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Author: Subject: Track width choice
coyoteboy

posted on 11/3/11 at 11:58 AM Reply With Quote
Track width choice

So I've been cruising around the web and books and the general concensus is that, when starting from scratch, there's no golden ratio of track width to wheelbase, and wider track is nice but basically is a world of diminishing returns. I know plenty of super cars have stupendously wide track (2m+) but they are trying to fit rather large engines and trannys in, and they're fairly large in wheelbase anyway making them a large vehicle.

Assuming you wish to stay within normal vehicular width limits in the UK you've a large scope to play with. Where would you begin the work of choosing a track width - by packaging (in order to keep your engine/tranny/wishbones reasonable length to maintain geom) or by some golden intuitive ratio? Actually cars of the "real" golden ratio (1.6108 IIRC) look pretty nice from an aesthetic point of view, but going with a vehicle of more square proportions will return a twitchy car, more long proportions a more stable car supposedly. The question is is that due to the ratio of track/WB or due to actual numerical values of one or the other? i.e. is a long car more stable regardless of track?

One thing that I like about the thought of a fairly wide vehicle is the fact that, while drag-limited at high speed, it provides the best possible platform for cornering, more room for a broad shouldered chap like me, and more room for engine bay bits.

What are your thoughts?

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liam.mccaffrey

posted on 11/3/11 at 12:23 PM Reply With Quote
I asked a similar question in 2004 linky

Hope that helps





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v8kid

posted on 11/3/11 at 01:33 PM Reply With Quote
Seem to remember the golden ratio 1.61:1 wheelbase/track.

My TR8 had a wide track and short wheelbase and was certainly twitchy and turned in well which was OK for fun but a pita when cruising as it wanted to turn in of its own accord!

Balance I guess is the answer hence the golden ratio - might be rubbish but it has some acceptance amongst "experts"





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designer

posted on 11/3/11 at 01:41 PM Reply With Quote
Yep, 1.6 to 1 is the recognised proportion.
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coyoteboy

posted on 11/3/11 at 01:44 PM Reply With Quote
Hmm thanks Liam, hadn't found that in the search. The opinions mentioned there are fairly common - i.e. it doesn't really matter as you can work around it (providing it doesn't cause packaging problems). Wider track = smaller weight transfer = more grip, surely? v8kid - yep, it does seems fairly common in faster/better cars but could be a styling influence?

[Edited on 11/3/11 by coyoteboy]

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tilly819

posted on 11/3/11 at 02:00 PM Reply With Quote
1.6:1 is a very commen ratio

basicly the smaller the ratio such as 1.2:1 will make the car very sharp in the corners but unstable at high speed
and a higher ratio such as 1.9:1 will be more stable at high speed but less responcive in the corners, it really does depend on what you are designing the car for and that is realy the first question you have to answer....... what am i going to use the car for??

I have chosen a raio of 1.7:1 for my Midi however i have a design speed of over 150 mph so i chose to lengthen the wheelbase slightly for a little more high speed stability, though my inital starting point was 1.6:1.

it is custamry to design a car inwards from the tyres however if you dont fancy having despoke drive shafts made it may be worth choosing a drivetrain and then this will position the width of your drive flanges and then choose your tyes and then some wheels and choose an offset and this will give you your track at the driven end (dont forget to add on the width of the discs sandwiched between the drive flange and the wheel)

once you have your track just multiply it by your ratio and that will give you your wheelbase

tilly





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coyoteboy

posted on 11/3/11 at 02:45 PM Reply With Quote
Cheers for the tips chaps, looks like I've gathered pretty "accepted" answers then and makes a lot of sense. As I'm creating the car for double-digit speeds and like my corners I know to keep the ratio at or below 1.6:1 then. As for ultimate measurements, that is going to depend on my powerplant choice which is swinging too and fro endlessly at the moment If I go car engine'd I'll just match the donor. If not I've got a lot more scope for fun and cost
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orton1966

posted on 11/3/11 at 03:56 PM Reply With Quote
Wide cars

Wide cars might have theoretically better cornering but in an S bend - Chicane situation you are forced into taking tighter turns so you loose all advantage. Therefore irrespective of ratio (anything around 1.5:1 to 1.7:1 will work well) smallest is always best, especially in real world situations. When was the last time you've seen a big supercar really hustling along a stretch of B road!
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coyoteboy

posted on 11/3/11 at 04:36 PM Reply With Quote
Most of the supercars I see are cruising through town, but I follow your point I'm not sure narrower is better overall though, at some point you become a bicycle and need to worry distinctly about roll.
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britishtrident

posted on 11/3/11 at 04:49 PM Reply With Quote
Change the track by 50mm you will hardly notice it.
Change the wheelbase by 100mm you certainly will.





