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Unsprung Weight - how does it apply to hot rods?

Discussion in 'The Hokey Ass Message Board' started by fiftyv8, Dec 10, 2008.

  1. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,394

    fiftyv8
    Member
    from CO & WA

    What does it do and is it our friend or enemy???
     
  2. Von Rigg Fink
    Joined: Jun 11, 2007
    Posts: 13,418

    Von Rigg Fink
    Member
    from Garage

  3. tpw35
    Joined: Feb 6, 2007
    Posts: 342

    tpw35
    Member

    Unsprung weight is anything that isnt sprung, tires, wheels, rearend, etc... It effects handling, this is one reason for lighter wheels, turning shocks upside down etc...
     
  4. 64 DODGE 440
    Joined: Sep 2, 2006
    Posts: 4,421

    64 DODGE 440
    Member
    from so cal

    It doesn't know if it's affecting a hot rod or a long haul semi.......the results are the same.

    The less you have, the better the tires remain in contact with the ground and in doing so, the vehicle will me more controllable.
     

  5. turdytoo
    Joined: May 14, 2007
    Posts: 1,568

    turdytoo
    Member

    Putting heavy cast spinner wheels on your car will have a negative affect on the HAMB.
     
  6. pwschuh
    Joined: Oct 27, 2008
    Posts: 2,827

    pwschuh
    ALLIANCE MEMBER


    UW is your enemy. Ideally you would have no unspring weight but that's not possible. So, if you're concerned about handling, you just make it as low as practical. If you only want to drag race, you're not so much concerned with this.
     
  7. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,681

    Bruce Lancaster
    Member Emeritus

    It is a HUGE problem for typical rods, because we tend to use parts from 4,000 pound cars on 2,000 pound roadsters...unsprung can easily reach ridiculous percentage of car weight with something like a '29 roadster and 9" Ford, disc brakes, etc. The need for strong running gear leads right to the heavy section in the junkyard.
    Early Fords stock are a study in reducing it; Henry made light cars, and hated US weight.
    A datum point: The Connecticut flathead stock cars, last flathead stock car racers, all cars seemed to be '33-36 Ford coupes. An experienced racer said that 9" Ford rears had been tried repeatedly by people looking for something cheaper than their QC's, but were always found to destroy handling on the light cars!
    This, by the way, is the main reason for IRS, not handling. Try to achieve 10% US weight on a 2,000 pound car and that's where you will be going...
     
  8. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,681

    Bruce Lancaster
    Member Emeritus

    A good illustration of the impossibility of controlling very high US weight can be seen in monster trucks on TV...you see the thing make what seems like a fairly minor bounce, and then just go on hopping til it turns over...the truck is simply too light to hold down its gigantic Army Truck axles and monster tires, and not much can be done about it because any shocks or springs used still have to attach to the bit that's too light.
     
  9. gnichols
    Joined: Mar 6, 2008
    Posts: 11,345

    gnichols
    Member
    from Tampa, FL

    I remember reading someplace (a wheel mfg advert?) that reducing the unspring weight by one pound was like shaving 6-8 pounds off chassis weight. Anyone confirm or deny that? Regardless, that's why I'm going to get the strongest and lightest wheels for my rod, probably Weld or something similar. Gary
     
  10. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,394

    fiftyv8
    Member
    from CO & WA

    I am glad I asked. Seems like you say we are adding rather than taking away weight.
    We seem to be shaving it off the bodies and frames but not the unsprung areas which would be to greater advantage.

    Food for thought!

    I got some 35 modified wires that could do with some weight loss.
     
