Unsprung weight??
12-16-2009, 07:48 AM
#1
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Unsprung weight??
I have spent my fair share of time searching and searching various forums to learn what I can about unsprng weight and the consequences running large tires. All I have learned is that to heavy can be bad and there may or may not be some formula the translates unsprung weight into sprung weight. What that means in English, I have no idea. Most of what I been able to find online actually pertains to hot rods and drag racing.
I am looking at three different tire sizes, all KM2's:
285/70-17 (by my math work out to be a 32.7x11.2x17) weighs 60lbs.
305/65-17 (32.6x12x17) weighs 67lbs.
35x12.5x17 weighs 68lbs.
My current setup is stock. I have 255/75-17 Goodyears SR-A's on stock rims and that setup weighs a total of 60lbs (on my home scale...which should be 38 lbs of tire and 22 pounds of rim if my internet research is correct).
What are the consequences of going from my stock 60lbs of unsprung weight up to an aftermarket 85lbs (60lbs for the 285/70-17 and 25 for aftermarket rims)? I am concerned about various factors in no real order...handling (both on road and off), gas millage and axle life to name a few. What if I add the extra 8lbs for the 35's? If I am already going up to the 285/70-17's then is the extra 8lbs that big of a jump?
I understand that the bigger you go to more "sluggish" our fantastic engine will perform. I understand the eventual concern for regearing. That deals with performance. Does that also handle the torque affects of my D30 trying to spin that extra weight around?
Thanks for your time and any help you can give.
I am looking at three different tire sizes, all KM2's:
285/70-17 (by my math work out to be a 32.7x11.2x17) weighs 60lbs.
305/65-17 (32.6x12x17) weighs 67lbs.
35x12.5x17 weighs 68lbs.
My current setup is stock. I have 255/75-17 Goodyears SR-A's on stock rims and that setup weighs a total of 60lbs (on my home scale...which should be 38 lbs of tire and 22 pounds of rim if my internet research is correct).
What are the consequences of going from my stock 60lbs of unsprung weight up to an aftermarket 85lbs (60lbs for the 285/70-17 and 25 for aftermarket rims)? I am concerned about various factors in no real order...handling (both on road and off), gas millage and axle life to name a few. What if I add the extra 8lbs for the 35's? If I am already going up to the 285/70-17's then is the extra 8lbs that big of a jump?
I understand that the bigger you go to more "sluggish" our fantastic engine will perform. I understand the eventual concern for regearing. That deals with performance. Does that also handle the torque affects of my D30 trying to spin that extra weight around?
Thanks for your time and any help you can give.
12-16-2009, 08:34 AM
#2
JK Jedi Master
If you are trying to make your Jeep more capable off-road, unsprung weight can't be a part of the formula. Most of the first steps to improvement involve weight added to the axles.
12-16-2009, 09:17 AM
#3
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So I see two issues, 1 unspring weight vs spring weight and 2 added weight tires vs axle strength.
1. I'm by no means an expert on suspension but my feeling has always been that for offroading purposes, unspring weight is a good thing. As you get more of the vehicles total weight in the alxes/tires, ultimately the lower center of gravity the vehicle will have reducing likely hood of a tip over.
But adding weight the vehicle anywhere makes it heavier and affects performace. MPG, etc.
2. I=(m*r^2)/2, even without anyweight on the tires and not considering the weight of the entire jeep, it takes power just to spin the tires (even if the tires are off the ground. This is called moment of inertia, in that formula increasing the mass directly increases this force, increasing the radius affects by a squared factor. What does that all mean. bigger tires affects things more than a heavier tire.
The better question to be thinking about is axle strength vs tire diameter. There is many threads on what a D30 can/can't handle.
But what you're thinking of doing is not uncommon, all the theory and math can't come near to saying what real life experience would say a D30 can/can't handle. As for regearing, that uses higher engine rpms to produce the torque necessary to turn the tires. There is a set amount of torque required to turn those tires, you can get that from raw engine strength (which our JKs don't have) or from higher enginer rpms and aggressive gearing (that we can do).
Sorry for the long winded explaination....hope it helps.
1. I'm by no means an expert on suspension but my feeling has always been that for offroading purposes, unspring weight is a good thing. As you get more of the vehicles total weight in the alxes/tires, ultimately the lower center of gravity the vehicle will have reducing likely hood of a tip over.
But adding weight the vehicle anywhere makes it heavier and affects performace. MPG, etc.
2. I=(m*r^2)/2, even without anyweight on the tires and not considering the weight of the entire jeep, it takes power just to spin the tires (even if the tires are off the ground. This is called moment of inertia, in that formula increasing the mass directly increases this force, increasing the radius affects by a squared factor. What does that all mean. bigger tires affects things more than a heavier tire.
The better question to be thinking about is axle strength vs tire diameter. There is many threads on what a D30 can/can't handle.
But what you're thinking of doing is not uncommon, all the theory and math can't come near to saying what real life experience would say a D30 can/can't handle. As for regearing, that uses higher engine rpms to produce the torque necessary to turn the tires. There is a set amount of torque required to turn those tires, you can get that from raw engine strength (which our JKs don't have) or from higher enginer rpms and aggressive gearing (that we can do).
Sorry for the long winded explaination....hope it helps.
05-02-2019, 01:53 PM
#4
The heavier the unsprung weight the more upward force there is when a wheel goes over a bump and wants to go airborne. The heavier the wheel, the harder it is for the suspension to quickly force it back down and keep it in contact with the ground, and the harder it is to get effective dampening to keep the wheel from oscillating up and down.
Heavier wheels travelling upwards at high speed until the weight of the body and the springs force them back down may also cause more felt impact inside the cabin.
off road at slow speed, I.e. rock crawling, this doesn’t become an issue until it really prevents the springs from keeping the tire in contact with the ground (traction). At high speed, too much unsprung weight could make the jeep uncontrollable over a bumpy surface. For the size tires you’re talking about, probably not much of an issue. Imagine going 70 over a washboard section of dirt road with 200+ pound 49x21 swampers, and things might start to get scary, especially with a live axle.
most go fast vehicles have ifs and lower unsprung weight. Baja racers tend to have smaller tires than rock crawlers and mud trucks, and many have ifs and irs. Independent suspension and lower unsprung weight are going to keep the truck level and controllable at high speeds over rough terrain better than a live axle with huge tires. They won’t be as strong or capable in rocks or mud though.
Heavier wheels travelling upwards at high speed until the weight of the body and the springs force them back down may also cause more felt impact inside the cabin.
off road at slow speed, I.e. rock crawling, this doesn’t become an issue until it really prevents the springs from keeping the tire in contact with the ground (traction). At high speed, too much unsprung weight could make the jeep uncontrollable over a bumpy surface. For the size tires you’re talking about, probably not much of an issue. Imagine going 70 over a washboard section of dirt road with 200+ pound 49x21 swampers, and things might start to get scary, especially with a live axle.
most go fast vehicles have ifs and lower unsprung weight. Baja racers tend to have smaller tires than rock crawlers and mud trucks, and many have ifs and irs. Independent suspension and lower unsprung weight are going to keep the truck level and controllable at high speeds over rough terrain better than a live axle with huge tires. They won’t be as strong or capable in rocks or mud though.