Steering and suspension analysis

[colinmseries]

I am writing an article for my car club magazine about the development of the car's chassis and would also like to explain some of its suspension and steering geometry with respect to how it feels in use.
The car has SLA front and back and I am looking to use one of the free softwear packages you summarise in the FAQs.
I have most of the books you recommend.
In particular, I would like to consider how features such as RCH and axis inclination migrate, and how scrub radius and mechanical trail change with lock. Any other points to consider gratefully received.
Repetition is not a problem, and for me is preferable to getting lost in a complex programe.
I understand to remember the role of compliance in a 40year old design with tall tyres being analysed by modern methods.
My question is, which of the packages would you recommend for a hobbyist such as myself?
I have a basic home computer and my professional work is in cryogenics, cars are a hobby.
I hope I may debate what I have found with yourselves in the future and collect your thoughts.
Thankyou, Colin.

 

[GregLocock]

Susprog3d is not free but it is cheap and has a great help system.

wishbone has the interface from hell and is only suitable for double wishbones. But it works very nicely and of course if you feel like programming can be modded in various ways.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

I should add that CarSim is the only free program I've seen that really translates back into how the changes affect the car and driver, but what you are asking for " RCH and axis inclination migrate, and how scrub radius and mechanical trail change with lock."" like to explain some of its suspension and steering geometry with respect to how it feels in use."

is very difficult without paying big bucks.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[colinmseries]

Ok thanks. Just so I am clear, do these programs show changes due to lock?

I can do side and front views using a drawing board but don't know how to accommodate the effects of lock. I have a homemade spreadsheet to calculate the real included angle the front upright resides in accounting for KPI and castor but I don't understand what happens as it rotates through lock.
The spreadsheet was originally made for calculating the real angle of my propshaft when running through two offsets.

Thank you, Colin.

 

[colinmseries]

I have started practicing with wishbone.bas before buying a program but noticed something that I can't see how to input. My front upright is the larger Alford and Alder type that Triumph used for years (TR6 etc.) and it does not use a lower ball joint as the outboard lower pivot.
It has a horizontal bearing for bump/droop in the end of the control arm and 1" further outboard a threaded bush inclined at the KPI for steering.
I am thinking wishbone assumes the two axis are concentric?
Can you tell me if this is going to work with wishbone?

Thankyou Colin.

 

[GregLocock]

I'v never analysed that type of suspension, I think I'd ignore the threaded bush for now.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[colinmseries]

Ok, can I better explain my confusion as it seems to me that wishbone calculates things like KPI, scrub, trail etc. from the chassis pickup and ball joint locations.
The threaded bush I refer to is often called a trunnion in old car circles and is everywhere on old Triumphs (Heralds, GT6, TR2/3/4/5/6). In effect the lower control arm is a little shorter than it would be if it used a balljoint because the horizontal pivot for bump/droop is inboard the KPI swivel for steering.
I have found a pic in the link below for you to look at. The trunnion is screwed onto the upright in the pic and the big hole in it is where the lower control arm pivots. The trunnion thread centreline lies on the KPI axis.
My thoughts were that if wishbone calculates the position of anything assuming the control arm pivots lie on the KPI axis, then the results will be wrong.
The top of the upright is no problem as it uses a balljoint.

http://my.voyager.net/~quadrant19/LTE-trunnion.jpg

I am wondering if this needs a program setting up that is more flexible than wishbone?

Thanks for your help, Colin.

 

[GregLocock]

Yes, if you were to try and model that exactly you would need something more flexible than wishbone. Quite which of the cheap/free packages, I don't know. I could do it in ADAMS, or Working Model 3D. There seems to be a redundant constraint in the mechanism, as described.







Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

Oh, ignore the last sentence, I see how it works.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[colinmseries]

Thanks for that. Is your thinking that I am a bit stuck doing it myself with respect to ability and cost and had better give up or pay a pro with the type of software you describe?
Colin.

 

[GregLocock]

No, I think you can do the entire job on a drawing board, or with meccano, so there is no desperate need to pay someone else. Incidentally susprog3d has an option for a trunnion style spindle, which is what they call one that uses a threaded steering joint.

The tricky bit in your whole project isn't establishing the kinematics ie " RCH and axis inclination migrate, and how scrub radius and mechanical trail change with lock", you could even do that in Excel, the tricky bit is really " to explain some of its suspension and steering geometry with respect to how it feels in use."



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[colinmseries]

Ok thank you Greg. I dont see how to do the lock part with a drawing board and dont want to be a nuisance. Also, if I'm successful with my chassis I fancied the idea of looking at other types with a program, so I think I'll try the free 30days with susprog3D.
With regard to my hope "to explain some of its suspension and steering geometry with respect to how it feels in use."
I was only thinking of a basic level that I could modestly justify in my article.
An example of what I hope to comment on is such things as the effect of lifting the back of the car to gain ground clearance. The car only has 3deg of static castor to start with and so lifting the back might explain why those owners that have often then complain of poor self centering.
I would like to come back to you with my findings and see how you rate my conclusions.
Colin.