29 replies to this topic

[John Brundage]

I am working on the suspension on a Lotus XV and as part of the project, locating roll centers etc.. The Lotus XV has a Chapman strut in the rear. The half shaft doubles as the lower control arm. There is an arm going from the frame to a pick up point on the bottom of the hub/housing casting. Puhn and Miliken seem to differ on locating/identifying the roll center for this rear suspension. Does anyone here have any thoughts on this?

[Greg Locock]

I am working on the suspension on a Lotus XV and as part of the project, locating roll centers etc.. The Lotus XV has a Chapman strut in the rear. The half shaft doubles as the lower control arm. There is an arm going from the frame to a pick up point on the bottom of the hub/housing casting. Puhn and Miliken seem to differ on locating/identifying the roll center for this rear suspension. Does anyone here have any thoughts on this?

The halfshaft forms a radius arm, so the IC falls somewhere along the axis of that. The strut forms a slider, so its axis must be tangential to the radius from the IC. I think usually you take the point half way between top seal of the strut and the piston?

I imagine that is what Milliken says?

Edited by Greg Locock, 18 October 2010 - 02:09.

[cheapracer]

http://forums.autosp...rformancetrends

Go to post 21.

[Greg Locock]

http://forums.autosp...rformancetrends

Go to post 21.

I'm not entirely sure that typing a geometry into a black box program really answers the question.

[cheapracer]

Greg, after reading the post properly you're absolutely right.

Just trying to be helpful

[Bloggsworth]

For a straightforward strut system - Draw an extended line at right angles to the shaft of the damper at the top mounting point; draw another extended line through the centre points of the hook joints (Universals) until it meets the line from the top of the damper. Then draw a line from the contact patch of the tyre to the intersection of the other two lines; where this third line passes through the vertical centre-line of the car is the roll centre, so the distance from there to the ground is the static roll centre height. The intersection of all three lines is the pivot point for the effective swing-arm length.

The "radius" arm on the Chapman strut turns the system into a sort of semi-trailing arm/strut system with a lot of pro-squat built in, as the centre of the UJs is the best part of 4 or 5" above the trailing arm mountings on the bottom of the strut (From memory, it was in 1964 that I last put new UJs in an Elite; weirdly enough, Max Mosley's Elite round at Len Street's one Saturday afternoon. I'd only gone there to buy some parts!). There must have been a fair bit of conflict going on in the system as the rubber balls at the monocoque end wore out pretty quickly.

Edited by Bloggsworth, 20 October 2010 - 08:56.

[cheapracer]

Just for my own curiosity does anyone have a picture of the setup please?

I want to see for myself what the RC does in this setup.

Even a rough mudmap would surfice....

(of course I Googled it)

Just found this

And from here with more info ...

http://www.bjorns-st...tus15_3_eng.htm

Edited by cheapracer, 20 October 2010 - 14:43.

[Allan Lupton]

Both Costin & Phipps and Campbell have a drawing of the Chapman Strut both as front and rear suspensions and both show a roll centre located at about 1/3 of the height of the inboard pivot of the wishbone/drive shaft. Neither offers a way of caclulating where that is.
Since Chapman (and Macpherson) struts have a upper point that gives no track change as the result of wheel movement, the kinematics of that could be the equivalent of an infinitely long upper link in a conventional double-wishbone system. Defining the fulcrum of that upper link could be difficult for Chapman Strut, as the upper point is usually well inboard of the lower outer pivot.
I see the link referred to above goes to some slow-loading stuff which includes Macpherson Strut so I should think that'd give similar results where the inset of top mounting is not as extreme as in the above photo.

[Bloggsworth]

If there was no track change, then there must have been plenty of camber and roll centre change associated with the layout, as the three are irreconcilable.

[Bloggsworth]

The drawings don't show the oil dripping off the rear discs....


You can see the conflict between the three fixed inboard mounting points. The radius arm and drive shaft could operate as a pair of wishbones, but that brings the movement into conflict with the fixed top mounting of the strut.

Edited by Bloggsworth, 21 October 2010 - 21:42.

