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How light is too light ?

Discussion in 'The Hokey Ass Message Board' started by jaxx, Apr 23, 2009.

  1. jaxx
    Joined: Mar 22, 2008
    Posts: 402


    All parts for the 25/26 dodge bucket Im building are gathered and finding the proper location on the car and Im finding that this thing will only weigh about 950 lbs curb weight. Is that too light for good braking ??? Ive heard that too light will slide the tires. Is that true ? Just a queation that I thought I might get a quick and easy answer to - thanks - Jaxx
  2. 34Chrysler
    Joined: Dec 16, 2005
    Posts: 300


    Use a big block mopar with iron heads, that will make it heavier
  3. 950 pounds? That's as light than an early sixties Jr Fueler. You should refigger.
  4. pasadenahotrod
    Joined: Feb 13, 2007
    Posts: 11,776

    from Texas

    What are you using for an engine/trans/rear combination...a VW transaxle and flat four?

    1500-1750# is more likely and believable unless it is the above!

    I have found that light cars, as in 1500-1750#, using modern radial tires rated for High Mileage, DO easily slide when the road surface is loose or sandy on pavement especially in a panic stop situation, don't ask me how.

    The hi-mileage rubber compounds are much harder than those in a cheaper lo-mileage tire and I believe that is the culprit. The good thing is that the cheaper lo-mileage rated tires used on very light-weight cars will get substantially higher mileage than rated depending on your driving habits...that is, no up to the curb/fireplug/fence/phone pole, etc. burnouts like the biker boys love so much and probably not a whale of a lot of Brodies/donuts like some rodders feel are so necessary for KOOLNESS.
    Last edited: Apr 23, 2009

  5. It's not too easy building a race car that weighs under 1000lbs.

    On a street car, might be possible with an all Aluminum 4 cyl,
    Aluminum case on the trans, light weight parts every where.

    Sounds like, maybe, you haven't weighed all the parts.
  6. Could You post some pictures and some words as to how you are doing the wire frame. Are you going to glass it (fiberglass) or clad it (sheet metal)?

    I am thinking about doing a 1929/1930 Plymouth truck project. Any Info would be helpful.

    Thanks Specs
  7. bibb86
    Joined: Mar 23, 2009
    Posts: 65


    I like wahoo's idea put a big block in there and that will solve your problems. but yes it is too light I had one that weighed in at 976 and it would slide all over the place.
  8. I think you're underestimating the weight of the fully assembled vehicle.

    Engine, Trans, Rear, Wheels, Tires, Brakes, Steering, Fuel Tank, Liquids, Radiator, Suspension Components, etc. Even nuts & bolts weight a ton when you add them all up.
  9. oilslinger53
    Joined: Apr 17, 2007
    Posts: 2,500

    from covina CA

    pounds, or kilograms?
  10. pasadenahotrod
    Joined: Feb 13, 2007
    Posts: 11,776

    from Texas

    Are you kidding? The Metric system hasn't reached Tennessee yet, has it?
  11. metalshapes
    Joined: Nov 18, 2002
    Posts: 10,909

    Tech Editor

    If you can build it at 950Lb I will be very impressed.

    This car weighs about 990 lb with Aluminum cyclefenders, super thin Aluminum bodypanels, a full tank of gas ( about 2 1/2 gal ), street equipment ( headlights, taillights, turnsignals, horn, batt, alt, muffler,parkingbrake,
    but no heater, no upholstery on the thin 'glass bucket seats, no windshield )
    Engine is about 210lb, i'm guessing the gearbox weight about half of what a V8 compatible 'box would, and the rear axle weighs a fraction of a 9" .

    Brakes are kinda small too, but they work really good.

    Solo 2.jpg

  12. A small number of European cars have been less than 1000lb as stock,

    It's not impossible to build a car that tips the scales at less than 1000lbs,
    I am hoping to keep my current build down to about that.
    the problem however is that while you can shed the pounds off the sprung mass,
    it more difficult to reduce the unsprung mass, thus very light cars can suffer with poor road adhesion due to the sprung to unsprung mass ratio, and hence under braking may slide.

    Fortunately lighter still means less M in a 1/2 M v squared, so reducing the wieght still tends to reduce stopping distances overall.
  13. Peter Mc Mahon
    Joined: Jan 1, 2009
    Posts: 199

    Peter Mc Mahon
    from Ontario

    I just scrapped a ford 272Y block and 3 spd. tranny at the local scrap metal place. If there scales are off I am sure it is in there favour. The motor and tranny weighed 770 lbs. Peter
  14. revkev6
    Joined: Jun 13, 2006
    Posts: 3,351

    from ma

    just looked at your gallery of pics. there's no way that car will weigh in at sub 1000lbs regardless of engine choice. that fiberglass body you are making will weigh a ton by the time you are done with it. I've built 1000cc minisprints that weighed in at 725lbs and a midget that weighed in at 830lbs. the things you have to do to get weight down that low are rediculous. even using a motorcycle engine/trans (about as light a combo as you could get) you are still looking at 1200-1500lbs min.

