Let's say one is interested in car stability under emergency evasive maneuvers / collision avoidance and such.

Things like suspension, COM height, tire traction, wheel base, track (is this the right term for distance between wheels on a given axle (is there a term for this like wheel base?)) may affect the car's agility and stability.

Holding other factors constant, and defining an aspect ratio of wheel base distance to track, is there an ideal aspect ratio that balances agility and stability (anti-roll over, oversteer, rear end swing out, etc)?

Let's say a car has a 108" wheel base and the track is 70" resulting in an aspect ratio of 108:70 or 1.54:1. Is this a longer, shorter or just right for optimal balance of agility and stability? It might be longer to allow for more (rear seat) passenger comfort. Or it might be just right.

Is there a way to compare this across cars / trucks?

I dunno the answer to your question, but I believe the term for distance between wheels is "track".

http://training.sae.org/seminars/c0414/


http://training.sae.org/eseminars/pd130702on/


There is a textbook on this site that answers your questions.

The subject is complex. A parametric view of factors is possible but challenging.

Google "vehicle suspension design parameters"

quote:

Originally posted by 4MUL8R:
The subject is complex. A parametric view of factors is possible but challenging.

You have a gift for minimalism
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I doubt you could compare even vehicles with similar wheel base/track & get good data (you'd have data, but there would be 10s of factors you couldn't control for).

You want better control in shitty situations?
1. ESC
2. Learn to drive (not a dig at you, take a driving course with a competent teacher - fun & you'll learn something).
3. Practice, practice, practice. How do you know when the car is getting 'unstable' unless you hang the ass end out a few times?

The most stable car in the world will be stable, until it ain't - then you better know what to do. An F1 car at 100mph is more stable than a grocery getter at 50mph (depending on your definition of stable).

I retract the question. I couldn't really understand the Overviews, let alone expect to understand the full complexity of a class or textbook

Just wondering if there was a simple rule of thumb, like wider tires is better for traction, longer wheel base is better for straight tracking, shorter wheel base is better for agility, etc. XX aspect is optimal for car heights, and YY aspect is optimal for truck heights.... Something like 50:50 front:rear weight distribution is somewhat ideal.

But sounds like it's way too complicated for even a rule of thumb....

It is not simple. Look at the different types of racing. Each vehicle in each type is set up different for what they race on. The surface of the track plays a part. Sand, gravel, dirt, asphalt to determine tire selection, suspension and brakes. These are key components in stability as well as wheelbase and track.

Take for instance circle dirt track racing. They use different width tires on either side of the same car. They are only turning left so a wider tire is used on the passenger side verses the driver side. A top fuel drag car will use the same size tire on both sides of the car. They want to go as straight as possible so a wider tire on one side would give more traction on that side and send them sideways.

For cars that turn left and right a wider track is better to spread weight across the car and keep it flatter in the turns. How sharp the curves are determines if a shorter wheel base is optimum. The sharper the turns the shorter the wheel base to get around quicker. A longer wheelbase gives a smoother ride though.

And then there is aerodynamics to factor in as well...

Generally speaking, a longer wheel base, wider track, lower Center of gravity will be better but it gets much more complex from there.
Trucks and SUV's are usually higher C/G than a car.
Practical things like keeping proper tire inflation, good tread, etc. are important too.
Most modern cars don't have any serious handling problems if driven at reasonable speeds and properly maintained.
Most street car dimensions are determined by marketing issues like passenger and cargo capacity, load rating, etc. Then there is styling so they can sell the things.

Small wheel base to track ratio can make for an unstable car for everyday driving. I drove VW Bugs for several years and loved the way I could sling the underpowered POS around curves. Then my wife 360ed my new Super Bug off the road with my first son in a laundry basket in the back seat. Not cool. If you are into drifting, be aware that should not be done on public roads.

quote:

Originally posted by konata88:
I retract the question. I couldn't really understand the Overviews, let alone expect to understand the full complexity of a class or textbook

Just wondering if there was a simple rule of thumb, like wider tires is better for traction, longer wheel base is better for straight tracking, shorter wheel base is better for agility, etc. XX aspect is optimal for car heights, and YY aspect is optimal for truck heights.... Something like 50:50 front:rear weight distribution is somewhat ideal.

But sounds like it's way too complicated for even a rule of thumb....

Long Wheel base = more stability, less agility
Short Wheel base = opposite

High center of gravity = more clearance, more rollover risk, worse handling
Lower center of gravity = opposite

50:50 is the 'perfect' weight distribution per BMW

40:60 front to rear is the ideal one for racing

To be honest, there are a ton of these general car knowledge facts. There will never be a gold rule of thumb or just one rule because it all depends on the type of vehicle you have and what your intended purpose is.

