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Suspension Group Exercise

orton1966 - 20/12/11 at 07:23 PM

OK I’m ready for the "depends on you priorities", go read Staniforth etc. comments but I have read this and others and I have considered but even still I would be interested in other peoples take on the suspension design process.

Can’t go the book route because it’s a midi and both the passenger and my own feet are between the front wheels but I am making my own uprights so outer mounting points are free (within the confines of a 15” wheel)

Yes I have made some progress myself and I’ll post the full dimensions when I get home but I was just interested how others, possibly with more experience, would approach the design process. Possibly someone has some proper software to analyse stuff and help fine tune what we come up with.

Anyway here are the parameters I’m working to:

Track – Std Sierra for no other reason that I might get away with using std drive shafts and I’ll go same track front and rear.

Wheels - 15” by 7” with 205 by 50 toyo r888 tyres, probably 108mm ford 38mm offset

Uprights – Thinking about 260mm between ball joint centres, 5 ish degrees of KPI and a small amount of scrub 10-12mm

Bottom inner wishbone mounts - 640-650mm apart to allow foot room as indicated above

Front ride height – 80mm

Rear 100mm

Roll centre heights (static) – Low’ish say about 50mm front, 75mm rear

Any thoughts on the direction to take for a light (400kg) track biased (limited road use) car

Neville Jones - 20/12/11 at 07:58 PM

This should be interesting.

I'll go get my popcorn and red wine now.

Now ready for the armchair professors to quote what they've read in a book, or better still, on wikipedia.

Cheers,
Nev.

matt_gsxr - 20/12/11 at 09:05 PM

quote:
Originally posted by Neville Jones
This should be interesting.

I'll go get my popcorn and red wine now.

Now ready for the armchair professors to quote what they've read in a book, or better still, on wikipedia.

Cheers,
Nev.

disappointing

nick205 - 20/12/11 at 09:26 PM

quote:
Originally posted by matt_gsxr

quote:
Originally posted by Neville Jones
This should be interesting.

I'll go get my popcorn and red wine now.

Now ready for the armchair professors to quote what they've read in a book, or better still, on wikipedia.

Cheers,
Nev.

disappointing

predictable

nick205 - 20/12/11 at 09:29 PM

Perhaps someone with CAD expertise could model this for you to illustrate it and aid comment/discussion?

orton1966 - 20/12/11 at 09:45 PM

I’m kind of hoping it’ll develop into a couple of different schools of thought to try i.e. “if absolute control of roll centre is paramount this is one direction” or “ if wheels vertical if paramount and you can control roll go this way”

bikenuts - 20/12/11 at 09:57 PM

Go on then I'll bite;
Bottom wish bone horizontal - length is determined by the track and footbox sizes you've already specified. Top wishbone 2/3 the length of the bottom and angled to give you the roll centre height you want. Keep your wishbones parallel to the chassis.It's not original but it'll work OK. I think your biggest problem will be the width of your footbox; it's going to leave you with short wish bones=larger camber changes in roll. Plus your feet are closer to the scene of the accident than I like.

nick205 - 20/12/11 at 10:14 PM

quote:
Originally posted by bikenuts
Go on then I'll bite;
Bottom wish bone horizontal - length is determined by the track and footbox sizes you've already specified. Top wishbone 2/3 the length of the bottom and angled to give you the roll centre height you want. Keep your wishbones parallel to the chassis.It's not original but it'll work OK. I think your biggest problem will be the width of your footbox; it's going to leave you with short wish bones=larger camber changes in roll. Plus your feet are closer to the scene of the accident than I like.

Ref. the lower wishbones, it's the pivot points that should be parallel to the ground, not necessarily the wishbones....

bikenuts - 20/12/11 at 10:27 PM

quote:
Originally posted by nick205

quote:
Originally posted by bikenuts
Go on then I'll bite;
Bottom wish bone horizontal - length is determined by the track and footbox sizes you've already specified. Top wishbone 2/3 the length of the bottom and angled to give you the roll centre height you want. Keep your wishbones parallel to the chassis.It's not original but it'll work OK. I think your biggest problem will be the width of your footbox; it's going to leave you with short wish bones=larger camber changes in roll. Plus your feet are closer to the scene of the accident than I like.

