Bump Steer ....on a Morgan Three Wheeler

[paul jacobs]

Right guys, I need some help here. Some of you may remember that I was rather taken with the Morgan Three Wheeler, and in fact I took delivery a couple of months ago.

 

It is great little car [actually, not so little, as it more or less the same size as my CSR, although the cockpit is much narrower], the big problem is Bump Steer, caused because the steering rack has to sit on top of the bellhousing, and therefore cannot be moved down, and the track rods then point down to the steering arms. This, as we know is a classic cause of bump steer, and the normal fix is to lower the rack, so that the track rods are level with the steering arms. This is obviously out of the question becasue of the placement of the engine, which cannot be moved.

 

So, big question for those who know their steering geometry is, what else can I do to try and eliminate this, given that nothing other than toe is adjustable? BTW, I have fitted a steering damper, and tried various tyre pressures, which does very little to sort the problem. The factory recomend 2mm of toe out on each wheel, so that then the wheel kicks in, it shouldn't be more than parallel. A little unusual to say the least, but an acknowledgement that they are very much aware of the problem.

 

Paul

 

 

[Paul Deslandes]

can you make a vertical pin, connected to the track rod end at the top and to the steering arm at the top so that the track rod is horizontal or whatever angle is needed - like a giant spacer. Depending on how long it would have to be you might have some quite high bending forces to contend with, otherwise it should work.

You could use hex bar and drill/tap the top end to match the thread on the track rod end and turn and thread the bottom end to fix to the steering arm.

[DJ.]

What response have you had from Morgan?

 

It sounds a major design problem that involves safety, and I would hope they are working on a solution which they should fit free of charge to your car.

 

Duncan

[SLR No.77]

Wot Paul said Always 2 ways to skin a cat, instead of moving the rack down you need to move the track rod ends up, same effect.

 

Stu.

[Martin J]

What a shame this problem is spoiling the car for you Paul 😔

I agree with Duncan that it does seem a bit of a design flaw....they must have carried out testing??

 

BTW I went to the silverstone classic this year & Morgan had the new Plus 8 there minus it's outer skin. A clever bit of work all in aluminium & the 4.6litre BMW V8 up front. It doesn't weigh much more than a ton, boy it must shift. £80K though.

[myothercarsa2cv]

I would suggest a spacer like Paul, this is what Arnie designed in to his steering rod ends, and it works alright.

But looking at this picture, the arms are about parallel with the lower arms? Are your uprights on upside down?

[paul jacobs]

I did wonder about the spacer idea, but for some reason I had it in the back of my mind that the steering arms should be close to or at the same level as the centre of the upright, maybe not though. The current steering arms, welded to the upright are indeed the right way up. As it happens, I have not used Arnies track rod ends, maybe I could find a use for them here?

 

Paul

[6speedmanual]

Raising the tre position upwards using a spacer or pin arrangement will also intrduce a significant twisting action on the steering arm which may not be in its designed capacity.

 

Morgan's recommendation to run a truck-load of toe so it comes into spec when the suspension does go into compression is complete bluster (Morgan stylee). In this condition you could get two detrimental effects. 1, poor feel and handling due to excessive toe-out. 2, still suffer the effects of changing steering angle on bump.

 

No wonder these cars have been described as having "squirrely" handling.... the suspension geometry is nuts! :-P

 

Peter

 

 

[Doug]

Morgan always used to use a sliding pillar front suspension arrangement on their three wheelers but the new cars look to be using twin wishbones, is that right? Might it not be worth talking to some of the old Morgan three wheeler specialists; after all most aspects of a motor car we're familiar with today were done 60+ years ago. Just a thought 😬

 

Edited by - Doug on 16 Aug 2012 09:09:04

[Jonathan Kay]

*arrowup* *arrowup*Such as Mulberry

 

Jonathan

[SLR No.77]

Jonathan, by chance is your middle name Google?

 

Stu.

[paul jacobs]

Yes, I should have mentioned that the Three Wheeler does indeed use twin wishbones not the old fashioned and frankly outdated, sliding pillar.

 

Morgans attitude is one which some of us in the Caterham family are used to, namely TADTS! They know all about it, and at this moment they are not planning to correct it, but I suspect it depends on how many complaints they get over time, but with an order book stretching ahead for over 12 months and rave reviews by all the journalists who have tried the car, why should they worry?

 

Some good comments Peter, thanks, and Jonathan, good searching, I will probably give them a ring, but they seem to be favouring sliding pillar, though I'm sure they will soon get interested in the Three Wheeler suspension.

 

Paul

 

 

Edited by - Paul Jacobs on 16 Aug 2012 13:02:40

[myothercarsa2cv]

Paul, do you have any close up pics of the setup? Might help some of the experts on here

[John Alston]

Went on a Morgan Factory visit last week and took this photo. Looking at the relationship between the upper wishbone and the track rod, they don't seem to be much out of parallel to me.

https://picasaweb.google.com/115521198178448675572

Hope it assists the experts.

