Spring lift calculations

[UKTJ]

Following on from the excellent new sticky on spring and shock specifications in the Resources section I have been working through what lift I might achieve on my TJ. My current thinking is the OME 2” springs, however, my calculations suggest I will achieve more than 2”, so I am hoping some body can check my logic.

My TJ is currently a stock 4.0l non Rubicon. That means it has a front corner load of about 719lbs and a rear corner load of about 653lbs. It currently sits on stock springs with a factory install height of 11” front and 8” rear.

In the near term I plan to add a Tomken winch plate weighing c.20lb and a Warn VR Evo 10k with synthetic line weighing c.73lb. So for the sake of argument let’s call that 40lb a corner at the front, taking the total front corner load to 759lb. No planned additional weight to the rear at the moment.

The OME 2932 front spring has a free height of 18.90” and a spring rate of 140lb per inch. With a corner load of 759lb that gives an install height of c.13.5” (18.9-(759/140)). That is c.2.5” over the stock 11”.

The OME 2941 rear spring has a free height of 15.53” and a spring rate of 140lb per inch. With a corner load of 653lb that gives an install height of c.10.9” (15.53-(653/140)). That is c.2.9” over the stock 8”.

Can somebody more expert than me in such things let me know if that all makes sense?

 

[CodaMan]

My head hurts

 

[UKTJ]

I was talking rubbish when I said I would not be adding weight to the rear, because I will be increasing the size of the spare and adding a beefed up carrier, so I actually think I will add about 35lb a corner to the rear. This gives me the following calculated lift.

Front: OME 2932 - free height 18.90” and a spring rate of 140lb per inch. With a corner load of 759lb that gives an install height of c.13.5” (18.9-(759/140)). That is c.2.5” over the stock 11”.

Rear: OME 2941 - free height 15.53” and a spring rate of 140lb per inch. With a corner load of 689lb that gives an install height of c.10.9” (15.53-(689/140)). That is c.2.6” over the stock 8”.

Hoping a blatant tag of @jjvw who seems to be the resident spring / shock expert and who has really helped move my understanding forward, will get me confirmation or correction of my calculation methodology.

 

[TheBoogieman]

Is it April 1st already?

 

[UKTJ]

Is it April 1st already?

?

 

[TheBoogieman]

?

April Fool's Day is an annual custom on April 1 consisting of practical jokes and hoaxes. You're making things way more complicated than they have to be. I buy 3" lift springs to lift my Jeep 3". If my heavy bumper and winch weigh it down 1," I add 1" spacers or 4" springs. Done.

 

[UKTJ]

April Fool's Day is an annual custom on April 1 consisting of practical jokes and hoaxes.

Still not with you.

 

[1o1proof]

Engineer, no doubt.

 

[SSTJ]

Don't listen to these haters. I like the way you're thinking this through. I'm not in a position to check your math or offer any lessons from experience, but I'm eager to see how this works out.

 

[UKTJ]

Engineer, no doubt.

Me?

 

[TheBoogieman]

Engineer, no doubt.

I prefer to keep things simple.

 

[Vasq]

I was talking rubbish when I said I would not be adding weight to the rear, because I will be increasing the size of the spare and adding a beefed up carrier, so I actually think I will add about 35lb a corner to the rear. This gives me the following calculated lift.

Front: OME 2932 - free height 18.90” and a spring rate of 140lb per inch. With a corner load of 759lb that gives an install height of c.13.5” (18.9-(759/140)). That is c.2.5” over the stock 11”.

Rear: OME 2941 - free height 15.53” and a spring rate of 140lb per inch. With a corner load of 689lb that gives an install height of c.10.9” (15.53-(689/140)). That is c.2.6” over the stock 8”.

Hoping a blatant tag of @jjvw who seems to be the resident spring / shock expert and who has really helped move my understanding forward, will get me confirmation or correction of my calculation methodology.

Makes sense to me.

 

[UKTJ]

Makes sense to me.

That's really helpful, many thanks.

 

[InOmaha]

People in the US are math and critical thinking averse. They tend to buy stuff, try it, and throw it away if if didn't do what they want.

