Messing about with track and wheelbase ratios.
Hot_Protein - 14/8/14 at 12:54 PM
Hi there Chaps
I have a technical question about handling in regards to track width and wheelbase and the golden ratio.
I have a MKIII 2.8 Ford Capri shell that I am building into a car of some sorts. I have had many Capris and love them. Although their live axles are
fun, they are a bit prehistoric. My solution is to use a suspension setup from another car that already handles well. I found RX8s to be cheap for
parts so I acquired an axle from a 2003(I think). The problem I am having is that the RX8 is a lot wider in track and longer in wheelbase. I can make
some arches to fit the wheels under or even an X-pack, however I would need to increase the wheelbase to keep the ratio the same as on the RX8. The
“Golden Ratio” is supposed to be about 1.6 but the RX8 has a ratio of 1.79 whereas the Capri has one of 1.85. I know that a longer wheelbase will give
me greater stability but less agility and a shorter one the opposite. What I want to know is; what are your thoughts on what would be best. Do I
shorten the rear suspension arms to match the track of the Capri or fit as is with the Capri’s original wheelbase or increase the wheelbase on the
Capri to match the RX8 track width? The ratio of the RX8 track width with the Capri’s wheelbase is 1.7. This is much closer to the Golden Ratio than
the others but I don’t think that is necessarily better. Apparently the RX8 had an increase in wheelbase to make it easy to handle for the less
experienced drivers. Having gone off a roundabout exit backwards in a Capri I would say that was me.
So to summarise: which track to wheelbase ratio is best for my Capri; 1.7, 1.79 or 1.85?
[Edited on 14/8/14 by Hot_Protein]
coozer - 14/8/14 at 02:28 PM
Not sure what your trying to achieve, the thing is set by the shell, don't think using the wider axle will give you any issues unless your
sticking a big block v8 and trying to make a Nascar racer.
Only issue for me would be the aesthetics, big wide rubber sticking out the back might be so 1980`s...
If your dead set on your 'golden ratio' then narrowing the rx axle to match the shell sounds like the best idea, or build a trick live axle
setup. May be old hat but there's plenty of bits around to get a real good setup. Plenty of live axle donors with good ratios and LSD as
standard, think 4x4. Hilux, l200 etc.
Hot_Protein - 14/8/14 at 04:07 PM
quote:
Originally posted by coozer
Not sure what your trying to achieve, the thing is set by the shell, don't think using the wider axle will give you any issues unless your
sticking a big block v8 and trying to make a Nascar racer.
Only issue for me would be the aesthetics, big wide rubber sticking out the back might be so 1980`s...
If your dead set on your 'golden ratio' then narrowing the rx axle to match the shell sounds like the best idea, or build a trick live axle
setup. May be old hat but there's plenty of bits around to get a real good setup. Plenty of live axle donors with good ratios and LSD as
standard, think 4x4. Hilux, l200 etc.
Well I just want to put an IRS setup on it really and as I got into the idea of building a locost I decided wishbones were the way to go. It would be
easier to put 2.8 gear on but it's so damn expensive. I paid £99 for my RX8 rear end delivered. Has brakes and hubs and a LSD. Also I feel that
if I need to replace parts they will be easier to find and thus cheaper.
I know relatively little about the science behind car design when it comes to the chassis but I am very interested in it. I know someone else has
fitted an RX8 rear suspension setup to a Capri but I don't know anything about how or why he did it or even if it made it to the road.
I don't want rubber poking out the sides of my car and I think an X-pack would ruin the lines but arches might be easier than shortening the
setup. However I would have to get it shortened by an engineering firm anyway so I needn't think about the complexities of such a task.
Sam_68 - 14/8/14 at 05:55 PM
Don't get obsessive about the 'golden section'.
Wheelbase to track ratio is only one of a number of factors that influence dynamic weight transfer, and it's purely coincidental that most
wheelbase to track ratios happen to be not hugely different to Phi. There is no mathematical or scientific reason to suggest that 1:1.618 is
any better or worse than a slightly different ratio.
43655 - 14/8/14 at 06:36 PM
Mine comes out surprisingly close at 1.68:1, track at ~1.58m, wheelbase 2.65m.
