6" space at either end and just about perfectly level.

 

[jjvw]

I bought a bone stock 99 save some 31's. I installed a 2" coil spacer in front to level it out and stop the tires from rubbing when flexed. IIRC that made for 4 3/8" from the tire to the edge of the flare. I yanked the spacers and installed BDS 2" lift coils. I now have 6" from the edge of the flare to the tire. So basically I gained about 4' in front from 2" coils. The rear I threw in a 1" coil spacer and not it sits at 6" space at either end and just about perfectly level.

Subtracting 12" and 8" from the current ride height of the springs is much more useful and more consistent in communicating a lift height. Measuring from tire to flare is chasing a whole bunch of moving targets.

 

[Gilaguy23]

Perhaps, but my point being worn out coils can make things seem off from what one might expect.

 

[jjvw]

Perhaps, but my point being worn out coils can make things seem off from what one might expect.

That is why measuring the coils is valuable. And it is part of the larger understanding of how spring rates and free lengths work with the vehicle sprung weights to create a ride height.

 

[Superjay5]

Weight. I’m guessing there is a 500lb difference from the factory options that were offered. Hardtop/full door vs.softtop/soft doors. Motor/trans options.. etc.

My rear changes ride height by about 1/2” depending if I have a full tank of gas or almost empty.

Add in a ton of aftermarket parts and jeeps can widely very in weight.

Manufactures can’t and it’s not even possible to account for all of that.

Spring length and ride height are two different things.

 

[LONGJP2]

I bought a bone stock 99 save some 31's. I installed a 2" coil spacer in front to level it out and stop the tires from rubbing when flexed. IIRC that made for 4 3/8" from the tire to the edge of the flare. I yanked the spacers and installed BDS 2" lift coils. I now have 6" from the edge of the flare to the tire. So basically I gained about 4' in front from 2" coils. The rear I threw in a 1" coil spacer and not it sits at 6" space at either end and just about perfectly level.

IIRC, the flares are not the same size.
The rear hangs down lower so the factory rake is less noticeable.

Speaking of rake, some lifts are designed to eliminate it.
So only the front springs lift the advertised amount.