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coyoteboy

posted on 11/3/11 at 04:55 PM Reply With Quote
That's pretty much the exact opposite I've read elsewhere
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britishtrident

posted on 11/3/11 at 05:46 PM Reply With Quote
That is the big difference between reading and experience
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britishtrident

posted on 11/3/11 at 06:14 PM Reply With Quote
To expand a bit in the past I have built racing Imps and Imp based Davrians with different front and rear track and wheel base combinations.

On the Imp the rear track was considerably narrower than the front (this was quite common on cars of this era) --- handling was great.
Same car suspension was unchanged but made the front track 4" wider fitting Firenza disc brakes ---- no difference in handling.

Built Davrian based Imp special saloon, 1 longer wheelbase exactly same suspension transferred over but mounted to give wider track at the rear, changes in spring rates handling OK but not great.

Built Davrian Mk8 as the above saloon but longer wheel base suspension transfered from written off Davrian salloon shell handling was superb from day 1.

[Edited on 11/3/11 by britishtrident]

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coyoteboy

posted on 11/3/11 at 07:31 PM Reply With Quote
The information I've read was from other peoples experience?

Not disregarding your info, it's valuable, but trying to identify causes etc...

The problem with your examples is you're chopping and changing chassis, suspension rates, possibly widening the track on a car with already optimal track width etc or widening the front track on a car where the dominant handling characteristic was the rear (rear track is normally narrower on most faster RWD cars). A fair comparison would be to take, say for example, a normal locost, then build an identical chassis with a wider *front and rear* track and compare the two. Without doing that there's a good few variables that could mess up the comparison.

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posted on 13/3/11 at 01:01 PM Reply With Quote
Read some amusing things on the net.

Car setup works for some doesn't work for others, there is no magical WB to track ratio and V8Kid, once they got a proper suspension guy into the factory Triumph Rally team, albeit towards the end, he sorted the TR8 out from a shitheap into a brilliant handling car according to Pond and Ekland.

What about the Stratos, one of the most successful Rally cars ever, The Renault Alpine, Lancia 037, MG6R4 etc.

Then there's long wheelbase cars that have been ultra successful, Opel Manta for one.

Coyote Boy, you need to understand what use you are going to pursue, if racetrack or open highways then wide as you can go regardless of WB and as someone says above, if backlanes, tight mountain twisties or gathering up your mates at your local roundabout then you probably don't want to see more than a 1600 track.

But don't forget that a low and centralized (longitudely) CG is just as relevant to what makes a car work in relation to tracks, the lower and further away from either side wheel, the less effect/importance the track width will have.

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britishtrident

posted on 13/3/11 at 02:00 PM Reply With Quote
Actually the Stratos was infamous for twitchy handling -- it was very fast due power to weight ratio, and great traction which came from weight distribution and long travel suspension but the short wheelbase and low polar moment meant it was only a useful tool when in the hands of the very best drivers at the top of their game.





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Doug68

posted on 14/3/11 at 04:26 AM Reply With Quote
My 2c on the subject.

Try and find a modern race car of any description in any series anywhere in the world, where if they are free to adjust the track that the wheels are not set as wide as possible on at least one end of the car.





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coyoteboy

posted on 14/3/11 at 11:29 AM Reply With Quote
Some good info here guys, cheers for the thoughts, all being absorbed. I'll see if I can find any series where they are free to choose track width and see how they get in wrt the course type.

As I say, fairly confident that I'm not really interested in a highway flyer and anywhere with a roundabout has far too many other people on the road to use potential, much prefer twisties, open country roads and probably small-ish track events for giggles.

