Lotus Elan

Rear insert alignment in top metal mount & axle position

PostPost by: miked » Sun Jun 21, 2009 9:43 am

I just got back to working on the rear of my track S4. It is 26 R type unused (NOS) black chassis with TTR "A" frames and all metal back end (top spherical mounts and inner and outer "A" frame bearing etc). I am having a trial build up of it all as you have to when you go metal.

It is all new kit!

I have question regards ligning up the lot.

I imagine that you dont want any forward or aft' stess on the shocker insert piston ????? Will wear out seal etc! I.e. the pistion should slide into the top metal mount hole in the middle and not under stress. Not requiring flexing to enter! The adjustment you have for this is the small top hat bushing for locating the rod end bearing in the inner "A" frame part. Lathe job! Also there is some scope for movement with the chapman strut and the washers that locate it in the spherical bearing on the outer part of the "A" frame.
Or am I wrong as this throws out your axle position????
It is good to do the same to each side, if the chassis and all the bits were identical, but I dont think they are.

I understand that the Chassis's can be out and also the "A" frames could have a very small natural twist (from heat of build) causing the strut to kick at a very slight angle giving top misalignment.

I am in a bit of a muddle. :? I have even had the camber gauge on the shocker tube (and the chassis leg uprights to see if the are perpendicular).

Have we any advice or is it best compromise because it is Lotus chassis. Should I be less concerned about a small amount of tension giving misalignment at the top of the shocker tube as it enters the top mount and more about the wheel/axlel position.

With all the tolerance's of the bits it is hard to decide what is a reference point. Should I be measuring rear axle postion in relation to the the front chassis leg wish bone pins as a reference for correct distance between front wheels and rear wheels. Then using the adjustment mentioned to give right whee/axle positions.

And I thought it was going to be quite easy. :o I have end up with one "A" frame set back ref' the other in trying to get a straight entry into the top mounts. This can not be right. I may have answered my own question. Perhaps the Chapman strut (and shocker insert) do not give a perfect 90 degree when bolted into the TTR "A" frame. Do they need a flex to assemble? We are only talking about 2 -3 mm here.

Any advice welcome. I have kind of got lost in tolerances. I may be over estimating the small amount of strain I may put on the spherical bearing.

PS I hope this is clear.
This is what happens when electrians start doing mechanical bits, they get all worried about stressing things. :? Nothing lost, as it can all be altered. Only washers and metal bushings etc. :D

Mike
Last edited by miked on Mon Jun 22, 2009 2:43 pm, edited 1 time in total.
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PostPost by: GrUmPyBoDgEr » Sun Jun 21, 2009 10:38 am

I'm not quite sure if I've fully understood your lengthy question Mike, but I think you're saying you have an alignment problem with the strut into the upper mounting.
I would have expected a spherical (Rose Joint) mounting up there to accomodate all of the possible adjustment that a fully Rose jointed suspension offers.
Stiction in struts, McPherson or Chapman is an inherent design problem & everything posible should be done not to aggrevate the situation.
Is TT not able to offer advice or a solution?
You are going deeply into 26R territory here & no doubt some of the racers out there will have the answers but not too many of them post on here.

Good luck
John
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PostPost by: rgh0 » Mon Jun 22, 2009 8:09 am

The strut should slide in to the top support with minimum side loads - in / out is not a problem but forward / backward could be a problem depending on how straight and square everything is. In the standard set up the rubber busshes can accomodate the normal tolerances. If you have everything rose jointed and with a top metal mount then the tolerances are critical to get everyting to line up without to much load on the shaft to twist the lower A frame to achieve alignment.

If they are not aligning then you need to measure everything until you understand what is the problem and fix it. changing the wrong thing will create 2 problems that may make the top strut align but may affect the rest of the geometry

cheers
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PostPost by: miked » Mon Jun 22, 2009 11:18 am

Thanks John and Rohan,

I will carry on and measure and keep checking. I had a word with TT. He agrees to carry on and put the time in to get the shockers lined up in the top mount without any forward and aft' stress. He used the word optimise. He mentioned about the "A" frames being hand made and not going to be as accurate as production pressed types on modern cars. I can quite see that the slightest twist can give a throw out at the top of the shocker rod.

I think I will be trying the spirit level across the bottom of the outer part of the "A" frame to see if I have got some small variation. I think I have on the right side as when the lot is assembled without the (inner "A" frame) rear top hat spacer bushes, the "A" frame springs to the rear of it's chassis mount. When you try to centralise so the shocker is perfectly at 90 degrees, it springs back, so there is some tension. No much though!

So it looks like an "A" frame twist. I have also been told not to get too hung up on distance between wheels but will measuring that too.

Thanks Mike
Last edited by miked on Mon Jun 22, 2009 2:45 pm, edited 1 time in total.
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PostPost by: GrUmPyBoDgEr » Mon Jun 22, 2009 12:25 pm

Hi Mike,

The twisted A-Frame theory seems logical.
Do you have access to a suitable surface plate (marking out table) or a large enough piece of plate glass.
If so it will be fairly easy to check for A frame twists.
It should be possible to "tweek" them flat with a bit of "gentle persuasion"

Good luck
John
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PostPost by: miked » Tue Jun 23, 2009 8:47 pm

John & Rohan,

I just nipped in the garage and dropped the right hand "A" frame off. Not got a proper surface but tried it on the kitchen work top as a start point. You can get about 2mm under the rear outer. Even the wifes work top can't be so far out. You can rock the frame. No wonder it wants to lean the shocker at the top.

Moral is check everything. Pity it is all laquered and passivated. Still never mind. I will check the left and deal with this. Bit poor really.

Mike
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PostPost by: miked » Tue Jun 23, 2009 9:02 pm

Just checked left side. Not as bad as the right but about 0.75mm to 1.00 mm and a enough to rock.

Mike
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PostPost by: GrUmPyBoDgEr » Wed Jun 24, 2009 8:39 am

I guess that's a fairish indication that you've got a problem but don't put too much trust in that work surface, you don't know how hard "her indoors" has been leaning on it with the rolling pin :roll:
Get hold of a big enough piece of 1/4" plate glass it makes a pretty good surface plate when well supported.
If it's Granite forget the above.
I'd have thought that the twist could be levered out cold without damaging the surface treatment with applied heat; it's the way they true up most fabricated frames.
Nevertheless it's good to hear you've got to the cause.
Cheers
John
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