After going online and bitching about why someone else didn't warn them they were making the wrong choices.

Measure twice, cut once.

The guy making the parts is doing all the same stuff you are. Because it's the right way to do it. There's a gear calculator app everyone in the 4x4 world uses, because it's free and handles the math for them. There's probably a spring app out there too.

I looked at the OME information in detail, because I wanted to tow a small trailer with the LJ. That adds 200-400 lbs of down force on the rear bumper.

Unloaded I gained 3.5 inches of lift in the rear with 2949 springs. 2.75" in the front with the 2933 springs. So I added a .75" spacer to get 3.25" and level things out.

 

[InOmaha]

The problem most people have is they get analysis paralysis because they go online and listen to opinions while gleaning some facts.

Set your boundary conditions and solve the problem from there. A street able jeep set up as a DD to tow small loads has a different set of boundary conditions to meet then a trailered rock crawler. They overlap in ability in some areas and not in others. Neither is "wrong". Just different.

People tend to preach what's the best setup for their conditions instead of listening to figure out the actual best recommendations. Dana 30 HP sucks, 231 sucks, you need these gears, blah, blah, blah.

I want a decent enough rig for mid level off road and still drivable on the interstate with a trailer.

If I want a hard core rock crawler, I'll buy one from one of the people selling them as they get bored of storing it.

My neighbor has a Jerp he gets to take out on nice days and drive around town. He trailers it to Colorado or Utah once or twice a year. And he drives his truck 95% of the time. I don't want that setup even though it's a great rig.

 

[CodaMan]

People in the US are math and critical thinking averse. They tend to buy stuff, try it, and throw it away if if didn't do what they want.

Why would you generalize like that?

As an Electrical Engineer I tend to overthink things, but I also know when the K.I.S.S. method is appropriate.

 

[eastbloc]

People in the US are math and critical thinking averse. They tend to buy stuff, try it, and throw it away if if didn't do what they want.

After going online and bitching about why someone else didn't warn them they were making the wrong choices.

Measure twice, cut once.

The guy making the parts is doing all the same stuff you are. Because it's the right way to do it. There's a gear calculator app everyone in the 4x4 world uses, because it's free and handles the math for them. There's probably a spring app out there too.

I looked at the OME information in detail, because I wanted to tow a small trailer with the LJ. That adds 200-400 lbs of down force on the rear bumper.

Unloaded I gained 3.5 inches of lift in the rear with 2949 springs. 2.75" in the front with the 2933 springs. So I added a .75" spacer to get 3.25" and level things out.

The problem most people have is they get analysis paralysis because they go online and listen to opinions while gleaning some facts.

Set your boundary conditions and solve the problem from there. A street able jeep set up as a DD to tow small loads has a different set of boundary conditions to meet then a trailered rock crawler. They overlap in ability in some areas and not in others. Neither is "wrong". Just different.

People tend to preach what's the best setup for their conditions instead of listening to figure out the actual best recommendations. Dana 30 HP sucks, 231 sucks, you need these gears, blah, blah, blah.

I want a decent enough rig for mid level off road and still drivable on the interstate with a trailer.

If I want a hard core rock crawler, I'll buy one from one of the people selling them as they get bored of storing it.

My neighbor has a Jerp he gets to take out on nice days and drive around town. He trailers it to Colorado or Utah once or twice a year. And he drives his truck 95% of the time. I don't want that setup even though it's a great rig.

 

[jjvw]

How did you arrive at the sprung weights? Not saying they are incorrect. I just don't know what they are.

 

[jjvw]

View attachment 235450

Don't worry. I got this.

 

[Mike_H]

I think you're on the right track. I've done similar. The issue you run into is that the math you do is only as good as the information you have to plug into the formula. Spring rates are not widely published. You notice how many blanks are in @jjvw's table. The ones that are there... Well maybe they are right, maybe they are not. It's not unheard of for companies to publish the wrong information, on purpose (see gen-right bumper thread). So, unless you buy the springs and measure them yourself, you don't really know.

So... You can try to figure it out ahead of time, doing so will increase your understanding, but in the end, it's still a crap shoot.