"For a bit over $1000, within a week we had our new driveshaft."
It never gets cheaper to race, does it.
Photography by Tim Suddard
After installing our engine and putting the car on the ground, we found some bad news: Our driveshaft was about two inches too short.
How could a Mustang driveshaft not fit into a Mustang? we asked.
As with most issues surrounding our Mustang’s drivetrain, the answer revolved around the Roltec transmission. This Chevy-based transmission is some 1.75 inches shorter than a Top Loader or Ford-based T-10.
After getting on the phone with Mark Williams Enterprises, they walked us through the process for measuring out a new driveshaft. (One tip: bury the yoke into the tail shaft and then backing it out 5/8 inch.)
Final answer: We would need a 55 3/8-inch driveshaft.
The next question: steel, aluminum or carbon fiber?
The steel driveshaft would weigh 19.8 pounds, aluminum would check in at 11.8, and the much more expensive carbon fiber shaves a couple more pounds.
Shaving eight pounds doesn’t sound like much, but it is nearly 50%. The static weight isn’t the real issue. Like a lightened flywheel, a lighter driveshaft helps the engine accelerate. Based on the horsepower rating of our engine and its intended use, the shop calculated that a 3.5-inch-diameter driveshaft would be best for us.
The price difference between aluminum and steel is about as much as the weight difference–imagine that. For a bit over $1000, within a week we had our new driveshaft.
Mark Williams Enterprises shipped us a beautifully made driveshaft that was perfectly balanced and built to withstand at least 1000 pounds of torque in a violent drag racing scenario. We feel competent it will serve our needs quite nicely.
"For a bit over $1000, within a week we had our new driveshaft."
It never gets cheaper to race, does it.
One thing made of carbon fiber that can be had fairly cheaply is tubes, makes you think about chopping up a junkyard shaft and affixing a carbon tube cut to length where the steel used to be...
Edit: Looks like Robbie had similar thoughts:
I made a similar faux pas with the Datsun. In my case I'd found a driveshaft via Dave at Futofab, sadly I'd measured wrong and was off by about the same 1.75"
Fortunately I simply moved the motor and tranny back in the chassis.
"Pounds of torque"? Why not the simpler "torques"?
Is that second picture just a demo of what the old one looked like when it was too short??
I seen an aluminum drive shaft break at svra road racing event. Warren Agor's 1976 Chevy IMSA GT Monza RaceCar trailerd hundreds of miles to pacific raceway in wa state and the aluminum driveshaft lets go first time out. He was done, no fix. How much did he spend to be there and get to that race?
I suggested he not to put another aluminum drive shaft back in or it would eventually happen again after those drive shaft welds fatigue and, they will, you can't stop it from happening.
Warren Agor's 1976 Chevy IMSA GT Monza RaceCar Link >> https://raceclass.com/rpm/ads/1453/1976-chevy-imsa-gt-monza-racecar-for-sale.html
When you weld aluminum it removes all the heat treating. Your left with raw soft aluminum. it is no longer 6061 or 7075 etc unless the entire thing is baked/annealed and re-heat treated.
A drive shaft is like a huge torsion bar twisting in both directions during acceleration and deceleration. This stresses the welds repeatedly every time it twist. Remember this is aluminum? Yup aluminum has MUCH LESS fatigue strength/resistance then carbon steel.
Leave the exotic stuff to the big $$ race teams who can afford to change driveshafts like I change socks.
Here is exactly what you need at this link and it is even affordable. Check it out >> https://www.strangeengineering.net/product/3-inch-chrome-moly-driveshaft-seemless.html/
My drive shaft for my ASA stock car conversion to road race car will run a 4130 chrome moly drive shaft and those few extra lbs of weight mean I can run it for years and not worry about it breaking at just the wrong moment after I trailer the car across the country and spend a huge amount of $$ to race.
NASCAR teams do run aluminum drive lines but only once, then change them out. I can't afford that so 4130 steel it's going to be for me.
That 8 lbs extra for the steel drive shaft with that weight in a 3.5" diameter is not going to carve off a bunch of acceleration and reduce lap time and is super cheap insurance.
In reply to robert777 :
Did you even look at the aluminum driveshaft in the pics before going off on your welded aluminum driveshaft rant? Or do you have any first hand experience with quality aluminum driveshafts to know how they are actually made?... I'll give you a hint: There are no welds. 
If you refused to buy any type of part that anybody ever had fail prematurely and ended their race weekend early, you wouldn't have anything to race with... Not even a steel driveshaft.
AngryCorvair (Forum Supporter) said:"Pounds of torque"? Why not the simpler "torques"?
Paid by the letter, bro
AngryCorvair (Forum Supporter) said:"Pounds of torque"? Why not the simpler "torques"?
Because then people would complain "why not ft/lbs torque instead of torques".
AngryCorvair (Forum Supporter) said:"Pounds of torque"? Why not the simpler "torques"?
They don't hate English speakers.
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