Through Our Eyes: Carbon and Aluminum Drive Shaft Overview

PST Driveshafts at RallySport Direct


With the popularity increasing the last few years of using aftermarket drive shafts , we wanted to go over some information in regards to the subject and cover some common questions that we see our customers ask us about.

The reason to change out the drive shaft to a Aluminum or Carbon Fiber unit is quite simple, it is to reduce rotational mass and increase response.

By replacing the heavy stock steel drive shaft with a lighter 1 piece unit you save about 1/2 the weight in most applications, and allow more energy to go into rotating the tires instead of rotating dead weight. Similar concept as doing a lighter flywheel and pulley, however other additional benefits are also to be had.

Many new modern cars are now coming equipped with light weight shafts for the reasons of performance, such as the GT-R and other high end exotics.
Aluminum Vs Carbon:
2 of the most popular materials to use when replacing the steel shaft will be either aluminum or carbon fiber. Below I will cover some of these basics.

1)CF shaft is smaller in OD to handle the same tq capacity as the aluminum counterpart unit. Means you have more clearance and requires less modification under the car to run a carbon shaft vs aluminum. Some applications require modification to heat shields or transmission tunnels to allow adequate clearance for the larger OD Aluminum shaft.

Reason for the larger diameter of the Aluminum shafts is that these shafts are purchased as raw aluminum from aluminum manufactures. These raw aluminum tubes are only available in certain sizes, so to get the adequate wall thickness of the tube to handle the torque the shafts are purchased in the larger tube size. With carbon shafts they are able to be manufactured at a smaller diameter with the desired wall thickness to meet the requirements of the load application

2) The CF shaft has a higher critical speed. For example an aluminum STi application shaft start to flex and vibrate above 155MPH do to the aluminum hitting its natural frequency. The CF shaft is closer to 195MPH. It might not sound like 40MPH is a big difference, but a Stg2 STi can get very close to 155MPH…to hit 195MPH you got to have a crazy fast car (well in access of 500WHP to hit that, and only a small hand full of tracks in the world have straights to accommodate that).

3) CarbonFiber has a much higher ability to withstand load in the application of fiber lamina direction compared to an equally as thick aluminum tube. This makes the shafts much stronger due to a higher yield strength required to load the shaft beyond a paint of failure. This will directly apply to shock load capacity of the shaft without fracture or damage.

For instance your typical 6061 aluminum has a Tensile Yield Strength of 40,000PSI, which sounds very high. Now Carbon Fiber is between 3,000psi-15,000psi(depends on the quality of the carbon, either industrial grade or military grade) in the fiber longitudinal direction. So you now might think that aluminum shaft will be stronger, but wait. The thing about carbon is you use more than 1 layer when creating a part, so after you stack 4-10 plies (depends on manufacture) you can see that your  tensile yield strength x number of plies = sky high yield strength! From there you can take into account the density of carbon fiber and can see the benefits. *Please note there is much more that goes into these shafts then just sticking plies of carbon fiber pre-preg together, but it gives you the basic idea.

Directly from ACPT
“All driveshafts twist to some degree when torque is applied. The resistance to this twist is measured as torsional spring rate. Standard carbon driveshafts have a torsional spring rate a little less than aluminum and about half that of steel. The advantage of a lower spring rate is less driveline shock and a reduction of stress on other drivetrain components, as well as increased traction.”

If you are having issues breaking your rear axles, or ring and pinion this might be a good way to prolong the life of those parts. Reason is that the carbon drive shaft will be like a rubber band, when launched hard it will twist slightly to absorb some of the shock and then spring back to its normal shape. The twist would be minimal (only a few degrees 5-10) on VERY hard launches, however that twist is absorption of energy. Thing that breaks many parts such as gears is shock to the part from abrupt and high application of power. F=M*A, so if you slightly reduce the acceleration of the power application you prolong life of components. *simple way of thinking this, you can put a 10lb hammer on your finger slowly with no damage, however if you slam the hammer you will do allot of damage. Similar concept with the carbon shaft twisting slightly, as it slows down the application of the energy on the component.

4) When the CF shaft fails, the fibers will come apart and un-wind. When an Aluminum shaft fails…well you have a 14lb sledge hammer spinning at over 10000RPM going to town under the side of your car and you just hope it does not penetrate.

The big concern many people have with the CF units is the CF shaft separating from the metal yokes. The adhesive that actually holds the 2 together, how its assembled, and how its cured is a trade secret of these manufactures as this will separate many of the manufactures and really make or break a manufacture.

That was a big concern for us as well when getting into the CF drive shaft market, however after many conversations with PST about they have yet to have a failure of the carbon or the adhesive itself. They state that the failure point is….the metal yokes actually. Not something that they have seen on import applications much in the past, but more or less on 1/4 mile domestic applications. And at that point they have a option to custom make a billet yoke if your car really needs it

5) A equivalent CF shaft will be about 5-10% heavier then a Aluminum counter part.

6) Aluminum shafts are only about 30-50% of the price of the aluminum unit.

So what one to get?
This strictly will depend on your application and budget to be honest.
Overall both aluminum and CF are significant improvements over the stock unit. But the CF will be better then Aluminum as its safter, smaller diameter, has a higher critical speed, a better elasticity, and higher degree of torsion before fracture/separation. However that is not saying that aluminum is a bad, as aluminum shafts have been used for decades before the composite technology got to the point where it was better and more affordable for this type of application.

Our Review of a shaft installed on our 2011 STi here at the shop:
Stock Driveshaft: 27.6lbs
PST: 14.2 lbs!

Weight Savings of 13.4lbs means that your engine spends less energy rotating the driveshaft and more energy on spinning the tires. This in turn decreases drivetrain power loss and increases power at the wheels. So even though your car is not making more power, its able to transfer more of it to the wheels increasing performance, drive-ability, and response.

After installing it on our 2011 STi shop car, we where nothing short of amazed. The fitment was spot on and was a direct factory replacement. Install time was about 10 min after the mid pipe was removed off the catback.

Test Drive Results where nothing short of AMAZING! The vehicle was much more pleasant to drive in very low RPM with no more bucking or shacking, and it was much more eager to accelerate even at partial throttle. Throttle input response was also drastically increased with less “dead” time between when the throttle was applied to when the vehicle started to accelerate. As for all out acceleration, well the thing is MUCH faster then compared to the way the car was with a stock driveshaft. The vehicle is much more eager to go through the RPM range at a much faster rate, meaning that its accelerating faster. The shifts where also crisper and faster as up shifts and down shift delays where cut to a minimal in terms of hard downshifting and fast upshifts.

We where curious to see the results the addition of the driveshaft made, so we did a quick dyno comparison of before and after.

Performance Mod List:
PST Carbon Drive Shaft
Cobb Tuning Accessport Running the 91 octane AEM Stage 2 map
Cobb Tuning Catted Down pipe
Cosworth Intake
Invidia Q300 Catback

Temps: High 30’s
Altitude: 4300ft
Fuel: 91 octane winter blend (talk about bad fuel)

As you can see, consistently through out the entire powerband both Tq and Power increased!

*sorry about the starting RPM range, it was getting later in the day and we didnt think twice about the starting RPM of the initial pre-driveshaft starting point. But you can see that after 3000RPM the gains are nice and constant.





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