Ref. the lower wishbones, it's the pivot points that should be parallel to the ground, not necessarily the wishbones....

True- but in an ideal world the pivots will be in line with the wishbone - and I can't keep a kink in my string

[Edited on 20/12/11 by bikenuts]

nick205 - 20/12/11 at 10:29 PM

Aye - I guess the ideal would be a spherical bearing in the centreline of the wishbone for the outer pivot point

orton1966 - 20/12/11 at 11:06 PM

quote:
Originally posted by bikenuts
Go on then I'll bite;
Bottom wish bone horizontal - length is determined by the track and footbox sizes you've already specified. Top wishbone 2/3 the length of the bottom and angled to give you the roll centre height you want. Keep your wishbones parallel to the chassis.It's not original but it'll work OK. I think your biggest problem will be the width of your footbox; it's going to leave you with short wish bones=larger camber changes in roll. Plus your feet are closer to the scene of the accident than I like.

Thank's for taking the first shot, I’ll draw that up and see what it does in roll, bounce, etc. Regarding the foot position, I confess I was originally torn but couldn’t get over, in my mind, the compromise of pushing every heavy component; me, passenger, engine/box etc. back nearly 1ft to achieve longer wishbones. I know there is a compromise but went for weight distribution over longer wishbones.

Regarding crash protection, the foot-box area isn’t very deformable if braced for suspension pick-ups anyway, so as long as I have a deformable crash structure in front of the front wishbone mounting protection should be very similar

phelpsa - 20/12/11 at 11:06 PM

quote:
Originally posted by Neville Jones
This should be interesting.

I'll go get my popcorn and red wine now.

Now ready for the armchair professors to quote what they've read in a book, or better still, on wikipedia.

Cheers,
Nev.

Now now neville, we've had this discussion about your patronising and unnecessary comments.

bikenuts - 20/12/11 at 11:57 PM

quote:
Originally posted by orton1966

quote:
Originally posted by bikenuts
Go on then I'll bite;
Bottom wish bone horizontal - length is determined by the track and footbox sizes you've already specified. Top wishbone 2/3 the length of the bottom and angled to give you the roll centre height you want. Keep your wishbones parallel to the chassis.It's not original but it'll work OK. I think your biggest problem will be the width of your footbox; it's going to leave you with short wish bones=larger camber changes in roll. Plus your feet are closer to the scene of the accident than I like.

Thank's for taking the first shot, I’ll draw that up and see what it does in roll, bounce, etc. Regarding the foot position, I confess I was originally torn but couldn’t get over, in my mind, the compromise of pushing every heavy component; me, passenger, engine/box etc. back nearly 1ft to achieve longer wishbones. I know there is a compromise but went for weight distribution over longer wishbones.

Regarding crash protection, the foot-box area isn’t very deformable if braced for suspension pick-ups anyway, so as long as I have a deformable crash structure in front of the front wishbone mounting protection should be very similar

You could go F1 and raise your feet over the lower wishbone, just depends on what you've got in mind for the back and how comfortable you want to be.

Matty Dog - 21/12/11 at 12:10 AM

I suggest plugging your figures into this programme:
http://sites.google.com/site/porschersk718/step8a%3Adesign
See what you come up with. It was created by a guy in NZ who is also building a mid-engined car.

Fred W B - 21/12/11 at 06:25 AM

What steering rack are you planning to use? Has to be considered when positioning inner wishbone mounting points if not custom or altered length.

Cheers

Fred W B

orton1966 - 21/12/11 at 07:30 AM

OK based on the numbers so far this is what I get, I'll do some pic's to show roll and bounce latter today

ceebmoj - 21/12/11 at 10:03 AM

quote:
Originally posted by Matty Dog
I suggest plugging your figures into this programme:
http://sites.google.com/site/porschersk718/step8a%3Adesign
See what you come up with. It was created by a guy in NZ who is also building a mid-engined car.

