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Two automotive theory questions:

  1. Does a stiffer (less compliant) suspension burn more gas?
    A year and a half ago, I installed a Bilstein B12 kit on my 2007 Corolla. The kit lowers the car by an inch with a set of Eibach springs with Bilstein B8 dampers. I seem to notice that on broken pavement, I have to push the gas more to overcome the potholes where as previously, the car was more likely to ‘glide’ over.

The way I explain this is to say that the forward momentum of the vehicle is interrupted by the pothole pushing the vehicle VERTICALLY UPWARDS. Thus this momentum (I’m not sure I’m using the correct physics terms here) which would have pushed the car horizontally to my destination, is being expended by lifting all that car weight upwards.

A more compliant suspension is able to absorb those bumps and carryon forward, which is more
fuel efficient (it’s not wasting energy with vertical momentum)

In the real world, this may end up being negligible but in theory this appears to be correct, right?

  1. For tiny cars, think: Smart Fortwo or Toyota’s discontinued IQ (or even a Honda Fit)
    with short wheelbases, the question is: does increasing the vehicle’s track via aftermarket
    wheels adversely affect handling (all else being equal)?

As an example, let’s say you put after market wheels on the Honda Fit, and since the aftermarket wheels are wider and have a lower offset than stock (stock offset being 53mm) this pushes out the wheels by 1.5 inches thus increasing the track by 3 inches total. All else being equal, this may adversely affect handling.

Another way to put it is that there may be an ‘ideal’ wheelbase to track ratio for best vehicle handling. Do you think this is correct?

No …

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  1. No, a stiffer suspension does not use more gas. An argument could be made that it uses less gas based on your momentum reasoning. What you don’t know is that the suspension is designed to allow the wheel to move back as well as up when hitting a bump. That absorbs forward momentum somewhat.

  2. Yes, increasing track with ill fitting wheels will spoil the handling. If the offset is not maintained, the center on the wheel moves away from the steering axis and causes bump steer due to increased scrub radius. The wider track will help handling by reducing the amount of weight transfer while turning if the scrub radius is maintained. There is no ideal track to wheelbase ratio, however. The amount and location of the mass in the car is far more important than the wheelbase itself.

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The way to understanding your first question is to think in terms of energy.
An ideal spring can absorb (potential) energy and later release it.

Suppose your car had no shock/strut dampers.
When it rolls over imperfections in the road some of the energy of forward motion is converted into energy of vertical motion, moving the wheels, flexing the springs, moving the the mass of the car.
At the back side of the bump or dip some of that energy will be put back into forward motion, but some will be repeatedly shuttled back and forth between the springs and the car’s mass, making it bounce.
Bouncing is bad.

Now add the dampers.
Dampers convert energy into heat, by way of friction.
Every time energy is transferred from spring to car mass the damper “steals” some away.
Dampers also remove some of the energy that could be returned to forward motion when the wheel rebounds from a bump or dip.

So yes, a highly damped suspension can increase effective rolling resistance on a “rough” road, but the effect is minuscule.

I will add that the air in the tire acts like a spring, while the rubber in the tire shell acts mostly like a damper.
There’s a school of thought with bicyclists that a highly inflated bike tire can increase rolling resistance on rough roads.
It transfers more of the vertical motion to the riders body, which acts like a damper, absorbing energy.
Whereas a moderately inflated tire will act more as a spring, absorbing the energy of a bump then later returning it to forward motion on the rebound.

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For #1, if you’re noticing lower mpgs, it’s because you’re driving your car more aggressively, now that it handles better.

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I would change that “highly inflated” to “over inflated” as it is clear from the graphs that higher is better…but only to a point.

Interesting article!

Fun trivia, on the Smart Fortwo, the wheels are a crumple zone. They’re designed to be sacrificial in an impact to absorb energy. Due to this, some tire shops will not work on a Fortwo that has aftermarket wheels.

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Soft vs stiff springs
I would think that the jolt of a pothole has different effects. For soft springs, the jolt sends the wheel upwards more readily than a stiff spring. The driver feels less jolt because the soft tire/spring/shock spreads the jolt out over a longer time span. In addition, I would expect the softer combo to respond more quickly to the jolt.
Also remember that a shock has different actions when compressed vs extended. It compresses easily, and extends with difficulty…with the idea to first respond to the jolt, and then to stop the bouncing.
Finally, consider the case of the ultimate stiffness: a rigid wheel/tire connected rigidly to the rest of the car. The horizontal component of the jolt will slow the car, and the vertical component will lift the car… neither will help the gas mileage.

Second question: is there an “ideal” wheelbase to track ratio?

First thought is: what is your definition of ideal? Best for corners? Best for top speed? Best for slalom?

My guess is that a manufacture will define this as something like “best for the average driver” (unless they are designing an Indy car). Then, the car would be set up and track tested for that goal, and presumably a drastic change in track would detract from that design.

And then there’s to question of “unsprung weight”, if you increase or decrease the total mass of the brake, wheel, combination it will change your handling.

For example, the Jag XKE came with wire wheels and the rear brakes mounted just inches away from the differential simply to reduce the unsprung weight and maximize handling. (And yeah, they were a PITA to replace the pads)

They’ll design it to be whatever they think will sell the most cars while staying within safety laws and liability protection.

That’s not necessarily going to be designed for an “average” driver, which these days seems to be a clueless dope more interested in playing with their smartphone than looking at the road. You won’t catch Koenigsegg designing the ideal suspension for that person. :wink:

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Go Karts are almost square and they handle very well. Kart suspensions are the tires themselves and a somewhat flexible frame.

Indy cars are not, F1 cars are not. Wheelbase is 3 or 4 times the track and more. Short wheelbase cars turn are better on very tight racecourses. Longer wheelbases are used for Indy, LeMans or Spa. Makes the cars more stable at high speeds. The original Mini spanked larger cars on tight rally courses.

But all these vehicles need to package all the things that make them run plus the driver. Length helps.

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A machinist at a company I worked for in the '80s, he an a buddy shortened a VW Beetle about a foot and eliminated the rear seat area.
They nicknamed it the “Twirling Dervish” because of its handling characteristic. :rofl:

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Early Subarus had inboard brakes in front.