Technical How much load is on wheelie bars?

I'm designing some wheelie bars for my altered wheelbase Chevy II. My wild guess is that there will be a few hundred pounds load on them. Does anyone know?

Car weighs about 2700, mild blown 427, 3000 stall converter gearing in the low-mid 3s. Plan to run 9" slicks. Rear end moved forward 8". I expect it to run somewhere in the low-mid tens.

 

I dunno , but if any of us had this question to ask we'd ask you!

 

i dont have a hard number for you, but I wouldn't be surprised if the number is closer to 500 with the added leverage of the relocated axle. -rick

 

More variables than constants in the calculations. Would probably require load cells in the wheelie bars

 

by "load cells in the wheelie bars", you mean try something with springs in it, and watch video to see how far the springs deflect?

My current plan is to have vertical telescoping tubes under the original rear spring shackle hole. Flat aluminum locating bars, coming straight back off the rear axle spring perch. I was guessing that I want to have springs with a few inches of travel, with a spring rate of around 50-100 lbs/in per side.

 

A simple force balance equation would be what ever is left over after subtracting the torque generated by the weight of the car at the CG point from the torque generated by the engine at the rear axle.

Fwb * Lwb = T - W*dcg

Fwb = Force at Wheelie Bar
Lwb = Length of Wheelie bar contact point from center of rear axle
T = Torque generated at rear axle
W = Weight of the car
dcg = distance from center of rear axle to CG of car

dcg is the hard variable to figure out, as the CG moves around as the rear suspension squats and the front tires lift. This is also a dynamic event, and the equation above does not take into account any loads generated by the wheelie bars being forced to decelerate the mass of the car as it is lifting and rotating about the rear axle. Long story short is make them stronger then you think they need to be.

Good Luck

 

I dunno, but I like your car.

 

"...above does not take into account any loads generated by the wheelie bars being forced to decelerate the mass of the car as it is lifting and rotating about the rear axle..."
Agree! Equation is for steady applied force. These things can be loaded suddenly and violently, with shock loads galore, and potentially up to entire weight of car plus effects of velocity...

 

Maybe beef up the rear frame rails to avoid wrinkling quarter and rear pillars, while testing.

 

squirrel,

I don't have anything to add to what has been discussed so far. However, I did want to comment on the apparent oxymoron: "mild blown 427". For some reason I am having a bit of a hard time reconciling that in my mind. That big 'ol blower and dual carbs sticking out of the hood isn't helping either.

By the way that is one mean looking Chevy II.

Thor

 

Maybe try using this equation with your best estimates to get your "baseline". When you go to design your wheelie bars, use a safety or "fudge" factor of say, 3 or 4. Then test it out!

 

Depends on how high nose is! With you I would say A LOT!

 

Are you saying the wheels will end up right under the rear bumper? That will unload the springs quickly. I have seen quite a few cars with all 4 wheels in the air with that set-up. I am sure you have too. The farther back the less pressure on the wheels.

 

I'd say do some test launches sans bars and go from there. It may turn out that it hooks just right as is. Or with a slight tweek to the suspension/CG. 'Course I'm just a backyard hack, not an engine ear. Car looks good.

 

I say just go with what's common- IOW reverse engineer 'em.

 

How well does it hook up, and how hard can it launch?

 

holy shit batman..........

 

Where'd my center of gravity go??!? I had it right here a minute ago...

 

Ah Pete Whackers pro mod. That thing got air bourne and snapped the mounts. And then bent the chassis on landing.

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It all depends on how well the chassis is set up, and traction. Way too many variables.

 

I can't do math calculations,but I learned the hard way.My 96" Altered lifted the front end really fast. That made the 39" wheelie bars act like a lever,which unloaded the rear wheels.We installed 68" wheelie bars.That was much better.I could then drive through the front wheels off the ground. Longer is better

 

Leaf spring style wheelie bars fit the car and are period correct. A lot easier to build. Check FX and MATCH BASH threads for pics.


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I've seen a wide variety of wheelie bars in those threads. Pretty much anything goes.

Thanks for all the input so far...I guess I just need to build something and see how it works.

 

That's funny.......but true.

 

If you get it wrong here is another case of ALL the weight of the car from bangshift.

 

Looks like there's no weight at all on those wheelie bars....at least not when the pic was taken

 

I learned 2 things very quicky....#1..you can't steer with the front wheels off the ground.....#2..setting it down gracefully is much more difficult than you think.

 

It would seem to me an impossibility to determine how much pressure was on a particular set of wheelie bars. It depends on so many variables. Not just the weight of the car, but how quickly it comes up against the bars, and how high it goes. If it barely hits the wheelie bars the weight or pressure would be minimal, but the same car hitting hard enough to pull all 4 wheels off the ground would obviously be putting hundreds of pounds (or more!) weight against the bars.
I think the old style wheelie bars with their vertical tubes enclosing the coil spring were built to allow spring changes. I'd guess most early users of that style wheelie bar experimented until the spring rate was determined to absorb the impact, not necessarily completely stop compression of the spring.

 

I think you're right....so the bottom line is, build it and see what happens.

 

And maybe take along several launch RPM chips to play with.