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DMSBD Tech Tips
Balancing Steering Cable and Front Kingpin Tension
–by Ian Carsten

Most racers advocate significant kingpin tension for maximum speed. But that makes the car steer poorly if the cable tension isn’t high enough. We experienced this when we first started racing at a faster, smoother track than we were used to. Both our drivers complained that they had a hard time keeping their cars straight.

We watched their path from the top and bottom of the track. It looked like they kept swerving from left to right once their cars had built up speed. Naturally, they didn’t do too well. Both drivers were pretty experienced and usually drove well so we didn’t think they were at fault.

The only real differences were: the speed and smoothness of the track and the significantly greater kingpin tension used. We increased kingpin tension because many of our fellow racers recommended it at this fast track. At the end of the day, we concluded modest steering cable tension combined with high kingpin tension was the problem. We don’t think the speed of the track caused this. It just made it much more obvious.

Here’s what we found. With higher front kingpin tension, the axle won’t move until it gets pulled fairly hard. If the cable isn’t tight enough, when the axle does move, it can pivot too far. That’s because any slack in the non-pulling branch of the cable allows the axle to pivot too far.

Suddenly, the driver has to correct by steering in the opposite direction. But the same over travel of the axle happens each time. The driver has to fight the steering all the way down the track. This makes the car travel a snake-like path. It’s unsafe and looses lots of time.

To compensate, you have to increase cable tension. When the cables are sufficiently tight, the steering will be responsive and not over steer. But the increased cable tension will spring the axle rearward. Now the front wheels are toed out and that will slow your car. You have to bend the spindles forward to compensate. For that you have to use a spindle gage, spindle stands, and bending tools.

Once the cable is sufficiently tight and the front spindles are parallel, your car will handle responsively and be easy to keep straight. This helps maximize speed. You’ll have to keep track of the cable tension. A tight cable can stretch for a while until it stabilizes. If it stretches, the cable will loosen making the steering sloppy again.

You’ll have to check the front axle with a spindle gage before each race day. If the cable loses tension, the axle will relax and return to its unstressed shape. That allows the front spindles go out of parallel. Usually all you have to do is tighten the cables until the spindles are parallel again as determined by the spindle gage.

It’s a good idea to apply some grease between the washers that slide over each other as the front axle pivots. For example, there’s about 3,274 pounds of clamping force between the washers on an unlubricated kingpin tightened to 164 inch-pounds.

Greasing between the moving washers helps reduce the steering effort and keeps the washers from galling where they slide over each other under kingpin tension.

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