[Greg Locock]

Ypu can see the conflict between the three fixed inboard mounting points. The radius arm and drive shaft could operate as a pair of wishbones, but that brings the movement into conflict with the fixed top mounting of the strut.

Kinematically I'd model that as two more or less horizontal arms with ball joints at each end, and a slider to the hub with a ball joint at the top.

All 'errors' in the relatiosnhip between those links come out as camber and toe changes, I think. That is, treat the strut and halfshaft as a pair, they obviously under-restrain the hub in toe, so the toe control link needs to point at their FVIC in front view, and I'm not too sure where in side view.

[John Brundage]

may this helps to visualize it:

The series 2 Elite is different than the XV.
The XV uses a single radius arm that goes from the bottom of the upright forward to a mounting point on the tub. At ride height the radius arm is parallel to the ground. The original radius arm has an s-bend to clear the tire before going forward. It runs parallel to the chassis center line. The radius arm is approx. 16" long.

[John Brundage]

Kinematically I'd model that as two more or less horizontal arms with ball joints at each end, and a slider to the hub with a ball joint at the top.

All 'errors' in the relatiosnhip between those links come out as camber and toe changes, I think. That is, treat the strut and halfshaft as a pair, they obviously under-restrain the hub in toe, so the toe control link needs to point at their FVIC in front view, and I'm not too sure where in side view.

Greg,
Please see my reply above. There is no toe-link per-se. While the radius arm does set toe, it is mounted fore and aft.
The toe-link in the Elite drawing above (the arm below the half shaft) does not exist on the XV.
Also, in the Elite, the inboard radius arm/link mount is in line with the strut center line. The XV is not. If you look at the angle of the XV strut in the picture in Cheapracer's post, you can visualize this.
I will try to take pictures and post them in the next couple days.

[cheapracer]

The front strut setup I experimented with recently isn't remarkably different from the Lotus 15's rear setup (I had never seen a 15's setup before this thread).

I had zero lateral scrub and my desired camber curve. It did require long LCA's and the outer BJ higher than the inner to achieve.

I'll try to find some pictures of it and the software screen captures.

[Greg Locock]

There is no toe-link per-se. While the radius arm does set toe, it is mounted fore and aft.
The toe-link in the Elite drawing above (the arm below the half shaft) does not exist on the XV.

That is what I meant as well, I hadn't actually seen the lower lateral arm(s) in that drawing !

[Bloggsworth]

The Series 1 Elite had its spherical rubber bush inside a machined block of aluminium bolted to the underside of the monocoque, the Series 2 had a hemisphere moulded into the monocoque and the ball was held in place by a pressed steel cup held on with 2 bolts.

[Tony Matthews]

may this helps to visualize it:

That illustration, whilst not actually misleading, is not as clear as it might be. It took me some time to realise that the 'toe-link' is in fact a radius arm.

[Greg Locock]

That illustration, whilst not actually misleading, is not as clear as it might be. It took me some time to realise that the 'toe-link' is in fact a radius arm.

So the leftmost arm is a triangle, or just a link? Anyway it certainly is not what was described in the original post.

[Tony Matthews]

Just a few words in favour of technical illustration - you need at least two, sometimes three views in standard orthographic projection to convey quite simple information, whereas a basic perspective sketch can tell all in one hit. This two-view needed a plan view to clarify the layout.

[desmo]

Perspective drawings are great, they convey spatial information in a far more intuitive manner than orthographic projections but must often be exponentially harder to produce and of course they distort dimensionally so that measurements cannot be taken from them. I wonder though how many errors are made reading from orthographic illustrations though because of their inherent lack of intuitive clarity?

[Allan Lupton]

Perspective drawings are great, they convey spatial information in a far more intuitive manner than orthographic projections but must often be exponentially harder to produce and of course they distort dimensionally so that measurements cannot be taken from them. I wonder though how many errors are made reading from orthographic illustrations though because of their inherent lack of intuitive clarity?

Measuring off drawings is not to be encouraged, so that can be taken as irrelevant, but if you use an isometric system you could do so if you had to.
Orthographic drawings are fine if you know how to read them - like maps or any other representation of three-dimensional objects on a single surface.

