Effects of skewed wishbone inner mount axes
Discussion
Hi Folks,
Just joined up and hope some of your learned members can help me find out the answer to a question that's been bothering me for ages. I've been planning to build my own design of sportscar for the past forty years and finally, having retired, have the opportunity to get on with it! However, there's one bee in my bonnet which I want to get out of the way before I fire up the Mig welder. For reasons which I dont want to go into at this stage, one version of my design would require the otherwise conventional wishbone front suspension to have the axes of the inner mounting points to the chassis viewed from above to be converging inwards towards front, possibly by quite a few degrees. My car design is mid engined by the way. The reason for the required convergence is due to my selection of components and space considerations. Despite considerable searching on the net and elsewhere, I have found scant reference to this and little or no reasoning when I have. Even some of the suspension computer programs I have managed to play with so far dont seem to have a facility which takes this into account. I have racked my little brain to work out what effects on roll centres, track control, camber changes, dive, bump steer etc etc etc such an arrangement would have but can't come to any firm conclusions without some help! I need to get this settled before ditching the idea in favour of something more tried and tested, but for all sorts of reasons I would dearly love someone to tell me it could work! The wishbones can be quite long if necessary, even to the chassis centreline at the front. Hope someone out there can point me in the right direction!
Just joined up and hope some of your learned members can help me find out the answer to a question that's been bothering me for ages. I've been planning to build my own design of sportscar for the past forty years and finally, having retired, have the opportunity to get on with it! However, there's one bee in my bonnet which I want to get out of the way before I fire up the Mig welder. For reasons which I dont want to go into at this stage, one version of my design would require the otherwise conventional wishbone front suspension to have the axes of the inner mounting points to the chassis viewed from above to be converging inwards towards front, possibly by quite a few degrees. My car design is mid engined by the way. The reason for the required convergence is due to my selection of components and space considerations. Despite considerable searching on the net and elsewhere, I have found scant reference to this and little or no reasoning when I have. Even some of the suspension computer programs I have managed to play with so far dont seem to have a facility which takes this into account. I have racked my little brain to work out what effects on roll centres, track control, camber changes, dive, bump steer etc etc etc such an arrangement would have but can't come to any firm conclusions without some help! I need to get this settled before ditching the idea in favour of something more tried and tested, but for all sorts of reasons I would dearly love someone to tell me it could work! The wishbones can be quite long if necessary, even to the chassis centreline at the front. Hope someone out there can point me in the right direction!
Hi, Thanks for that! Yep, I also remember the old Rootes cars having their suspenion angled outwards towards the front as well. The bottom wishbone of some Rolls and Bentleys is angled forward so much that the front mountings are virtually on the chassis centreline. One reference from a technical paper written back in the seventies (cant remember who by) simply said that the inner wishbone pivot axes could be angled in just about any direction (in the horizontal plane that is) to avoid things like engines with very little adverse effect. But no hard and fast physics was given! I can see how you could get away with it in a big heavy Bentley Turbo R (they must have had some reason to do this in any case) but my concern is for a sports/racing application for something which, dynamically at least, will be similar to (but entirely different from!) say a Lotus Elise. Why is this so important to me? Well, all I can say at this stage is 'I have got my reasons!' I am a professional engineer (although not in the automotive industry) and have studied this seriously for years now, even made a working model of the set up, but I guess what I am looking for is someone to say either 'Yes, there's no problem with that!' or 'You stupid b*****d, of course that wont work because it will result in massive *****, can't you see!' Anyway, maybe someone will know - I'm prepared for the embarrasment!
The Dallara F3 car has the front bottom wishbone front legs about 50mm from the centre line. I would have to get back to my old notes, but I'm sure it helps to control roll centre movement, certainly camber control is improved.
Sounds an interesting project.
Not sure about MIG welding though..!
All the best
Graham
Sounds an interesting project.
Not sure about MIG welding though..!
All the best
Graham
Hi Graham
Thanks for your reply. Will certainly check out the Dallara and other formula cars for clues. I'm only an educated amateur but certain things seem clear in my mind. Firstly, I reckon longer wishbone arms or links in general result in less roll centre migration, subject to well selected geometry of course. This is straight forward to devise and demonstrate when the pivot axes are in parallel to the chassis axis. But when the inner suspension axes are converging, things get a bit more complicated! I suppose the most extreme form would be when the axes are turned by 90 degrees - then you effectively have a leading arm scenario with the roll centre pinned at ground level. Maybe, as far as roll centres are concerned, the answer for my skewed axes lies somewhere in between - but I haven't been able to fathom out the maths yet! (Should have listened more at school.) I really want to make my idea work, if for no other reason than I like to do things a little bit differently to everybody else! Have to admit, the idea came initially from designing a car which relied on using all the existing parts from a single donor, not from any brainwave about making a better suspension system. But the more I thought about it, I could actually see some potential benefits. As you say, at least some cars do utlise such layouts, presumably with some intended benefit in mind and I'm gald you pointed out the Dallara, that being an out and out racer. Its just that I desperately want to be shown WHY and what are the potential pitfalls of such an arrangement. My goal is (and has been for all my life!) to produce firstly my own car as my little contribution to the motoring world. But secondly, perhaps eventually to make a business out of it and offer it as a kit, if it had any racing potential. I know, dreams, dreams, dreams. But I really think I have arrived at the point where this dream can be realised. Anyway, thanks again and if you or anyone else has got any hard information, I'm all ears!