    btw, the reason a light car skids during breaking has nothing to do with it's weight. It is the weight of the suspension in relation to the rest of the car. Its been a while since i did the numbers but generally the lighter the car the higher the unsprung weight becomes a percentage of the total weight. You can only lighten the suspension up so much (a certain size wheel and tire combo only come in so many weights) with the light sprung weight cannot control the unsprung weight. It's something sports car guys deal with alot and maybe you can find one on here that can explain it better than I.
  15. KreaturesCCaustin
    Joined: Sep 3, 2008
    Posts: 1,258

    from Austin, TX

    I may be inclined to agree with many posts here that say you've underestimated the total weight. My Kawasaki Vulcan 1600 has a dry weight of 675 lbs. Either way, you will need to deal with the stopping issue on such a light weight vehicle.

    Sprung and unsprung weight are easily explained. Things such as wheels, tires, brakes, axles spindles, etc, that are placed below or outboard of the springs are considered "unsprung". Meaning that they are coming pretty much in direct contact with the road surface. The frame, body, interior, fuel, driver, etc that is attached to the other end of the springs is considered sprung weight.

    Ideally, you'll want your wheel/tire and suspension combination to be as light and 'soft' as possible in your scenario. Aluminum or magnesium wheels, soft, light-weight springs, slightly underinflated tires and a light duty shock will all help your stopping and handling characteristics on such a light vehicle by balancing that ratio of sprung and unsprung. If thought out and exectuted correctly, you'll be fine. Think about it this way; my 675 lb motorcycle, with my 220 lbs on it stops very well on dry streets with only two small tires and two disc brakes doing the stopping, but it's utilizing some pretty state-of-the art, high tech suspension to do it. Don't expect that sort of performance from outboard brakes, straight axles and tall, skinny, bias-ply tires made of a hard compound.

    Lighten your wheels, tires and suspension and eat a lot of pizza before driving. Hope that helps :D
  16. Phil1934
    Joined: Jun 24, 2001
    Posts: 2,717


    I see from earlier posts you plan a fiberglass body with 225/A904/7-1/4". I was shocked I could barely move a tube axle with spring, hairpins and discs, so that's 200#. Figure 250# for rear and spring. 100# for frame. Add 500# for engine and radiator, 150# for trans and 100# for body. Another 50# for gas and tank. 150# for seats, steering, pedals and master cylinder, shocks, windshield. Add 150# of tires and wheels. That's 1650#. You might be up to 150# lighter. Then again if you use 3/4" plywood for floor and firewall, that's 75#/sheet.
    Last edited: Apr 26, 2009
  17. wsdad
    Joined: Dec 31, 2005
    Posts: 1,259


    More mass = longer stopping distance. Less mass = less stopping distance. The same applies to turns. It's harder to change the direction or speed of something that has more mass. The lighter you can make it, the easier it will be to turn or stop or go. Make it as light as you can.

    There are some cases where a lighter car will cause the tires to skid worse than a heavy one. For instance, if the road is covered with a layer of water or slush. If you have a light car and a heavy car with exactly the same width tire, the lighter car would tend to hydroplane before the heavy one.

    Imagine putting super wide tires on your family car. That is how your light car will handle. It will give you more traction on dry pavement and less in a heavy rain.

    Incidentally, on a wet road that doesn't have a layer of water on it but is just wet, the wider tires (or lighter car) would actually have better handling. It would also handle better on ice or snow (unless there is only a thin layer of fluffy snow). It would handle worse in slush.

    I'd make it as light as possible. It will accelerate, brake and turn quicker under normal conditions.
  18. OK, for the sake of argument, let's assume Jaxx is right and he does come in under a grand. Now if he wants to add weight to enable stopping, handling and ride quality, what is the best way to go about that?
    I remember sticks of lead clamped to the front axle of our digger to keep the front end down, not too practical on the street.
    A friend's sprint car, that has to weigh 1575 with driver has a 1/2" piece of steel under the floor to help keep the weight up. I've often wondered how difficult it would be to pour molten lead into typical 2x3 mainrails to add weight. Mike
  19. mustangsix
    Joined: Mar 7, 2005
    Posts: 1,326


    1150 lbs w/ full fuel.

    Attached Files:

  20. Shifty Shifterton
    Joined: Oct 1, 2006
    Posts: 4,964

    Shifty Shifterton

    The best way wouldn't be adding weight. It would be either going to a skinnier tire to increase pressure on the contact patch or a stickier tire. There's good reasons why small cars run small tires.

    One of the main problems is that most available car springs are gonna be 2X too stiff and make it skip all over, which would then be blamed on light weight. Get that suspension to cycle and low weight won't be an issue.

    good luck
  21. KreaturesCCaustin
    Joined: Sep 3, 2008
    Posts: 1,258

    from Austin, TX

    Yes and no. You also must take into consideration what kind of traction you'll get. A lighter vehicle will have less traction due to the coefficient of friction to the pavement. I drive a tractor-trailer for a living. An empty truck takes longer to stop in a skid than a full truck. (been there and have the stained underwear to prove it).