[Tony Matthews]

Orthographic drawings are fine if you know how to read them - like maps or any other representation of three-dimensional objects on a single surface.

Yet several posters who I would expect to have no problem reading a standard engineering drawing have been slightly mislead by this two-view. Every engineering drawing that I have seen has something like "Do not scale" writ bold in one corner. All I'm saying is that this was not the best way of showing how a Chapman strut works.

[Allan Lupton]

Yet several posters who I would expect to have no problem reading a standard engineering drawing have been slightly mislead by this two-view. Every engineering drawing that I have seen has something like "Do not scale" writ bold in one corner. All I'm saying is that this was not the best way of showing how a Chapman strut works.

The first comment was more or less my thought, too.
Those of us old enough to have learned first angle projection for three-view at school and then had to change to third angle at College of Knowledge can still get confused, but we keep quiet about it.
I have always admired those who were not confused and could translate three-view into perspective and in particular into cutaway. Apart from those who work(ed) for magazines and became household names, there were many others working in industry (I remember in particular Andrew Coates at Hawker Siddeley Hatfield in the 1970s).

[Bloggsworth]

The drawing posted is not an engineering drawing, it is only for illustrative purposes, probably from the service manual, why is it being discussed and, to a degree, criticised for not being up to snuff? It serves perfectly well to demonstrate the general arrangement.

[Tony Matthews]

The drawing posted is not an engineering drawing, it is only for illustrative purposes, probably from the service manual, why is it being discussed and, to a degree, criticised for not being up to snuff? It serves perfectly well to demonstrate the general arrangement.

You are quite right, it is not an engineering drawing, but it follows the discipline, and it is not immediately clear what is going on. It was done, I think, to illustrate different suspension systems. I only mentioned it in passing, I appreciate that there is a thread on another forum for this subject.

Edited by Tony Matthews, 23 October 2010 - 10:37.

[Greg Locock]

http://greglocock.we...cledynamics.htm

Here's an oblique view of the most complex geometry easily represented by that drawing. I suspect John's car does not have the purple and orange links, consequently the geometry of the red link is very important.

[cheapracer]

http://greglocock.we...cledynamics.htm

Here's an oblique view of the most complex geometry easily represented by that drawing. I suspect John's car does not have the purple and orange links, consequently the geometry of the red link is very important.

I worked on an Elcat (?) once that didn't have the purple and orange links so I suspect you are correct.

[John Brundage]

http://greglocock.we...cledynamics.htm

Here's an oblique view of the most complex geometry easily represented by that drawing. I suspect John's car does not have the purple and orange links, consequently the geometry of the red link is very important.

Thank you for the link.
The grey is the halfshaft
As noted above, the XV does not have the orange and purple links.
The red link is parrallel to the X axis. Toe is adusted by varying the spacers on either end of the rod end at the chassis end of the red link.
At the present ride height, the angle of the strut (green link) is 45 deg. The halfshaft is approx 11 deg, with the differential lower than the center of the rear wheel.
Rear wheel camber is about 3 deg. It is different side to side--2.5 deg on the other side.
The series 3 Lotus XV did have 3 different pick up points for the top of the strut. This car originally had this set up, but now has a single mount as is shown in the picture on an earlier post. I plan to going back to that arrangement.

[Greg Locock]

As noted above, the XV does not have the orange and purple links.
The red link is parrallel to the X axis. Toe is adusted by varying the spacers on either end of the rod end at the chassis end of the red link.
At the present ride height, the angle of the strut (green link) is 45 deg. The halfshaft is approx 11 deg, with the differential lower than the center of the rear wheel.
Rear wheel camber is about 3 deg. It is different side to side--2.5 deg on the other side.

If the red link is parallel to the x axis then toe control seems a bit unconstrained, ah I guess the bolts through the bushes in that link are in the y axis?

[John Brundage]

If the red link is parallel to the x axis then toe control seems a bit unconstrained, ah I guess the bolts through the bushes in that link are in the y axis?

Yes