  11. brandon
    Joined: Jul 19, 2002
    Posts: 6,368

    brandon
    Member

    its still very important to the drag guys...why use a wheel thats 10 pounds , when you can use a 7 or 8 pound one ...it all adds up...i talked to a stocker class drag guy at bowling green . the more i looked at his car , the more trick and light weight stuff i found...its free hp...though , not usually cheap..:D this guys ride was subtle trick.... titanium studs and lug nuts , every bolt under the rearend , aluminum hubs, and hats on the brakes...i can't remember the exact figure he saved on weight , but it was 3 figures on the over all package...... brandon:D
     
  12. 29nash
    Joined: Nov 6, 2008
    Posts: 4,544

    29nash
    BANNED
    from colorado

    As old as the invention of the wheel. Have you ever rode on a farm wagon, with no springs?
     
  13. AnimalAin
    Joined: Jul 20, 2002
    Posts: 3,416

    AnimalAin
    Member

    Low unsprung weight is good. Low moment of inertia is good, as well. The smaller and lighter the wheels,tires, and rotating brake mass, the better. Of course, there is a tradeoff with traction and stopping ability. Might be the reason engineers get the big bucks, figuring out how to balance numerous competing priorities to an optimum. Did I mention that cost is always part of the equation, as well?
     
  14. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    Quoting myself:
    " Unsprung vs. sprung weight have no difference in their effect on acceleration or top speed. There is no “1-10” rule (or any other ratio) where 1 lb. removed from unsprung weight “has the same same effect as” a higher amount of sprung weight. Any benefit from weight reduction towards increased MPH or reduced ET will be exactly the same as if the weight were removed from the chassis. Weight removed from an unsprung component, such as a rear wheel or axle housing, may affect traction if the wheel is not under control during launch.
    Lighter wheels & tires do have a very small additional benefit due to the lower amount of power required to rotate them (true of all rotating components), but this is not due to their classification as unsprung weight. "
     
  15. BigChief
    Joined: Jan 14, 2003
    Posts: 2,084

    BigChief
    Member

    We're getting unsprung weight confused with rotating weight here.....The scale (or the laws of physics) doesn't care wether or not the weight is sprung or unsprung. Any reductions in unsprung weight have a far greater positive effect on handling than reducing the overall weight of the car does.....conversely, ANY reduction in weight off the car is going to get you an increase in performance in acceleration and handling.

    There are no really hard and fast 'rules of thumb' that can be accurrately applied to the phrase "for every one pound off the wheel is like getting three pounds out of the trunk". To even begin sorting this all out you'd have to start thinking in terms of "equivalent mass" which refers to a portion of car mass which would have the same effect on acceleration performance as the rotating inertia of that component that is accelerated, rotationally, as the car accelerates. Herb Adams' Chassis Engineering Handbook probably has a reasonable example as a guide....Herb says ....basically 1lb removed from components rotating at engine speed is equivalent to 15lbs removed from the chassis and 1lb of mass removed from components running at driveshaft/wheel speed is the same as removing 3lbs off the chassis. Again, loose figures. There's some measurement of actual components on your car and many calculations needed to see what your gains would be. Another good example Adams' used as a reduced rotating inertia problem was a 600hp V6 vs a 600hp V8 being run in the same car with the same on-track weight. All things being equal the V6 would accelerate faster than the V8 due solely to the lighter rotating mass.

    -Bigchief.
     
  16. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    Still too vague and misleading.
    1 lb. off the center of the clutch has almost no effect; 1 lb. off the diameter of the clutch has an enormous effect. This is why a driveshaft tube isn't cost effective - it runs too slowly, and it's too small.
    The "mean radius of gyration" (not the diameter) is very important, but Mr. Adams has omitted it - not because of ignorance, but to make it easier to read. The actual math is very annoying, and a complex 3-dimensional shape such as a crankshaft pretty much needs a 3-D drawing to calculate rotational inertia.
     
  17. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,394

    fiftyv8
    Member
    from CO & WA

    Well at least we know now that it is worth reducing unsprung weight if possible, probably make shocks and tires last a little longer to.
     
  18. BigChief
    Joined: Jan 14, 2003
    Posts: 2,084

    BigChief
    Member

    Yeah.....thats where the phrase - (There are no really hard and fast 'rules of thumb' that can be accurrately applied to the phrase "for every one pound off the wheel is like getting three pounds out of the trunk".) - comes in.
     
