JoelP
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| posted on 30/9/04 at 09:01 PM |
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book roll centre
it the front roll centre on a book chassis any good? im thinking of copying the dimensions into another design. would there be any easy improvements
if i have an ARB to help keep it level?
thanks in advance for any comments (except comments of 'thick twat', these wont be appreciated...!)
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pbura
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| posted on 1/10/04 at 02:25 AM |
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The book roll center is about 3-1/2", maybe a little high to go with your IRS, which often have lower RCs.
Suggest you determine where your rear roll center is, then see about putting the front one somewhat lower.
Give me a holler (U2U) if you'd like a hand doing some modelling.
Pete
Pete
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leto
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| posted on 1/10/04 at 07:12 AM |
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The book geometry is fair but there is room for improvement. Also it will only work well with the Cortina upright so if you plan to use another
upright you should redesigned it.
Cheers!
“I'm gonna ride around in style
I'm gonna drive everybody wild
'Cause I'll have the only one there is a round”. (J. Cash)
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Bob C
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| posted on 1/10/04 at 11:35 AM |
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"Complete and utter BULLSHITE"
Syd - I'm sure thethread was only started 'cos we hadn't wound you up lately ;^)
Now.. No bl00dy swearing boys.......
Bob
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JoelP
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| posted on 1/10/04 at 11:45 AM |
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quote:
Originally posted by Syd Bridge
Heeeeere we go again!
Roll centres. Complete and utter BULLSHITE!!
Cheers,
Syd.
i see. do you mean complete pooh compared to other things like camber change in bump, droop, and roll, or just absolutely irrelevant?
in which case, i shall rephase my question:
is the front suspension dimensions on a book chassis any good in terms of camber control during bump, droop, and roll? would it be good enough to copy
the dimensions in to the FRONT of a mid engined vehicle (fooled ya pete - this is for the next project!)
thinking about it, since it will have a stiff ARB, i will make the bones more even length and more parallel and rely on controlling roll more.
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stephen_gusterson
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| posted on 1/10/04 at 01:42 PM |
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tescos
thats my roll centre
atb
steve
just ask, im full of good advice like this. Syd isnt the only person to have that opinion.
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mackie
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| posted on 1/10/04 at 02:52 PM |
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Why are roll centres bollox?
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stephen_gusterson
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| posted on 1/10/04 at 03:00 PM |
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my take on it is unless you are going to race competitively, then does it really matter?
A further take on it, is are you going to be able to make something at home thats better than 40+ years of cateringvan development?
At the end of the day, most of us are amatuers. The more you deviate from the base design without knowledge, the more likely you are to f&*k it
up.
Again, in 4+ years I have never ever once at all, in the slightest, seen anyone post
'my locost handles like shyte - how can i make it better'.
KISS
atb
steve
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Volvorsport
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| posted on 1/10/04 at 03:24 PM |
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get some mitchell software , and do your own , it doesnt take much , what it will show up are things like toeout/in on turns and roll etc , so you can
eliminate as much of that as possible .
trying to keep a good position on roll centre will have the same meaning as not leaning the spring damper units over at 45 degrees aswell .
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JoelP
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| posted on 1/10/04 at 09:12 PM |
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i quite see your point syd. completely agree. i put my thinking cap on and worked it out anyway, most of the uneven length stuff and the
unparallel-ness is to keep the wheels upright in different situations, i think i will use a big ARB and make it more simple.
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Bob C
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| posted on 1/10/04 at 09:57 PM |
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Sorry Syd, just remembering it seemed to get a bit personal last time which detracts/distracts from what's potentially an interesting topic.
My personal experience of home-made IRS designs is a single vehicle I built 12 years ago when there was no www & I just guessed at pivot locations
& the result seemed to work just fine. The internet has given me access to slightly more information (a little information is a dangerous
thing????) so with the locost I'm building now I used "roll centre" software to help determine where to place the pivots. Will it be
any better/worse - I don't know yet (and probably ain't good enough a driver to be able to tell ) but for the time being I fell happy that
I can justify every dimension of my suspension on the basis of at least "something"!
