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Bondo and race cars

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  • Bondo and race cars

    Just trivia stuff that some here may like or even encounter if working on retired race cars.

    I'm on an old Ford forum, focused on FE series 352-390-406-427-428 engines. One of the members is an expert & scholar on 1960's era Grand National and NASCAR race cars. He's restoring another genuine Holman Moody Ford car right now. In a recent post a couple of days ago, he discussed and posted pics of how the underside of some cars- sheetmetal and chassis elements- were routinely filled and smoothed with body filler as a cheap way to improve aerodynamic characteristics within the rules. Actual metal modifications or belly pans were not allowed, but body filler slathered on & smoothed was. I know bondo isn't a favorite word around metalshaping, but it used like this under an old race car may be more indicative of race team strategy than of cheap and shoddy repairs.

  • #2
    filler gets quite heavy when used on mass, plus in a fire it burns and the possible chance of it falling off, also has anyone tried to apply a large amount in one go? you would have to turn the car over or it would want to drip off before hardening. gravity still works even when trying to cheat aerodynamics. maybe he is right i've never restored any race cars or NASCAR's but i've been involved in making some cars fast, and weight is something i always avoided adding
    thanks neil


    • #3
      Yes. Losing weight adds horse-power!

      One reason why I dream about making an aluminium bonnet and doors for my Alpine,

      Cheers Charlie


      • #4
        yup, I know.. 10 lbs = 1 HP and 10HP = .10 seconds in the !/4 mile. Sprung weight vs unsprung weight, etc. Not discussing weight reduction. That's why I posted the info. Thought it might be a surprise to some and help explain such bondo if they ever encountered it in the restoration/recreation of a related project.

        I expect that Holman Moody and whomever else did this was able to eliminate enough weight in other areas to still use bondo under the car and make mandated race weight. This wasn't being done by a naive amateur team. This was being done by the best in the business who were well connected to Ford, at least in 1967 and 1968. Dr. John Craft C5HM is a well-recognized scholar on the period, the teams and their methods.

        If the link doesn't work, I can grab some pics to post here.

        The cage is standard NASCAR 1 3/4" 90 wall (mostly).  Nothing bigger. The stamped, rear unit body "frame" rails are still in place and not narrowed.


        • #5
          Thanks cliff, good stuff. I recall reading an article from Smokey Yunic years ago where he did this on Hudson nascar racers.
          cheers Steve


          • #6
            Most of the nascar stuff was done quickly and these cars got re-skinned almost weekly. In the 60’s lead was filler of choice and very heavy. It was used to balance cars and to meet weight requirements. Bondo to smooth them out. I restore nascar and drag racing cars that ran hemis. I’ve ran into all kinds of interesting things


            • #7

              Did you know that bondo was used by North American Aviation during the production of the P-51 Mustang? Called "Acme gray surfacer and Acme glazing putty" was used on the front 40% of the wing cord to smooth all of the rivet heads and to final sculpt the wing profile to match profile boards. This helped insure laminar airflow over the wings decreasing drag and increasing overall speed. When you see original wings or proper restoration, you will see the rivet heads disappear as you approach the leading edge. You will also note that the wings were painted silver and not left bare aluminum like what was found on the fuselage. I'm sure the aircraft and racecar industry, evaluated closely weight verse the speed penalty of Bondo.



              • Ron Kirkpatrick
                Ron Kirkpatrick commented
                Editing a comment
                That old bondo was heavy too. I was told by aircraft restorer that it helped balance weight in aircraft. I’m not sure how true that was but it sounds like early aviation