How to Add a U-Joint to a Steering Shaft

One of our favorite things about driving our Formula 500 single-seater on an autocross course is the quick, direct steering. But there’s more to the equation of going fast around the cones in an open-wheel car: Driver comfort and proper leverage on the controls are serious variables, too.

On a road course, slow hands and small control inputs keep the car at the limit and minimize speed loss from friction. But in an autocross situation, a driver sometimes needs to “force the issue” and transfer the car’s load more abruptly than on a road course. 

Since our F500 was going to be an autocross-only machine, we didn’t mind making changes that could make it less competitive at road racing. We removed the driver-side protection to free up more elbow and shoulder room, and we formed a new seat to replace the KBS chassis’ fully supine–but highly aerodynamic–factory seating position with a more upright one. 

Finally, we raised the steering wheel up off our lap and into a more street-car-like position. This gave us even more leverage and the ability to drive with our entire upper body, not just our forearms. Removing the steering shaft from the rack and relocating it was easy, but then we had a problem: The shaft no longer lined up with the rack.

The solution: Add a universal joint. 

Steering U-joints, also known as Carden joints, redirect the rotation of a shaft, but that redirection comes at a price. As you can see in the accompanying chart, the output shaft’s speed can vary greatly throughout a single rotation of the input shaft. For something like a driveshaft, this can be a serious situation–especially once you get past a 15-degree offset and the speed variation becomes dramatic. But even with a steering shaft, it should be taken into consideration. 

Manufacturers recommend that a steering U-joint not exceed a 35-degree offset. If it does, use a double U-joint instead. Our joint was running through just a bit over 20 degrees, and since we rarely turn the wheel more than 180 degrees, we didn’t worry much about speed variation.


Adding a U-joint to a steering shaft is usually as easy as cutting the shaft, welding or bolting in the joint, and reinstalling. A single U-joint usually doesn’t need any additional shaft support.
To support our steering shaft, we mounted a 20mm Aurora rod end to a tab welded on the front roll hoop. This, in conjunction with the U-joint, actually gave us some height adjustability for the wheel so we could further fine-tune the cockpit.


This chart shows the speed differential of a U-joint’s input shaft and output shaft throughout a single rotation. As you can see, high offsets are best avoided.

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Comments
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Bob Moore
Bob Moore
3/5/20 12:45 p.m.

I'm confused between the description of "raising" of the steering wheel and the image of the lower steering shaft showing the shaft apparently u-jointed to allow it to be dropped (pointed downward from the steeply-upward angle of the rack input shaft). My first question was going to be why not rotate the rack in its mounts, but it's a little more complicated down there. Anyway, food for thought on a good topic. Thanks.

fe1rx
fe1rx New Reader
3/7/20 12:10 p.m.

U-joints should be installed in pairs with the two joints installed 90° out of phase, unless the angle change between shafts is really small, in which case one joint is ok.  Assuming the deflection angle of the two joints is the same, the two joints cancel each other's rotational speed error and the sum of the joints becomes constant velocity.  The tradeoffs are the free play introduced by 2 joints instead of 1 vs. the rotation speed error.

Anyway, my main point is that when mentioning two U-joints, it is important to remember to install them 90° out of phase.  

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