Also I'm sure that there are many ways of "skinning a cat", this system seemed to work for me. I appreciate that the important thing
is to keep the outside wheel vertical in turns - that's a direct quote from late fifties colin chapman.
Re Steve's point about the lack of threads "my locost handles shite", it may be that a lot would - if the suspension weren't
so hard! I've had a go in a few now & generally the ride is so hard that at 60+ on normal roads the car is bouncing off the road a lot of
the time & you can't see properly! (OK I'm old & soft, there must be others out there...)
I'm not trying to stir it, just pointing out the alternative view that the roll centre bobbins has its merits!
When my car's on the road I'll find out if handling is one of them! I feel reasonably confident!
Cheers
Bob
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cymtriks
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| posted on 2/10/04 at 09:25 PM |
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Just for info
Standard Caterham Deon
front 60mm
rear 120mm
Experimental Indy Caterham
front 30mm
rear 65mm
Elise
front 30mm
rear 75mm
If you're going to build it yourself why not build it right? Put the roll centres in the best position. If they don't matter much then
this won't hurt. If they do matter then you'll get better handling.
Syd, please tell us why you think roll centres are wrong.
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blueshift
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| posted on 3/10/04 at 09:14 AM |
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quote:
Originally posted by cymtriks
Syd, please tell us why you think roll centres are wrong.
I wouldn't bother cymtriks, have you not seen syd go off on one before? If not, consider the phrase "arrogant patronising slanging
match" versus "intelligent debate".
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dblissett
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| posted on 4/10/04 at 07:40 PM |
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rear irs
hello syd my car handles like poo cos its been in the garage on stands for the last 8 months while i have tried to sort out this very sort of crap
its never been on the road yet and yes i agree that the more we go off book the more problems we make for all us home builders
on balance tho with pbura's help i now have a rear irs i feel more confident about taking on the road and i suppose thats all that matters
dave
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pbura
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| posted on 5/10/04 at 03:08 AM |
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quote:
Originally posted by dblissett
on balance tho with pbura's help i now have a rear irs i feel more confident about taking on the road and i suppose thats all that
matters
Thanks, Dave, and I hope it works out OK!
Re: Roll Center Height
Caterham recently had the suspension of its SV-R designed by a company called Multimatic, and the front and rear roll center heights are similar to
those cited by Cymtriks for the Elise. Is this the experimental Caterham you referred to, Cymtriks? Full report here:
http://www.ukintpress.com/vehicledynamicsexpo/images/vdipdfs/oconnell.pdf
Pete
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Hugh Jarce
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| posted on 5/10/04 at 03:32 AM |
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quote:
Originally posted by Syd Bridge
I am saying no more.
Cheers,
Syd.
That's the best comment in this thread so far!
The pay isn't very good , but the work's hard.
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MikeRJ
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| posted on 5/10/04 at 09:49 AM |
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It seems pretty obvious to me that the location of the roll center wrt the COG is going to determine the amount of body roll under any arbitrary
condtions and hence the amount of camber change. Do you not consider camber change in roll important?
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stephen_gusterson
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| posted on 5/10/04 at 10:00 AM |
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surely, roll centre is down to pick up points on the chassis - if what I read here is to be believed.
I know bugger all to make a tech argument.
My one observation is :
using given hubs, with a given ride height, surely where you mount the pickup points is pretty much fixed for you?
If the hub lower joint is say 150mm from the ground, and you want a 150 high level lower 'bone its gonna be fixed 150 from the ground. Same for
top bone.
Unless you have so whacky wishbone angles.