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hughpinder

posted on 15/3/11 at 08:59 AM Reply With Quote
The method I used was:

1. Choose engine and transmission - using a standard set up is far cheaper and easier - This determies the overall width and track.
2. Work out minimum space to give sensible clearance to everything - dont forget pedal movement etc. This defines the minimum length, and I doubt its going to be much less than 1.5* track width anyway. Some of the things you can do to reduce this involve lifting things, which adversely affects the height of the COG.
3. Make the length anything you like thats longer than step 2! In general cars with a longer wheelbase are more stable/less twitchy on the road and less affected by bad surfaces, and lift/dive under acceleration and braking are less, wheras shorter wheelbase will be the opposite.

All in my opinion of course
Hugh

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Neville Jones

posted on 15/3/11 at 10:34 AM Reply With Quote
Mr.Britishtrident, STOP trying to be an engineer!!! You're talking a load of uninformed boll*cks regarding Davrians and polar moments.

Polar moment is the measure of how much weight is at the extremes. High polar moment has weight closer to the extremes than a LOW polar moment.

The first Davrians had a HIGH polar moment, due to the engine being an Imp item, and hanging out the back.

Later Davrians had a lower polar moment, when the engine was mated to inverted VW and other transaxles, and the engine placed in front of the rear axle.

I've seen Davrians with DFV's and hewland transaxles, down to the Imp powered cars. When set up properly, all were competitive and handled well.

Modern racecars try to achieve the lowest polar moments that can be achieved. This then facilitates a car which will change direction quickly, giving sharp turn in and exit behaviour, and preferable dive and squat characteristics.

The argument on wheelbase and track has gone on forever. A longer wheelbase car with a high polar moment will behave like a whale, when compared to one with a lower polar moment.

Shorter wheelbase and max track, with low polar moment, will give a car that will be a little twitchy, but ultimately a better cornering car and faster all round.

The wder car carries with it the extra form drag of the wider bodywork, but this usually overcome by the better all round handling.

Well, that lot above comes from someone who is involved in the design of LMP cars. If you know better, you should be applying for a job with one of the big factory teams.

Cheers,
Nev.

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coyoteboy

posted on 15/3/11 at 11:33 AM Reply With Quote
Cheers for the info Nev, I'd not have known the difference between the cars mentioned.

I suppose wider track also increases polar moment but not as much with an open-wheel type vehicle as most of the kit is around the longitudinal axis of the vehicle rather than in the sidepod area. Drag is of little interest to me as about the only thing it'll affect is motorway mpg and it will be an exo type vehicle I was just unsure as to whether I'd read through the calcs about track width and read too much into them by assuming wider is always better to a sane limit (by not putting it in context). I suspect the choice between BEC and CEC will make more difference to the polar moment than track width and wheelbase but I like to be fully aware of the parameters I'm dealing with.

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nick205

posted on 15/3/11 at 12:42 PM Reply With Quote
Unless you want to enter into modifed or custom built driveshafts, then your track width will inevitably be defined by the donor vehicle components. Better to select a suitable donor vehicle based on suitability of components and track width, then work out what wheel base the new car will have - I would have thought.






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coyoteboy

posted on 15/3/11 at 02:02 PM Reply With Quote
My donor could be anything or any multiples at this stage - I'm not limiting myself to a specific donor. If I'm going to the effort of designing a car from scratch I'm not going to stop for a pair of 150 shafts, but I see your point Currently on the books are everything from audi V8/tranny, MR2 turbo down to twin 750's one driving each side akin to the furore original. Each has benefits and problems.

[Edited on 15/3/11 by coyoteboy]

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v8kid

posted on 15/3/11 at 03:06 PM Reply With Quote
Driveshafts are fairly easy to modify the tubular ones are mild steel and weld easily. Beware solid ones if ther are tempered as welding can cause brittleness try scratching first.
An overlooked effect of longer wheelbase on mob engined cars is increacing wheelbase generally reduces the weight on the front wheels leading to corner exit oversteers. Assuming same drivetrain that is.
It can get a bit extreme even with shorter wheelbases when using front wheel drivetrains at the rear so it needs watching
Nice to see Neville contributing but I wish he'd stop sitting on the fence
cheers!





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coyoteboy

posted on 15/3/11 at 03:12 PM Reply With Quote
Yes that was my major concern with the MR2 tubby setup, the block and box C of G is only ~6 inches in front of the rear axle and a good 30% of the vehicle weight.
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