  19. My Torque Thrust II's and new tires knocked off 12 lbs. off the rear alone but my tire wear has increased significantly followed by a severe lack of traction...!

    Why??????????

    I replaced my 40 Ford drum brakes for vintage drag disks using a Magnum Hub and JFZ calipers which helped me shave off 42 Lbs. of reciprocating mass from the front end.

    I'd like to reduce the weight of my Winters Quick Change and rear disk brakes to get the Roadster on an unsprung weight diet!

    Car weighs 1,840 Lbs. right now and I suspect the Quick Change is close to 175 lbs.
     
    Last edited: Dec 21, 2008
  20. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,394

    fiftyv8
    Member
    from CO & WA

    Yeah that traction thing always get you in the end.
     
  21. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,394

    fiftyv8
    Member
    from CO & WA

    Nobody really mentioned drilled front axles to reduce front unsprung weight!!!
     
  22. lowsquire
    Joined: Feb 21, 2002
    Posts: 2,567

    lowsquire
    Member
    from Austin, TX

    at the outerlimits of reducing weight, drilled beam axles may be of benefit, but theres a hell of a lot more hefty items to shave down first!!consider the weight in hubs, brakes,wheels..theres a lot more scope for weight saving there than half a pound of holesaw shavings!!

    the only reason i drilled my front axle is because its against the law here. :p

    [​IMG]
     
  23. autocol
    Joined: Jul 11, 2002
    Posts: 589

    autocol
    Member

    shit yeah lowsquire. that made me laugh...
     
  24. striper
    Joined: Mar 22, 2005
    Posts: 4,498

    striper
    Member

    Ha, Ben. I like the little addition. Just in case the cop thought he might let you off, when he takes a closer look you've sealed the deal! Beautiful.

    Pete
     
  25. striper
    Joined: Mar 22, 2005
    Posts: 4,498

    striper
    Member

    BTW guys, while the inertia and rotating mass stuff is interesting, the question was about unsprung weight and how it affects hot rods. I'd like to read more about that.

    Pete
     
  26. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,681

    Bruce Lancaster
    Member Emeritus

    The less the better. Usually analyzed as % of total vehicle weight, so it gets to disastrously high fast on something like a Ford roadster with 9" rear and big discs and fat tires...
    I have seen discussions offering rule of thumb cutoffs for good and bad percentage, but don't remember. On a light rod, good is going to be nearly impossible with common components as most parts that are strong enough are made for cars that weigh twice as much. IRS and IFS probably only realistic path once car gets real light. Rodders tend to use components made for a 5,000 pound Ford wagon, say, on a 2,000 pound A...
    I think a crude statement of part of the problem is that a light car cannot provide enough inertia to anchor the suspension over heavy running gear...like those monster trucks, or the '34 Ford stock cars with 9" rears. Suspension movement then lifts the car that is supposed to be holding it against the ground...
     
  27. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,394

    fiftyv8
    Member
    from CO & WA

    Good point Bruce.
     
  28. drpushbutton
    Joined: Oct 28, 2008
    Posts: 43

    drpushbutton
    Member
    from Kansas

    Because the tires and wheels are easier to rotate now. Kinda like going to a lighter flywheel; the engine gains rpm easier. Now, so do your tires and wheels. If you figure out how to plant the tires the car will be quicker though.
     
  29. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    Because the tires and wheels are easier to rotate now

    Read several previous posts: has nothing to do with rotation.
     
  30. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,681

    Bruce Lancaster
    Member Emeritus

    I think unsprung weight and this discussion are mostly about handling/ride effects, not the acceleration aspects of rotating stuff...from the dragstrip accel point of view, unsprung is probably less important...those cars have a nearly perfect road surface and go in a straight line. It is in handling behavior that unsprung weight fights the car.
     

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