I think some times that people take car building way too seriously. Its been done for 40+ years to this design, and there are loads on the road. Its
not F1. Many people seem to sell their cars in less time than they take to build em!
atb
steve
[Edited on 5/10/04 by stephen_gusterson]
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Bob C
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| posted on 5/10/04 at 10:38 AM |
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Yo Steve,
yeah the hub end is determined by what hubs you use, but at the chassis end you have about a square foot in which you can design your pivots to go
anywhere you want! Who says the lower wishbone has to be level? It's one of those general rules of thumb, that and "make the top wishbone
a bit shorter". Then you worry about vertical separation at the chassis - make this a bit less than at the hub so your camber change in roll is
sensible (nb tiger avon rear doesn't do this). you're still left with a large area of uncertainty - maybe it doesn't matter? The
software is free off the web & easy to play with - what's to be lost by using it???
It's certainly easier thanresorting to string (string computer - was that in Staniforth's book?)
Syd - I know you weren't going to say any more (teehee :^) but honestly I'd be interested to know the system you advocate. Previously
you've said (I think, paraphrasing) "consider what's happening to the outside tyre" - which is a bit esoteric, a somewhat more
detailed procedure would be appreciated.
Cheers & beers
Bob
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Cita
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| posted on 5/10/04 at 10:39 AM |
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Hi Sid what do you mean by "bring the inner pivot point a bit closer together as the outer pivot points?
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Bob C
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| posted on 5/10/04 at 10:40 AM |
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Aha - got your reply in while I was writing my post
cheers
Bob
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Peteff
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| posted on 5/10/04 at 12:13 PM |
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"Than" the outers Cita.
I think the inners closer than the outers gives you better camber control on the outside wheel when cornering so you can keep the tyre flat to the
road. That's my theory and I'm sticking to it. I don't worry about things I don't understand, but I don't deny their
existence. Roll centres, fairies, Bigfoot, air and the Holy Grail are all things I think about occasionally but I don't get too wound up about
them.
 
Rescued attachment wishbonepos.jpg
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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Hugh Jarce
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| posted on 5/10/04 at 01:31 PM |
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I second Bob C's observation: Who says the (front) lower wishbone has to be level?
If you have the oportunity to optimise roll centres why not take it?
If the inner end of the lower wishbone dips down even one degree, then the front RC will be below wheel centre height which has to be a good thing,
especially if the car has a live rear axle.
And Syd, does "Make them as even in length as possible." mean they should be equal length if possible? I don't think even you would
believe that. Pick your words more carefully, because there are people here who take a lot of stuff as gospel.
The pay isn't very good , but the work's hard.
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MikeRJ
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| posted on 5/10/04 at 01:42 PM |
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Sure you can make a system that has equal length and/or parallel wishbones and it may make some folk happy enough, but quite simply it isn't
going to perform anything like as well as a properly designed system.
Sid, you have thus far given no reasons for your belief that roll centers are "bunk", yet I have several books in front of me given lots
of reasons why they are important. Would you like to expand on your side of the story a little?
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MikeP
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| posted on 5/10/04 at 04:02 PM |
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To echo a common sentiment (from a different perspective):
If the tires and pivot points have no compliance, there's no doubt the vehicle will roll about the virtual roll center. The roll center can
(and often does) move, so this is not a static calculation.
Tires and pivots do have compliance, so the calculations are at best an approximation. It's risky to think that, with the analysis tools and
build tolerances most of us have at our disposal, we can design our suspension system (geometry, shocks, springs) with pencil and paper to have it
perfect the first time (ignoring for a second there's no such thing as perfect, only compromise). We hopefully can get close enough that the
adjustability we build in can do the rest.
I've never heard of a serious manufacturer that designs, builds and ships without a test cycle. To be absolutely sure this needs to be done on
a track at the limit of adhesion. While our cars may handle better than anything we've ever driven, many of us street drivers will not find out
what happens at the limit until it's too late. Finding out there's trailing throttle snap oversteer in a panic situation on the street
can be pretty deadly...
I've had to make changes (NA builder), but I try to stay as close to the original design as possible for this reason (not that I'm
comfortable Champion did all of this testing, but I'm sure he's done more than me).
Like Hugh I was confused by the assertion that wishbones be equal length. I thought it only make sense for high performance cars with virtually no
roll (F1?), as it keeps the tires better positioned under braking (front) and acceleration (rear). But I am willing to learn  . Please share Syd
.
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