Unsprung weight and Tire/Wheel choices

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 1Blazin71, Nov 17, 2001.

  1. 1Blazin71

    1Blazin71 1/2 ton status

    Aug 23, 2000
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    Los Angeles
    Can anyone enlighten me on the effects of unsprung weight and tire / wheel choices?

    I'm putting a 14Bff under my 71 and am somewhat undecided on a D60 vs. 3/4 ton spindles for the front. I'm leaning toward the D60 and so I plan to put 16" wheels on the 14B now so the same wheel will fit the 60 without having to grind the calipers.

    The truck sees mostly 2 track, sand, and moderate rock crawling, and is also my wife's daily driver for her 8 mile commute (god bless her, we dont have to have 3 vehicles).

    The 14B will probably be adding at least 300 Lbs of unsprung weight to the rear so is it a lost cause to be worried about saving wheel and tire weight by going 16x8 on alloys or should I save some $ and go 16x10 in a steel wheel?

    I'm leaning toward either the TSL Radial or the TSL SSR, both of which my 4x4 mechanic buddies argue are too heavy and are suggesting something like the BFG MTR on 16x8 alloys.

    Any thoughts?

    Matthew Meyer
    '71 Ochre K5 - 4" Lift 35"s
    ZZ4 350 - 700R
  2. GONZO2

    GONZO2 1/2 ton status

    Jul 17, 2001
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    formerly Great Falls, MT now Lompoc, Ca
    my philosophy is any weight saved is good.
    if you can afford it go for the allow wheels. i think they look better too.

    <font color=blue>85 blazer</font color=blue>
    <font color=blue>88 blazer</font color=blue>
    <font color=blue>99 30th ann. TA</font color=blue>
    <font color=blue>79 TA</font color=blue>
  3. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

    Jul 23, 2000
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    E-town baby!
    It would be nice to light and nimble...but with what we do with these trucks it's not worth worrying about (IMHO). I did read that rotating mass is even more critical to vehicle performance than unsprung weight. The article I read said that every pound of rotating mass added acts like 8 pounds of weight performance wise. So if you add tires and rims that are 20 lbs heavier it would make the truck feel 480 lbs heavier. When i swapped from 35" radials to 36" TSL's (easily 20 lbs heavier) I didn't really notice much performance loss. The TSL's are quite a bit taller so I think gearing would be more to blame than rotating mass.


    <font color=green>Dyslexics of the world...UNTIE!</font color=green>
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  4. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

    Mar 5, 2001
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    Austin, TX

    Something else to consider.....maybe the wheels is not where you should be looking to shave precious pounds?!?!?

    Those 14-Bolt rear drums are HEAVY!! I have the 13" ones....they are massive and very heavy, even the 10-inchers are a lot of rotating mass. Switching those over to a lightweight rear disc setup would save FAR more weight than the (Alloy vs Steel) wheel idea....IMHO. [​IMG]

    How BIG of a tire are you thinking about by the way?

    I was thinking about going to a 16" wheel for D60 caliper clearance issues too. Not many tires to choose from on a 16" wheel though. I decided to go with 16.5" for better selection.....once I realized that I could get a D.O.T.-approved INTERNAL beadlock system to keep them from popping a bead when I air-down.

    If you are willing to do the internal-beadlock thing (around $200 a tire) you really do open up your tire options a lot over a 16" wheel.

  5. Stephen

    Stephen 1/2 ton status Moderator Vendor

    Sep 15, 2000
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    Carbondale Colorado
    OK, this gets a little long and may be hard to understand the first time around, so you may need to read it again, but I think it's worth posting and reading.

    There's a couple of things you need to watch with heavy parts:
    one is just the weight, which you'll have to carry up the hill, we'll go over that later.

    the other is the weight placement, weight up high is generally bad for tip over, rear weight is going to make it hard to climb, etc.

    With wheels and tires, both are important but the placement is more important. A large heavy tire, meaning the actual tread is wide and heavy, will have a HUGE effect on the performance because of a little physics effect called moment of inertia. Basically an object in motion doesn't want to stop or start, it just likes to do what it's doing already. This is why heavy trucks are hard to stop and why heavy trucks suck at drag racing.

    The way this applies to wheels and tires is that the farther out the weight is from the wheel center, the worse off you are. The extra weight farther out will create more resistance to stopping and starting the rotation of the wheel. In fact, mathmatically speaking, the radius factor in the moment equation is squared, while the mass factor is not, so in this case, size is more important that weight. Going to a larger diameter tire, even if it's lighter that your existing tire, will cost you acceleration and braking performance. This also means that your choice of aluminum or steel wheels isn't going to have nearly the effect on braking or accelerating as your choice of tire size. The drum brake to disc swap would have an even smaller effect since they're pretty close to the center of the axle.

