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Calling mechanical engineers. Changing steering column geometry.


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This is for my LHD 914 which I believe has exactly the same steering geometry as the 911.   Because of this, and due to the 914 being wider and the seats further apart, the steering wheel is offset 40mm towards the centre of the car    I want to correct this and move the column mount so it is straight ahead of the driver seat.   Here is a picture of the mount and that marks shown how far off the seat centre it is.  

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I’ve been doing a lot of reading about UJs and it is not the easiest subject!

First question.  The input and output angles (steering wheel and rack)of the 911/914 steering column are not the same, I believe this is the reason that the UJ’s are fitted out of phase to compensate.    Can someone confirm this?

The column mount also angles the top section of the column slightly towards the centre of the car    It’s a very small amount so it doesn’t really correct the massive offset but am curious as to why?  Anyone?

The easiest method is to maintain the orientation of all of the column sections and make the centre section longer so that the mount moves 40mm left but it will also have to move towards the driver  .

Now, the question for the proper mech engs on here.  if I wanted to change the column to an arbitrary angle, how would I calculate if the change affects the phase compensation of the UJs?   There must be a formula for working this out.

This video helped me get my head around it:
 

 

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Does this help?

https://grassrootsmotorsports.com/articles/how-add-u-joint-steering-shaft/

I always thought two UJ's should be fitted out of phase. My Marcos has two UJ's at 90 degrees with a rose joint support between them. As long as they are out of phase then you should have enough adjustment to move things without causing any issues. Having said that I still have a bit of a thick head after Christmas!!

As an aside I did, many years ago, fit the shafts out of phase by mistake and the steering was not far off unusable as it would 'lock' at certain angles.

 

Edited by Roy M
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Saw this quote "it's interesting that there are such contradictory views on phasing but your approach of "in phase" but "spline at a time until smooth" makes good sense" on this site https://www.gt40s.com/threads/steering-intermediate-shaft-phasing.51663/ where the initial post shows that there are different schools of opinion. Obviously a more complicated subject than I thought and possibly a physical model rather than calculations is the answer?

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^. At the end of the video I posted they show a small phase angle causing non linear rotation on the rig with equal angles of input and output shaft.   The same effect happens when the input and output shafts are at different angles.   Therefore it seems logical that the Porsche column (which has non equal angles) is ‘corrected’ by the offset of a few splines.  This must just be maths but perhaps isn’t the sort of thing to be doing today!  😂

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Can you not maintain the same angle on the upper UJ when extending the intermediate shaft and compensate with a shorter boss to bring it back to the dash? You should be able to work out orientation relative to stock pretty easily to see if this is possible with an acceptable wheel position. 

Until I read your post I never realised when driving my 914 that the wheel is offset. Are you sure this is a problem that needs fixing? 
David 

Edited by flatsix777
Didn’t make sense
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^ Yes, that was the plan but I'm also trying to fit a power steering motor in there so it’s tight and then I started thinking about adjustable columns and whether I can change the angle.

Starting from scratch with the interior so am approaching this as you would with building a hot rod by starting with driver position.  With it all stripped back,  the offset is quite noticeable and a lot more than the RHD 911 which has never bothered me.     Because the car has no clutch, it feels odd to have the accelerator in the middle of the car and the brake to the right.

If you look at the first photo I posted, there is a strengthening flange attached to the dashboard lip above the mount.  A bit of a coincidence that hat my corrected centre mark is exactly in the middle of that.  Almost like that's where the wheel was meant to be!

 

Edited by Jonny Hart
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On 27/12/2020 at 09:22, Jonny Hart said:

^ Almost like that's where the wheel was meant to be!

 

That’s interesting. I’ve read on 914World that the early 914s, I think 70-72, had a different column to the later ones. There is lots on there about replacing early columns with later ones and also 911 columns. I don’t recall too much as my focus was  on replacing the upper bearing but I wonder if the mount position changed early vs late? 
David

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2 hours ago, flatsix777 said:

That’s interesting. I’ve read on 914World that the early 914s, I think 70-72, had a different column to the later ones. There is lots on there about replacing early columns with later ones and also 911 columns. I don’t recall too much as my focus was  on replacing the upper bearing but I wonder if the mount position changed early vs late? 
David

I don’t think so.   I reckon the reason is that the whole front end is based on the 911 so the geometry is the same.   Trouble is, the 914 is wider than a 911 and the seats narrower and further apart.    

Given the troublesome birth of the 914, it would not be inconceivable that it might have been designed to have its own suspension but then they bailed.   Seems weird that it has its own rear setup with coil overs and then another car’s front. 

 

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  • 2 weeks later...

Ok, I found some equations.  

https://www.sdp-si.com/catalogs/D757-Couplings-Universal-Joints3.php

At a glance, these are only for cases where the input and output shafts are in the same plane.  On the 911/914 this is not the case.

Perhaps I am over thinking this as modern cars with steering height adjustment are also changing the input shaft plane which theoretically upsets the steering dynamics.  

 

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