However this is coincidental, the car it was based off was about 400mm narrower.
I really wouldn't think it was worth worrying about, perhaps the slightly shorter wheelbase will make it less steady at high speed, but the wider
track can only be good really
CNHSS1 - 14/8/14 at 07:57 PM
Stanniforth has an interesting section in his books (Race and rally source book and competition car suspension book) on the golden ratio, and a list
of cars and their respective ratios.
I would agree that its not essential to hit the ratio as its only one part of a cars handling. Weight distribution and polar moment (weight ideally
within the wheelbase and close to centre line) are just as important but its not a bad thing to keep it in mind.
Uber short wheelbase cars with a wide track often handle well but do suffer from nose lifting under hard acceleration which same car with a longer wb
wouldnt for instance
ettore bugatti - 14/8/14 at 08:31 PM
1.7 should be fine, similar to a BMW 1M.
A Toyota IQ has a low ratio as well
Sam_68 - 14/8/14 at 09:36 PM
quote:
Originally posted by CNHSS1
Stanniforth has an interesting section in his books...
Allan Staniforth was a bit of a bugger for that sort of thing: he wasn't terribly hot as a mathematician, and had a tendency to cling to any
fixed point of reference he could find, like a drowning man clinging to the flotsam of a sinking ship.
Sometimes he clung to the right bit of wreckage (fixed roll centre locations; the dynamics of which I actually explained to him later in his life),
sometimes not (his speculation that a horizontally displaced roll centre might be used to generate 'downforce' by levering the chassis into
the ground displayed a laughable lack of grasp of even the most basic physics!).
I wouldn't rely on him too much as a mathematical Guru.
quote:
Originally posted by CNHSS1...Uber short wheelbase cars with a wide track often handle well but do suffer from nose lifting under hard
acceleration which same car with a longer wb wouldn't for instance
Therein lies the true relationship:
long wheelbase = less longitudinal weight transfer under braking/acceleration
wide track = less lateral weight transfer under cornering
The ratio between them influences (all other things being equal) how sensitive the car is to transitions between the two.
BUT:
a) The relationship is progressive. There's no 'magic number' and if you do the calculations, you'll find no significance on the
value of phi. The 'optimum' value, even if you could isolate it, would be different for a Hillclimb car, a Le Mans car, or a luxury
saloon.
b) All other things aren't equal, so even if there was a magic number, it would depend on lots of other magic numbers.
As just one example, the ratio of pitch stiffness to roll stiffness also figures heavily in the calculations so, even if it existed, the 'golden
ratio' would instantly be corrupted as soon as you introduce an anti-roll bar (which increases roll stiffness but has no effect on pitch).
CNHSS1 - 14/8/14 at 10:26 PM
Allan may have been less than the purest mathematician but he built some great handling hillclimbers and improved a great many more. Proof in the
pudding as they say.
Ugg10 - 14/8/14 at 10:28 PM
If you are looking for a chassis to drop a capri on then have a look at the dimensions of the tvr cerbera - from carfolio -
Capri mk3 2.8
Wheelbase - 101
Front track - 53
Rear track - 54.5
Cerbera
Wheelbase - 101
Front track - 57.6
Rear track- 57.9
A set of wide arches should cover the extra 2" each side. You could even use the ajp engine uf you are brave.
Also have a look at what whitspeed do with their space frame escorts, maybe the cerbera and capri can be done in a similar manner.
[Edited on 14/8/14 by Ugg10]
Hot_Protein - 15/8/14 at 12:25 PM
quote:
Originally posted by Ugg10
If you are looking for a chassis to drop a capri on then have a look at the dimensions of the tvr cerbera - from carfolio -
Capri mk3 2.8
Wheelbase - 101
Front track - 53
Rear track - 54.5
Cerbera
Wheelbase - 101
Front track - 57.6
Rear track- 57.9
A set of wide arches should cover the extra 2" each side. You could even use the ajp engine uf you are brave.
Also have a look at what whitspeed do with their space frame escorts, maybe the cerbera and capri can be done in a similar manner.