Cheers John A

[Stationary M25 Traveller]

Have a photo linky

[paul jacobs]

Thanks John and Ian, that picture still doesn't show clearly what the layout is and

I've had a look through stacks of other photos, but because the rack is behind the engine and on top of the bellhousing, none of them really show the layout clearly.

 

I will take some specific photos over the weekend and post them in an effort to show the problem.

 

Paul.

 

Edited by - Paul Jacobs on 16 Aug 2012 16:39:20

[myothercarsa2cv]

John's photo shows how out of parallel they are to my untrained eye - looks like you'll need a 2" spacer on top! Could you get someone to fabricate a braced extension welded on to the existing steering arm maybe, so that a spacer and long bolt isn't needed?

[EFA]

Better photo:

 

http://www.carmk.net/media/2011/07/2011071308281063.jpg

 

 

The issue of bump steer is not related to any parallel with a wishbone, unless the wishbones are:

1) parallel

2) equal length

 

On the Morgan above, neither of these is true.

 

Also, in my opinion, extending the steering arm vertically by 2" (as suggested) using anything other than high grade forged steel would be a fatalistic move, as only with a component from forged bar stock will the input and reactive forces through the steering be perpendicular to the material axis of strength.

 

Even then, any such design would need review by a competent mechanical engineer, before fitting.

 

 

To fix the Morgan problem, someone needs to model the whole suspension in a CAD app, where the range of movement can be animated, and bump measured.

[Bricol]

Looks to me to me more due to relative positions of pivots - suspension and track rod where it attaches to the rack.

 

However, the Lancia Stratos, and several of the replicas, used Fiat/Lada front uprights with an extension of around 4" long from memory down to the upright. My worry with it wasn't the extension, but the extra leverage and bending moment on the link arm. Mine didn't give any problem, and to be fair, I've not heard of any on other's cars.

 

Bri

[EFA]

Bri,

 

You have a point. The rack knuckles should be more in line with the wishbone inner bushes. A shorter rack is the first thing to fix here.

[Paul Deslandes]

I think you're spot on there Bri. From that view, assuming the knuckles are further out than the inner pivots, as the wheel rises the track rod ends will be pulled inwards relative to the vertical link to give outward going toe. No wonder its lively.

[richy]

So, where are the rack knuckles on that picture? Behind the top damper bolt?

Richy

[EFA]

Richy, Remember it has to turn right and left. Its fair to assume they are probably halfway along the gaitors.

[richy]

OK, thanks.

Richy

[6speedmanual]

Bumpsteer is caused by the arc swept by the tre ball joint being differnt from the arc swept by thepoint on the upright at the height of the tre being a, different in radius or b, the segment being swept not the same. (or both)

 

The inner rod ends of the rack do not necessarily have to be at the same "width" position as the wishbone inner ends. Nor do the wishbones and trackrod (as Arnie has said) need to be parallel. It is just that often they are close to these two conditions in a double wishbone set up beacuse that helps to reduce the problem conditions.

 

What I'm saying is: just making the rack shorter or longer or moving it up and down may not resolve the problem.

 

Modelling it on CAD might be useful, but equally measuring the bumpsteer using traditional B/steer guage and whilst doing so carefully observe the movements of the suspension and trackrods may reveal what needs doing.

 

Also, going back to Paul's original post. You describe the car as having bumpsteer, but what is the actual symptom you are "suffering"?

 

Is it a change of handling characteristic and steering feel as the suspension rolls into the corner? (available of a completely smmoth track as the suspension goes into "Bump") Or are we REALLY just talking about kickback/fidgeting in the steering or the car being turned off course by bumps, potholes, ridges, road paint, surface friction changes, braking on split friction L/R, etc??

 

The reason I ask this is that from Arnie's photo I can see two things

1) The trackrod is only about 2/3 the length of the top wishbone which would give the unequal arcs discussed above and in this entire thread and certainly some bumpsteer in the geometry.

 

But what might be far more significant

 

2) There appears to be a HUGE ground offset. difficult to measure but it looks in the order of arount 130mm. That is a big lot indeed!

(on a SIII on 6" rims ET24 it is approximately 50mm [i have not measured it, just by eye] The design of the 7 is already "a lot". Many cars run close to zero ground offset or even negative values.

 

This huge ground offset on the MOG will give all sorts of issues with the contact patch forces trying to turn the upright.

[To try to understand the effect, imagine driving your 7 with 80mm wheel spacers added to the front wheels!]

 

It is quite possible to have a geometry with quite a lot of bumpsteer but almost no steering kick or feel. Also possible to have a set up with zero geometric bumpsteer but terrible kickback, tramlining, instability under braking if there is a high offset.

 

Hmmmm.....

 

Peter