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Paul_D's Avatar
 
Join Date: Jun 2010
Location: Ojai, CA
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914/6 Race Car Steering Trouble (long)

Hello Group - I'm new to this forum but have been Porsche owner since 1986. A friend recently purchased a 914/6 race car from the Seattle area. I test drove the car at Willow Springs 2 weeks ago. There was a serious steering problem that we've been trying to track down ever since. In addition to severe bump steer, the steering was sloppy and loose in the near straight ahead position, but became very darty and over-responsive in the corners. With the wheels off the ground the steering was stiff and felt like it was binding in places.

In addition to general wear-and-tear, we found some crude alterations. The steering knuckles had been heated and bent down, presumably to compensate for lowering, and the rack had been spaced up. With these changes, the tie rod ends angled down at about 10 degrees with the car on its wheels. I'm guessing this is the root of the bump steer issue.

Also an intermediate steering link (the angled shaft) had been made up using the two splined ends of the factory shaft, welded into a tube, but off-center and not perfectly straight. We made up a new shaft, and replaced all the bearings and seals. The car hasn't been driven again yet, but I see a major problem taht I don't understand.

The Cardan type u-joints like on the Porsche steering, are not constant velocity. The greater the operating angle, the greater the variation in rotational speed of the output shaft with a constant speed from the input shaft, right? Now, with a double u-joint in the system, if set up right, the shaft speed differentials are cancelled out and the parallel shafts - the one with the steering wheel and the one connected to the pinion - should always rotate at exactly the same speed. From everything I can find, the correct set up for a double u-joint shaft is for the joints to be "in phase", meaning that the u-joint half connected at each end of the shaft should be on the same plane - kind of like a dog bone. However, when we made the shaft up that way, there is clearly a problem with the shaft speeds. In other words, in one position, say close to straight ahead, rotating the steering wheel through a 20 degree arc produces rotation of the pinion of less than that - about 15 degrees. At other angles, a 20 degree rotation of the steering wheel gives about 25 degrees of rotation at the pinion. This is what you'd expect with a single u-joint in the system, but shouldn't happen with the double joint.

Now I just checked the steering on my '71 911T, and am surprised to see that the factory steering link does not have its u-joints in phase, but they are out by something around 30 degrees (just eyeballing). The steering works beautifully on the 911, but I don't understand why it should if the u-joints are out of phase. I'm prepared to take the steering link out of the 911 so we can match the angular offsets of the u-joints, but I'm not sure that's the right thing to do.

I'm also concerned that the PO's alterations to the steering may have increased the angles to the point where they exceed the design limit of the joints. I notice on a "blueprint" drawing of a 914 that the factory steering column does not point straight ahead, but is slightly angled toward the center. It is also angled down. On the race car, the steering wheel has been lowered, and the column is closer to level with the ground, and it appears to point straight ahead - which would increase the angle at the joint. So I'm concerned that we're still barking up the wrong tree - if the steering shaft angles are wrong no amount of playing with u-joints and all the rest, will help. Yet it's hard to believe that the car was raced in this condition.

Any input would be appreciated.

Thank you.



Old 06-15-2010, 05:10 AM
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I might be onto the reason Porsche sets the U-joints slightly out of phase with each other...
If the pinion shaft and steering column were parallel and on the same plane, then the u-joints on the intermediate shaft WOULD have to be set in phase with each other to have constant velocity of the other two shafts. However, the shafts are only parallel (roughly) when viewed from above, but they are inclined at different angles, viewed from the side, resulting in different compound angles formed at the 2 u-joints. So Porsche must have calculated an angular offset of the joints to minimize the problem. On my 911, the 2 joints are offset on the intermediate shaft by 3-4 splines. I had guessed at 30 degrees, but it is less than that.
Does anyone know how to calculate relative positions for double u-joints where the for a system with unequal angles? The simple solution would be to copy the factory setup, but if the steering has been altered, as it has with this car, then the factory setup may not be right.

Thanks.
Old 06-15-2010, 06:35 AM
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The 914 I used to have was lowered about as much as could be and never had the mods you seem to be describing. If it were me I would take it to a reputable race prep shop and have them look at it in detail. I would also try to find the shop that did the work on the car to see if what was done could be explained. I would feel that the joints in the steering shaft are not the whole issue but that will take a suspension expert to find out. If the car came from the Seattle area, from what I remember up there they do not have a track like Willow Springs with the long turn 8 and the long front straight so the alignment might be setup for a quicker turning car similar to an auto-x setup and that will most surely make it dart all over the place and feel unstable.

The couple of hours a shop will charge would be money well spent since the car looks great but if it is a terror to drive then it will cause problems when in a race.
Old 06-15-2010, 07:32 AM
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Quote:
Originally Posted by Paul_D View Post
the rack had been spaced up.
That is part of your problem. The (cheap) rack spacers work but they also change the geometry of the steering coupler inside the car.

I just went through the same issue with my car. I ended up tossing the spacers. The correct way of doing this to remove the rack spacers and go back to the stock geometry on the steering shafts.

If you need to eliminate bump steer due to a much lower (than stock) car go with a tie-rod spacer kit, like the one from Smart Racing (pic below). That way, there's no need to bend the arms on the spindle either.

Btw. the complete steering rack of your 911 should be a direct bolt in to the 914. They are virtually identical. However, the shaft connecting the steering column to the rack (between the two u-joints) will be slightly different in length.

Andy

PS: Congrats on buying the car, i have seen it many times at the track and chatted with the owner a few times ...

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Last edited by SirAndy; 06-15-2010 at 11:56 AM..
Old 06-15-2010, 11:51 AM
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Thanks John, and Andy. I had noticed that spacing the rack up would change slightly the angle of the lower steering u-joint. I'd also think it outs a strain on the joints. They don't seem to be adjustable on the splines to make up for the rack being higher. The pinch bolt goes through a groove on the shaft and has to stay in that position.

I believe I worked out the geometry problem with the linkage. In order for the variable rotational shaft speeds of non-CV type joints to be cancelled out in a double u-joint configuration, the two drive angles must be equal ( say within 1 degree).
On the 911 and 914 steering design, the two angles lie on planes that are inclined at an angle to each other. The compensation for that is the angular offset of the u-joints on the middle shaft. The amount of offset is equal to the difference in inclination angles of the 2 planes. For the same reason, the steering column and pinion shaft are not parallel to each other when viewed from above. The steering column angles toward the middle of the car by several degrees in order to open up the angle of the upper joint to match the lower one.

I now believe that the root cause of the problem is that the altered steering column in the race car is aligned parallel to the centerline of the car. With the pinion shaft also parallel to the centerline, it becomes impossible to equalize the angles formed at the 2 u-joints. Thus the binding and unequal rotation of the steering wheel vs. the pinion shaft. We plan to re-fabricate the steering column support structure and tack-weld it at approximately the stock 914 angle and dummy-up some linkage that can be adjusted in length in order to find the exact lengths and position which results in equal angles at the joints. When it all looks good we'll weld it up tight.

You might say it would be easier to copy the stock set-up, but the structure of the race car dash, which is all integral with the roll cage is totally different. It is also very hard to accurately measure dimensions and angles of the steering linkage elements even in a stripped down racer.

Thanks again for helping me think this through.

-Paul

Old 06-16-2010, 05:12 AM
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