Nice build I have never seen before.

bikenuts - 21/12/11 at 12:41 PM

quote:
Originally posted by Fred W B
What steering rack are you planning to use? Has to be considered when positioning inner wishbone mounting points if not custom or altered length.

Cheers

Fred W B

If your making your own uprights this is less critical as you can adjust the height of the rack to get the pivots in a line then position your steering arm on the upright to suit - should only be necessary to alter the length of the tie rod.

scootz - 21/12/11 at 02:38 PM

Design?

Pah!

Just throw it all together and drive round the issues!

bikenuts - 21/12/11 at 02:49 PM

quote:
Originally posted by scootz
Design?

Pah!

Just throw it all together and drive round the issues!

Whilst selling the bits that didn’t stick at a loss???

I can talk; I’ve 6 different sets of rear lights in the garage and I’m still not happy. To be honest design is what I do for a living; my own car is being developing in a much more “suck it and see” maner, it is after all a hobby.

orton1966 - 21/12/11 at 08:42 PM

I've put the numbers in the suspension calculator provided, dispite my best attempts I do get some variation form my cad model, I think this is down to the difference of entering as x any y coordinated, for the mounts, rather than setting wishbone lengths but with some playing I think I could get them to match. Now I’m going to try moving the top inner mounting point in a clock type fashion to see which directions brings positive results i.e. less roll centre movement.

bikenuts - 21/12/11 at 09:59 PM

quote:
Originally posted by orton1966
Now I’m going to try moving the top inner mounting point in a clock type fashion to see which directions brings positive results i.e. less roll centre movement.

Looks like you're OK for camber gain so my guess would be a longer top wishbone might be worth investigating. I'd look for a set up that gives about the same camber as static when you've got an inch or so of bump and a couple of degrees of roll; hopeful a more realistic condition than looking at bump and roll in isolation.

orton1966 - 22/12/11 at 06:58 AM

OK here is the same set-up but showing "possibly" a more real dynamic situation. So 2 degrees of roll, 20mm or overall bump plus an additional 10mm on the outside wheel and -10mm on the inside. I guess this could be like turning in whilst still rolling off the brakes. I guess positives are yes that the roll centre has moved but in a good direction, plus the outside wheel has the same camber as it was static, good things??? Anyone like to comment?

bikenuts - 22/12/11 at 10:54 AM

I'd say that's an excellent starting point - but I'm biased Time to have a look at the rear? I guess packaging will be more restrictive but you're after broadly similar characteristics; you don't want front and rear trying to do wildly different things. I think you may need the wish bones more parallel to give adequate clearance for your drive shafts, which will tend to raise the roll centre, but you'll probably want it a little higher at the back so that's no bad thing. Try the same lengths as the front, might cut down on jigging?

Big question: When do you plan to start building? You could easily spend all winter tweaking things looking for a perfect solution, if that fits your schedule fine but I'd start looking to join the dots with a chassis ASAP; that's a whole new can of worms

orton1966 - 22/12/11 at 10:19 PM

quote:
Originally posted by bikenuts
I'd say that's an excellent starting point - but I'm biased Time to have a look at the rear? I guess packaging will be more restrictive but you're after broadly similar characteristics; you don't want front and rear trying to do wildly different things. I think you may need the wish bones more parallel to give adequate clearance for your drive shafts, which will tend to raise the roll centre, but you'll probably want it a little higher at the back so that's no bad thing. Try the same lengths as the front, might cut down on jigging?

Big question: When do you plan to start building? You could easily spend all winter tweaking things looking for a perfect solution, if that fits your schedule fine but I'd start looking to join the dots with a chassis ASAP; that's a whole new can of worms

I plan to start cutting metal in Jan, I have a outline cad model, although some parts still aren’t properly dimensioned and have just starting doing a full size mock up of critical areas like the cockpit, engine bay and suspension mounting areas so that I can finalise dimensions and check component packaging, I’ll get some pic's up soon