Thanks for your reply. Will certainly check out the Dallara and other formula cars for clues. I'm only an educated amateur but certain things seem clear in my mind. Firstly, I reckon longer wishbone arms or links in general result in less roll centre migration, subject to well selected geometry of course. This is straight forward to devise and demonstrate when the pivot axes are in parallel to the chassis axis. But when the inner suspension axes are converging, things get a bit more complicated! I suppose the most extreme form would be when the axes are turned by 90 degrees - then you effectively have a leading arm scenario with the roll centre pinned at ground level. Maybe, as far as roll centres are concerned, the answer for my skewed axes lies somewhere in between - but I haven't been able to fathom out the maths yet! (Should have listened more at school.) I really want to make my idea work, if for no other reason than I like to do things a little bit differently to everybody else! Have to admit, the idea came initially from designing a car which relied on using all the existing parts from a single donor, not from any brainwave about making a better suspension system. But the more I thought about it, I could actually see some potential benefits. As you say, at least some cars do utlise such layouts, presumably with some intended benefit in mind and I'm gald you pointed out the Dallara, that being an out and out racer. Its just that I desperately want to be shown WHY and what are the potential pitfalls of such an arrangement. My goal is (and has been for all my life!) to produce firstly my own car as my little contribution to the motoring world. But secondly, perhaps eventually to make a business out of it and offer it as a kit, if it had any racing potential. I know, dreams, dreams, dreams. But I really think I have arrived at the point where this dream can be realised. Anyway, thanks again and if you or anyone else has got any hard information, I'm all ears!
Yes, you are right to point out some potential caster change. But I think the amount we are talking about with fairly long wishbones is minimal. I'm just wondering how it will change with steering movement. The more I think about this, I reckon I am just going to have to construct a full size working model of the front of my proposed chassis and physically measure what happens. I did make a quarter size mock-up in wood and coat hangers and suspender belts (!) once, but apart from getting a general idea about how to iron out bump steer (which didn't end up being a problem) I couldn't measure things like camber and caster changes very accurately if at all. Wish I had access to a suspension program that could allow for converging pivot axes - does anyone know of one that doesn't cost an arm and a leg!
dave de roxby said:
Yes, you are right to point out some potential caster change. But I think the amount we are talking about with fairly long wishbones is minimal. I'm just wondering how it will change with steering movement. The more I think about this, I reckon I am just going to have to construct a full size working model of the front of my proposed chassis and physically measure what happens. I did make a quarter size mock-up in wood and coat hangers and suspender belts (!) once,
Sounds like a Staniforth design - were you using the suspender belts as the springing medium?dave de roxby said:
but apart from getting a general idea about how to iron out bump steer (which didn't end up being a problem) I couldn't measure things like camber and caster changes very accurately if at all. Wish I had access to a suspension program that could allow for converging pivot axes - does anyone know of one that doesn't cost an arm and a leg!
In terms of software, don't get too hung up on looking for "suspension software" - basically you just need something with reasonable motion simulation characteristics. So long as you can get a plot or tabulation of X,Y,Z data against a known input (e.g. bump deflection or steering input) then a quick spreadsheet will give you your information on bump steer, castor, and so on.For a free tool you could try freeCAD - not the prettiest, but does some useful things, not least of which giving you some inertial and dynamic (response) data to work with. In an "ideal" world for quicker optimisation you need a software tool which is fully parametric (i.e. you can change the coordinates of just one point, say, and then re-run the analysis); I'm not sure if you can do that with freeCAD or not. Oh, and in case the name wasn't obvious, it is free.
Also might be worth looking at the free version of CarSim. You don't seem to be able to get the free educational version from their site any more (well a quick look didn't see it for version 7), but older versions are available online - have a look at http://www.me.utexas.edu/~lotario/VSDC/CarSimEd/ for instance for version 4.5.1.
dave de roxby said:
All very useful, thanks. (Wasn't WEARING the suspender belt, silly!)
No, no you misunderstand. Allan Staniforth (apart from writing some interesting books) used to design and race / hillclimb his own single seaters called Terrapins. He also sold plans for them.Some Terrapins used rubber straps in tension as the springs - essentially large (and precisely specified) elastic bands. For a quick model of a similar set-up, a suspender belt (or at least the suspenders themselves) could have been good "quick fixes"
Here's one of Staniforth's books.
Sorry for jesting. Yes, I remember reading at least some of Staniforth's book quite a few years ago. Must revisit. Nothing wrong with elastic as a suspension medium, cheap, light, adaptable - more people should think outside the box. I made the first model mainly to see how I could arrange the steering arms and minimise bump steer. Unfortunately, the joints were a bit Heath Robinson and so I couldn't rely on fine measurements, especially at quarter size. But it showed me that it would actually move up and down without tying itself in knots. Don't know why I didn't persevere at that stage - well actually I do - divorce, fights with bank manager, selling houses etc etc. But that's all behind me now. YIPEE!
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