    Now that I've said that, one of the mags did a dragstrip test a while back with identical tires and steel vs. alum wheels and found a measurable performance difference between the two, alum. let the truck stop about 1/2 car length shorter than the steel wheels. If the simple wheel change had that much effect, imagine going from a nice light 33" BFG all terrain to a steel belted 38" swamper SX. It's no wonder big tires feel like they really suck power and are hard to stop. That's why I look seriously at brakes when I consider bigger tires. You can make up for the acceleration loss with gearing, but gearing doesn't help out your brakes.

    Think what this does to the stress on the axleshafts also. Imagine the extremes, sitting in a mud hole with a 28 spl. 10 bolt with a 33 x 9.50 BFG compared to a worn out 44" TSL. (the reason I say worn out is so that it has no traction, just inertia.)
    With the 33" tire, you have a low weight tire with a small diameter, so it's not hard to spin the tires, so when you romp on the go pedal the wheels spin and that's about it. It doens't take much to get it started, especially with no traction.
    With the 44, you romp on the throttle and the tires don't want to move due to the fact that they have a huge "rotational moment of inertia" (lots of weight a long ways from the hub). So the poor little 28 spl axleshaft starts twisting up before the tire even tries to move. Eventually the axle's going to snap because you're going to try to accelerate the tire faster than the axleshaft can stand. That's why we need big blocks and 14 bolts.

    Now, if we ignore the moment of inertia part, the actual weight is also a factor, but much smaller. Taking 80lbs off the axle by going to discs will help climb and accelerate, just because you're not carrying it with you. Same thing with wheels and tires, you'll move around easier if you're not carrying the weight of heavy wheel and tires with you.

    Your suspension will also react much better with less weight on the axles. Imagine hitting a bump in the road and the axle starts to compress toward the frame. If it's really light, it'll fly up a little, then the spring and shock will stop it and send it back down to the road. If it's really heavy it's not going to want to stop and it's going to take longer to slow it down and stop it. That's why IFS trucks ride better than solid axles on the road, they don't have much unsprung weight so the suspension is much more reactive and you get better "feel".

    Now, in the real world...........
    Tire size helps us in so many ways off road that we basically ignore the on road drawbacks until they get too expensive or dangerous.
    Wheel choice often comes down to what we can afford, or what's available for our trucks. We just can't get a 15" alum wheel to fit on an 8 lug axle, so if that's what you want, get steel.
    Most guys would rather run good drum brakes till they're worn out than replace perfectly good drums with discs just for the weight. If you're in a lot of mud or sand, you go to discs for durability and safety.
    We can try to save as much weight as we can on our trucks, but lets face it, it's hard to start with a 4400# truck and make it a light trail rig so we have to be realistic.

    basically in the real world, we can keep all this weight stuff in mind but it's not always the only factor and it may not be the most important one.

    Specifically Matthew, put whatever tires you want on the truck, it's not going to make that big a difference. You should decide using other factors than weight, like durability, cost, traction for your kind of terrain, etc. Same with the alum wheels, if you have the dollars, buy them, you can't go wrong losing the weight off the truck, but it's not that big a deal performance wise in the overall scheme.
    Don't bother with the D60 unless you're running over 36" tires. It's not a waste of money for small tires, it's just not the most efficient use of money on these trucks. 3/4T running gear can work well for you. If you just want the D60, by all means go for it, but I don't think you don't have to have it. There's lots of other info on here about the D60 thing if you search for it, so I'll leave that alone.

    Hope this helps, it's a little technical and long winded, but I think it's worth the read.

    Making the world better, one truck at a time.
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  6. BorregoK5

    BorregoK5 1/2 ton status

    Sep 7, 2001
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    San Marcos, Ca USA
    Bravo, that was the best explaination I've ever heard of moment of innertia and how it relates. Somebody paste that in the tech section quick!

    I started with nothing and I still have most of that left! - <a target="_blank" href=http://www.echobit.com:81/k5/> Pictures</a>
  7. 76chevy

    76chevy 1/2 ton status

    Aug 16, 2000
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    I think you should change your user name to <font color=green>Professor</font color=green>.

    <font color=red>Spirit of 76</font color=red>
  8. JunkYardCrawler

    JunkYardCrawler 1/2 ton status

    Sep 9, 2000
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    Tucson, AZ
    just woundering but whats your education?

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