    A super-light vehicle will stop faster on dry pavement under optimal conditions, but add a little rain or loose material to that equation and it changes dramatically. Running the tires way under-inflated will help that to some extent, though.
    Last edited: Apr 24, 2009
  22. PasadenaHotRod's - what a cool SoCal nickname - comment on tire durometer (rubber hardness) is a good one to keep in mind.

    Hard enough to find the 'right' tire and most of us probably don't think too much about durometer.
    Even so, if we're using a fairly common size it's something to keep in mind.

    You gotta watch it when you weigh lightweight cars - and by lightweight I mean from 3600# on down - on a big scale.

    Big truck scales and similar don't reflect very low weights as accurately as they should.
    Maybe not so nowadays with all the digital stuff, but back in the day, the CHP guys would weigh us on the truck scales if things weren't too busy and my 3200# Shoebox coupe would come in at 3600-3700#.

    Dragstrip scales at various strips averaged out at 3200# or so.
    Stock trim, no front bumper (tow bar), Rocket Motor, Caddy trans, TJ T&R and that was about it.

    One of the best ways to weigh a hot rod is to weigh each corner on a 1000# feed store balance scale.
    Add the weights and you'll have an accurate total.
  23. Screamin' Metal
    Joined: Feb 1, 2009
    Posts: 506

    Screamin' Metal
    from Oklahoma

    Dawm Skippy.....he's in the wrong business......ya need to be in Racin'!!!!!! On, lets see...... a big block with all the parts....about 400-500, the trans....about 200.....hum.....wheres the seat go????????????
  24. Ok I admit I don't know about trucks in NA,
    but here in Europe Trucks all have load sensing axles,
    which apply more braking on the trailer axles when loaded,
    so un-loaded verses loaded truck brake performance is not a valid model.

    Plus again since the trucks axles are still the same mass,
    (not lightened by removing the load)
    the unsprung to sprung ratio has changed massively,
    hence with no load adhesion is reduced.

    As someone said earlier this is why small light cars have small light wheels, tyres suspension etc.

    Fact is that the more mass the more energy you have to put in to accelerate,
    and the more that has to be converted to heat by the brakes to slow the plot down again.
    Hence a light vehicle can always be made to accelerate faster than a heavy one,
    and slow down quicker.

    So there can never be a 'too light', but there can be bad set ups like high unsprung mass and poor front rear wieght split which may limit how light you can actually go before other problems over rule.

    I am pretty sure I remember years ago there being a tax break in the UK for cars under 8cwt, which would have been 896lbs.
    We still have a class B1 which is cars under 550Kgs.
  25. There is a lot of mis-information here, as usual. There is some truth to the statement that a heavier vehicle increases friction on the tires for braking. It also increases the forward momentum and inertia that is required to brake. These two force vectors are orthogonal (at right angles to each other). Since Mass is a linear multiplier for both traction and inertia, a heavy vehicle and a light one should both stop in the same distances. And if the brake systems are engineered for the amount of weight that they are attempting to stop this is generally true, especially for street driven vehicles. As long as the available traction of the tire is not overcome and the brake system is up to the task, light and heavy vehicles will stop in approximately the same distances. The heavier the vehicle becomes the more likely it is to overpower the tires and the ability of the brakes to stop the vehicle as more work is required and more heat is produced. Conversely the lighter it is the harder it is to mange the braking power without overcoming the available traction.
    So the answer to the OG question is NO the target weight is NOT too light, although I doubt is is achievable.
  26. Harry Bergeron
    Joined: Feb 10, 2009
    Posts: 345

    Harry Bergeron
    from SoCal

    The big problem is rear tires locking up before fronts. The cure is either two master cylinders with a balance bar between or pressure limiters for the rear brake circuits.

    The make adjustable ones for road racers, a little spendy, but if you want to experiment, lots of PUs, vans and FWDs from the 80s had them. The guys in the junk yards probably don't even know what they look like, since I've never heard of one failing.
  27. Actually this is LESS of a problem with a light car vs. a heavy one. The reason cars tend to lock the rears first has to do with weight transfer. The less weight to transfer the less this becomes an issue. It is also a direct result of spring rates that allow the amount of transfer.
  28. stude_trucks
    Joined: Sep 13, 2007
    Posts: 4,755


    Jeez, 950lbs? My 392 hemi weighs close to that by itself. What is your car made out of, paper mache?
  29. Jeem
    Joined: Sep 12, 2002
    Posts: 5,885

    Alliance Vendor

    Seems to me if it IS that light and you want to add weight for whatever reason, you now have a choice as to WHERE you add the weight. Food for thought....
    Also, if you want to add weight, go to the Taco Bell drive-thru, eat in parking lot, repeat. You'll be heavy in no time.

    I'm so freakin' smart.
  30. JeffreyJames
    Joined: Jun 13, 2007
    Posts: 16,612

    from SUGAR CITY

    Back early 60's a Buick 215's was commonly used to keep the weight below the legal limit to where fenders were required. I think the limit was 1800 lbs so if you are half of that I would say that it's pretty f'in light.

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