Random Aerodynamic Oddity of the Day Thread

[kach22i]

Originally Posted by Bicycle Bob

.......ice....snow ....I've never needed 4WD, nor a tow.

Same here, but it's like other luxuries that you can live without, like heat and A/C. Once you have AWD/4WD you tend to use it and enjoy the illusion of invincibility.

I say illusion because after a big ice storm the vehicles in the ditches are usually the yahoos that thought they were invincible in their 4WD full sized pickup trucks.

[RedDevil]

4WD cars roll more smoothly over speed bumps than 2WD cars do. But in winter I'd rather have 2WD with snow tries than 4WD with all season tires. And you lose about 10% in FE.

[Bicycle Bob]

Originally Posted by RedDevil

4WD cars roll more smoothly over speed bumps than 2WD cars do. But in winter I'd rather have 2WD with snow tries than 4WD with all season tires. And you lose about 10% in FE.

What is your sampling method for those speed bumps? I would expect that more unsprung weight would degrade the ride. My velomobile has 1 WD, very low unsprung weight, and goes over speed bumps like a hydraulic Citroen.

[RedDevil]

The wheels going over a speed bump travel a greater distance than the wheels on a flat surface.
When they are not driven they temporarily speed up, forced forward by the mass of the car. But the driven wheels don't speed up much as they are connected to the engine, which tries to maintain its revs and has a large rotational mass anyway - so the car has to slow down a bit; especially on the upslope.

With 4WD this effect is spread over 2 axles, so when the front wheels hit the bump and slow the car the rear wheels resist the slowing down and force the car over, and likewise when the rear wheels climb the bump the front wheels will pull it over.

There are formidable speed bumps where my parents live, and when my dad bought a Subaru the way it kept going at steady speed over those speed bumps was the thing that impressed me most.

With a velomobile there is very little rotational mass in the wheels, even the driven one. If it also has smooth wheel travel it could float over speed bumps as if they were hardly there.

[Bicycle Bob]

^^ So, a sample of one. That's not too convincing, especially without trying different loadings fore and aft, which affects pitching frequencies. To get over a bump at speed, a wheel has to store a lot of energy in its spring, which can only come from momentum of the vehicle. Low unsprung weight lets you recover more of that energy on the back of the bump. I have not studied the rolling distance factors, because nobody else has written about them. I suspect that compliance built into the system, and the differential between the axles will minimize that quite well.

[RedDevil]

So what? I know what I feel. I understand how it works.
Don't confuse statistics with proof.
A sample of 1 is adequate to prove a point, when it is not about a single value but a functional situation.

But I've driven many other 4x4's since and notice the effect every time, and I can tell whether a car has FWD, RWD or 4WD by just driving it over a bump.
And if you recognize how it works you can do that too.

It has nothing to do with unsprung weight.
It is not about how much the car moves up and down.

it is about how much or little the car jerks backward and forward when going over a bump.
2WD: - a big jerk when the driven wheels go over the bump, none when the others go over the bump
4WD - two smaller jerks for each set of wheels.

You'd hear the revs change when the driven wheels go over a bump too, especially with a stick shift.

[Bicycle Bob]

Can you find any published author who agrees with you?

[RedDevil]

Hey, I don't have to convince myself of what I know and understand.
Feel free to believe it is pure BS if that makes you happier, I don't care.

[Bicycle Bob]

"Knowing and understanding" can pave over huge logical gaps. There were still tenured professors teaching about ancient animal migrations via "land bridges" when all the younger ones had adopted continental drift.

[cRiPpLe_rOoStEr]

Originally Posted by RedDevil

I can tell whether a car has FWD, RWD or 4WD by just driving it over a bump

I never even lurked about it, yet this makes sense. Well, most of my driving experience has been on FWD anyway.