Wide tires in the dry...GOOD

Wide tires on slush and heavy rain? Not good

narrow tires in gravel and snow? Good!

wide tires on gravel and snow? less good!

For turn response:

Low car...good

Tall car, truck...bad

Long wheelbase...ok

Short wheelbase...better

Tall+short wheelbase...BAD

long metered suspension travel...good...

short stiff suspension on shitty roads? not good.

riding on bumpstops...BAD

Wallowing around on a tall vehicle with a long travel suspension, wide wheels with a short wheelbase?...BAD

NOW

wide tires with a short wheelbase, moderate suspension travel, low car in dry? FUN

wide tires with a short wheelbase moderate suspension travel with a low car in the snow? Less fun.

I think you get the idea. It's common sense. DUH

I get the qualitative aspects. I guess I'm looking for a number beyond the qualitative understandings like shorter is better, wider is better, etc.

If BMW M5 and Toyota 4Runner are respected in their categories, maybe I should just assume that as an baseline aspect ratio?

The physics of racing may help with some of your question

http://phors.locost7.info/file...hysics_of_Racing.pdf

quote:

Originally posted by konata88:
If BMW M5 and Toyota 4Runner are respected in their categories, maybe I should just assume that as an baseline aspect ratio?

I don't think the 4Runner would be a baseline in the small SUV class when it comes to "emergency evasive maneuvers". A better example might be one of the Mazda SUVs

With the M5, you have to factor in the factory set negative camber of it's wheels/tires

I'm clearly being too simple minded about this. I guess there is no answer. The genesis of the question was after watching some videos of Subaru vs VW and others in slalom type testing. Mostly on the supposed merits of AWD. But even the Subaru was not impressive to me. The demo of KDSS was interesting and on topic.

So I came to wonder if we could just magically change the aspect ratio of the Subaru, what ratio would be optimal? Change nothing else about the car other than wheelbase and or track. Or hold one fixed and vary the other. What combo would be best?

I understand that many variables are in play here. Maybe they can't be ignored. But again, what if he held all other factors constant?

KDSS is a cool system that Toyota uses in some of their off road offerings. Basically it adjusts the stiffness of the sway bars. It can go from stiff for on-road driving to disconnected in its most extreme off road setting. It allows more wheel travel and articulation in off road driving conditions and flatter cornering in on road conditions.

It's really impossible to even give general guidelines if you don't have a specific purpose for the vehicle. Taking your examples of the 4 runner and M5, the stock 4 runner tires are going to be terrible for off roading, and good grippy off road tires won't grip or handle as well as the stockers on the road.

The stock tires on the M5 would need to be replaced with some wide slick racing tires to get the best performance on a race track. Those tires would be dangerous on the street. If you wanted to drag race the car, the best tires would be skinny and light up front, with huge, meaty slicks on the back that can compress for a better contact patch when you launch. Those tires would also be terrible on the street.

Good snow and rain tires are going to be narrow with lots of siping that can cut through and make contact with the pavement, and good mud tires are going to be wide with massive lugs that will clear mud well and stand up to spinning through mud and rocks.

quote:

Originally posted by 4MUL8R:
http://training.sae.org/seminars/c0414/


http://training.sae.org/eseminars/pd130702on/


There is a textbook on this site that answers your questions.

The subject is complex. A parametric view of factors is possible but challenging.

I look forward to reading this. I'm sure center of gravity/weight height and suspension would play a huge part.

good lord what a question OP. There are WAY more factors than that involved (main one being the person behind the WHEEL). This is an odd question, and never ever heard anyone talk about "car aspect ratio"

Do you have another question/issue maybe that you're trying to get to the bottom of? Tesla is the answer for you, if you want to avoid accidents.

I think ya'll are overthinking this

I know that there are a lot of factors and this can get technically very deep. On the other hand, I think most of you can generally take a look at a car/truck and think -- hmmm, that looks narrower than it probably should be for that height. Or, wow, that's car has a relatively wide track, must be handle curves pretty well.

Let's say I'm thinking about buying a Suzami model that kinda of looks like a Jeep. Aspect ratio looks about normal for cars. But then the COM is considerably higher. So, they want it to handle and be agile like cars but have the breakover similar to a Jeep. Ok, how did they compensate? Wider tires with rim offset outside the axle? Stiffer suspension? Anti-sway? KDSS like suspension? Nothing, hmmmm, maybe it's prone to being less stable?

Or if I'm looking at a 4Runner and trying to figure out is KDSS useful for highway driving or just a gimmick there -- it's really just for rock climbing despite the demo's. Is the 4Runner already compensating for height with a wider track and lower aspect ratio? Or might it actually provide good value and augment stability?

I'm not making purchase decisions. This is just scientific, conceptual curiosity on what is optimal and then what tradeoffs are made for different purposes and what other compensations are created?