[Edited on 14/8/14 by Ugg10]
That sounds very expensive. Also I'm going to try not to have this thing SVA'd. Nice idea but I was looking for proven engineering from the
likes of big car manufacturers. I just don't trust TVR stuff. Especially their engines.
quote:
Originally posted by Sam_68
Don't get obsessive about the 'golden section'.
Wheelbase to track ratio is only one of a number of factors that influence dynamic weight transfer, and it's purely coincidental that most
wheelbase to track ratios happen to be not hugely different to Phi. There is no mathematical or scientific reason to suggest that 1:1.618 is
any better or worse than a slightly different ratio.
I just wanted to know that I wasn't about to totally screw up the handling by changing the ratio. I guess it's not all that important and
shortening the wishbones seems to be the best option for the car.
Thanks for all the info guys.
Sam_68 - 15/8/14 at 03:25 PM
quote:
Originally posted by CNHSS1
Allan may have been less than the purest mathematician but he built some great handling hillclimbers and improved a great many more. Proof in the
pudding as they say.
Interesting, then, that the Terrapins were very much at the 'short wheelbase:wide track' end of the range (and I'm absolutely certain
that he would never have recommended 'improving' a car by altering its wheelbase:track ratio to match the Golden Section)?
I'll see your 'proof of the pudding' and raise you a 'practice what you preach'! 
To be fair to Allan, and before we get carried away, it's worth pointing out that all he had to say was that aesthetically the
Golden Section wins [because] it is considered to the the ideal proportion visually. Which is perfectly true, but nothing to do with vehicle
dynamics...
The list of ratios he gives in the Race and Rally Car Source Book, even excluding ultra-short wheelbase Karts, show over 30% variation, and his
own Terrapin Mk.7 is right at the short wheelbase end of the range.
quote:
Originally posted by Hot_Protein
I just wanted to know that I wasn't about to totally screw up the handling by changing the ratio.
It has to be said that you are more likely to screw up the handling by changing the ratio to match the Golden Section, if it's at the cost
of other factors.
Personally, my priorities when drawing up geometry run something like (in approximate order):
* Control of dynamic roll centre locations
* Steering geometry
* Roll axis inclination (albeit 'optimum' roll axis inclination reflects to some extent on wheelbase:track ratio)
* Appropriate camber control
* Packaging
* Wheelbase:track ratio
...but I actually start from a loose assessment of what I want to achieve in terms of weight transfer characteristics (suspension design is an
iterative process, so you end up back at weight transfer when you do the fine tuning, too).
I don't know about the Cerbera, but the Griffith's geometry was bad to the point of being potentially dangerous, and made me rather
anti-TVR, to say the least.
ettore bugatti - 15/8/14 at 07:03 PM
TVR Cerberas were the first TVRs with a dedicated front upright using Chrysler/Jeep bolt-on hubs. Personally, I do like this construction. Not too
dissimilar to the S1 Elise.
Regarding suspension kinematics. It would be better to use the RX8 setup unmodified. But I do understand that from a bodywork/IVA point of view you
don't want that.
Narrowing the RX8 IRS would ruin the suspension kinematics and you probably still have to cut the floor and rear chassis legs to get it to fit (a no
no if you dont want an IVA).
A very good option is to build an deDion axle, 4 links, coilovers and panhard rod.
And other option is to construct a subframe which bolts to the leaf springs and diff mount and then mount the diff, suspension arms and coil springs
to that subframe.
Ford developed for the first Mustang a bolt-in IRS conversion as a feature for the GT350, but never made it.
Currently Duane Carlings T5 offers something similar:
Hot_Protein - 16/8/14 at 12:34 AM
That's a very interesting setup for the Mustang. I hadn't thought about using the existing mounting points for an IRS setup.
Anyway, I looked at my RX8 IRS and I don't think that shortening it would be such a good idea. I wasn't thinking about the drive shafts when
I considered shortening. Also I want to keep things a simple as possible. And cheap too. So arches it is then. This width of track will give me a 1.7.
I always wanted to increase the track width when I was younger and thought that wide tyres and wheel spacers were the way to go lol.
Now I just need to work out what height to mount the whole lot at.
Thanks again for your input guys.