How wide is too wide for a vehicle?

[dzine]

I am starting the design of a custom vehicle project, and have been looking into the suspension geometry.

I have the ability to make the track as wide as I want to (within reason), for the highest performance possible.
The vehicle is intended for 80% track, 20% street. I expect the COG to be in the vicinity of 3-400mm.

Does anyone have any thoughts on this? The standard Impreza track with is about ~1650mm, but as I am need to make new half shafts and suspension arms, how wide is too wide, ignoring all things like extra sprung weight etc. I am thinking I would like around 1900-2000mm.

thanks for the advice

Hayden

 

[mcgyvr]

In the states maximum standard vehical width is 102 (2590mm) to 98 (2489mm) inches depending on state laws.

 

[NormPeterson]

There might be some requirement to fit clearance/running lights for road use once the width exceeds some value (for some reason, ~80" comes to mind).


Norm

 

[dzine]

I was hoping the discussion might go more along the lines of things like aspect ratio, either 'track to wheelbase', or 'wheel to track width' (or something).

Hayden

 

[patprimmer]

Hayden. You don't say where you are. Local road laws vary greatly from country to country and state to state. That would be a very good starting point.

Once that is established and the minimum maximum parameters are clearly defined, it them makes sense for us to consider the problem further.

What is within reason.

What aspect of performance. For a salt flats or drag racer, as narrow as possible is normally the highest performance for aero reasons. On tight rough tracks with reverse camber corners, very wide improves stability but hurts aero.

If no restrictions are applicable, about 100' wide would be very stable.

You really need to think this through before you ask if you want useful help.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dzine]

Thanks Pat, some good points there, particularly regarding aero.

Vehicle is mainly intended for circuit racing, with a small amount of street use. This means that I would not want it to be full legal width (about ~8ft), because it would become impractical to drive around easily. I was thinking somewhere in the range of 1.6-2m wide.

Something such as the Koenigsegg has a vehicle width of around 2000mm, whereas the Ariel Atom is around 1700mm.
For a given power, weight, etc, could I assume that the wider vehicle is always better for cornering, and the narrower vehicle is always better in straight line performance? Is the handling related to wheelbase at all? I was intending mine to be around 2400mm.

Sorry if the statements and questions are a bit vague, I am doing my best to explain

Hayden

 

[NormPeterson]

What sort of track duty is this intended for? Paved courses? Dirt? Various classes and different sanctioning bodies are apt to have their own limits that you'd have to satisfy. If it's for autocross or possibly rallycross (think autocross on dirt), there probably is some point, after which any additional width hurts slalom performance enough to offset the theoretically improved upper limit on lateral g.

Extra track width involves extra chassis width (unless this is some sort of open-wheel effort), which in turn affects the magnitude and CG of the the sprumg mass as well as that of the suspension and drive components that you mentioned.

I wonder if CarSim would be of any help here.


Norm

 

[dzine]

'Track use' would be circuit racing on a ashphalt sealed track, in a class yet to be specified, but probably racing with groups of 10-20 cars, so passing ability is important.

Thanks for comments about the width for auto-cross. the intention is that extra width would be just in the suspension arms and driveshafts, so in essence it's an open wheeler.

 

[evelrod]

From a practical point and perhaps to prevent re inventing the wheel...You might check out some of the local "bull ring" race tracks and clubs. In traveling around to my races, I like to take in the Saturday night local races...Some pretty odd race cars, some are WIDE!

Rod

 

[bikesrfast]

Weight and torque play a significant part in it all. Example - If its a up to 300hp 4 cyl for example (you mentioned Impreza?), a 1800mm track should surfice taking aero, stability, manueverability etc into account for a sedan based trackday car.

 

[dzine]

I am thinking about the ~1800mm mark might be what I will settle on. Without going through full simulation based on differing widths, it seems to be a decent compromise.

Thanks for the help.

 

[patprimmer]

What size are NASCAR cars. Would you need more stability or more manoeuvrability than a NASCAR racer.

1800 sounds a bit wide to me, especially if the CG is low and you have less than big V8 power.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dzine]

'Big V8 power' compared to big '4 cylinder turbocharged power' would only make the car heavier. Manuoeuverable? Hmmm, If I want to go around in a circle maybe.

 

[patprimmer]

Whatever

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dzine]

Ok, just re-read my mesage and yeah, was a bit average.
Are NASCAR's supposed to be based on the production variants? I would imagine that they are restricted in the body size to within the bounds of the original (or within a percentage maybe).

 

[evelrod]

NASCAR has perhaps THE most restrictive rule book of any organization, including F-1. There is virtually no part of the car that is not spec something or other! AND, don't knock the "roundy round" bunch. I'm just a vintage racer these days but I've done a bit of oval, dirt and paved, in my time...trust me, it's tougher than it looks. Check the progress of some of this seasons "rookie" Indy champs.

Rod

 

[patprimmer]

My implication was that NASCAR has about twice the power and reasonably similar aero drag to the car you are contemplating.

As they run very high speeds on an oval track, aero is important re lift more than speed.

As they turn one direction only and almost constantly, stability is relatively important, but manoeuvrability is relatively unimportant, therefore you should not need to be wider than a NASCAR racer. I do not know any specifics of their rules, other than the intent is aimed toward parity between American brands and there was a recent kerfuffle because the Japanese were allowed to race.

Also, if you have a flat 4 engine the CG will be a bit lower than a V8, making extra width less productive.

I would also look at dimensions of prototype sports cars and the old Can Am series cars. They also have a lot higher power to weight than your car and manoeuvrability and stability is just as important. I have not checked, but I would be surprised if either are over 1800 wide.


Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dzine]

Thanks Pat, some good points there, especially regarding the CoG etc.
I have a feeling also(dont know for sure) that most race cars are defined by their production equivalents, or a rule book to obtain inter-brand parity, like you mention.

Which is why I find it interesting that most 'supercars' are around 2000mm wide.

Here are some widths:
Pagani Zonda C12 2055mm
Lamborghini Murcielago 2045mm
Ferrari Enzo 2035mm
Bugatti Veyron 1998mm
Saleen S7 1990mm
SLR McLaren 722 1908mm
Lamborghini Gallardo 1900mm
Porsche 911 Turbo 1853mm
Ultima GTR 1850mm
Radical SR8 1800mm (Nurburgring record, 6.55)

 

[patprimmer]

Supercars are not race cars, they are mutcho ego strokers, so styling might play a bigger part than performance.

On a very tough track in Australia, the Holden Monaro HRT 427 (sold in the USA as a Pontiac GTO) regularly beat exotics like Ferrari and Porsche in the annual 24 hour production car race at Mount Panorama, Bathurst NSW Australia.

http://en.wikipedia.org/wiki/Holden_Monaro

https://www.aaa.asn.au/roadtests/reports/245.pdf

http://www.bathurstregion.com.au/mountpanorama/?id=1008

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dzine]

Yes, I dont think the term 'supercar' really signifies much in the real world, aside from the fact it proves you can afford to have a very inconvenient car

Interesting that the Nurburgring lap record holder isnt all that wide (1800m). The concept Monaro you mentioned was 1824mm, and the new Commodore's are 1900mm wide.

What I am beginning to think, is that the width isnt actually that critical at all, and suspension setup is far more important. Probably not much point going beyond 1800 I guess.