Bogus Windshield Washer Physics and advice!

In the April 12 2008 show, Tom and Ray gave this advice to someone with weak windshield washer shpritser height. Get a small hose clamp from an aquarium store and use it to squeeze the washer hose to increase pressure and spray height. I’m pretty sure this is flat out WRONG.

Because of the Venturi effect, this has the OPPOSITE effect when other than the end opening is squeezed. Consider stepping on a flowing garden hose.

Why am I the only one seeing this?

I don’t think I’m wrong. I had 1 year of MIT physics just like Tom and Ray!

What does everyone else think. Wolfgang?

I agree with you. It will just restrict the flow some, maybe not even noticeable.

If they need more pressure, get a new / better pump.

But then I “only” went to Colorado Tech, not MIT (thank God).

Fluid Dynamics
_ Law of Continuity–states that the product of the cross-sectional area times the velocity is constant, e.g., cross-sectional area and velocity are inversely related.
_ Basically, this simply means as cross-sectional area increases, the gas velocity decreases, and vice versa.

The Bernoulli Principle & Equation
_ Based upon the law of energy conservation

_ Simply stated, at a constant flow rate, the sum of the potential energy and kinetic energy per unit volume of fluid and lateral wall pressure at one point in the system will equal the sum of these factors anywhere in the system.
_ Remember, we are dealing with incompressible fluids, as well as rigid tubes…there?s more…

If we place a restriction in this tube...note what the restriction...

_ lateral wall pressure decreases
_ central velocity increases
_ central pressure increases
_ PE and KE on both sides of the restriction remain unchanged (Energy Conservation!!)

AHEM: Now that I have everyone’s attention,

The bottom line is that when fluid comes upon a restriction, central fluid stream pressure and velocity increase…then when it reenters the original size channel (tube), central stream velocity and pressure decrease. This must happen to maintain constant energy. Squeezing the stream prior to the jet will not add energy. And only by adding energy, changing the spray pattern, or changing the orafic size can the spray be changed.

One variable is pressure. If the orafice is partially plugged, pressure can build within the tube while the spray is decreased. The additional energy is converted to heat energy. Conversely, unplugging the orafice can increase the spray at teh expense of pressure within the system. Again, energy is consevred.

Oh, yeah, Venturi simply studied Berniuli’s work, caught the “lateral wall pressure decreases” and used that for his principles as applied in carburation. Smart fellas, both of them.

I never went to MIT. I drove by there, wondering about the geniuses inside, but I never went there.

just a small thought,google will not help you when you do not know how to spell,much less you spouting fluid dynamics,JUST NOT FEELING IT,maybe you should have stopped ,and learned spelling,before throwing out,ummm! the bernulli,or burnulllli,(you smell what Im standing in.)orafic size.

have a wonderfull dayyy,dai.

Exactly! The key words from this wiki-like excerpt are:
“Squeezing the stream prior to the jet will not add energy. And only by adding energy, changing the spray pattern, or changing the orafic size can the spray be changed.”

That’s what I am saying. An aquarium hose clamp, by definition, is prior to the jet, cannot add energy, and thus can NOT change the spray height.

The orifice must be changed, but that is NOT what Tom and Ray recommended, of course.

Thanks. Glad you agree. MIT was great, btw. Not as beautiful as Colorado. By far.
It was early 70’s for me and for Tom and Ray, none of whom were physics majors.

Let me get this straight…you’re attacking a tough-in-cheek explanation of the physics because I misspelled the guy’s name??? And doing it with a fully garbled reply???

Let me guess…you didn’t go to MIT either.

By the way, I didn’t get that from Googling. I used a part of a physics lecture I have on file. Sorry to disappoint.

Have a vunderful dai.

Excellent summation. Sometimes I think T&R get too wrapped up in their family history to remember the question.

Thanks, TSM. Your post made it easy.
Anyway, what’s with Wolfgang? Shouldn’t HE weigh in on this? How does one get that to happen? And what about Tom and Ray themselves. I don’t think they’re above an on-the-air reconsideration of the facts, especially if it included a brief Wolfgang session, and an apology to the sensibilities of this old fellow tech alumnus!


Thanks. Who’s Wolfgang? Didn’t he used to play piano?

bear with me…I don’t get out much.

Wolfgang is the Harvard physics professor whose job it is to listen to Car Talk and weigh in on physics-related “differences of opinion”. Last one I heard was the question of equality of a 60 mph wall crash versus two 60 mph cars hitting head on.
He’s nowhere near an MIT physics professor :slight_smile: but he’s ok and quite personable.


Cool. Yeah, I’d like to hear from him too.

Something just occurred to me. Perhaps the fella that originally asked T&R for the advice simply needs a new pump! An aftermarket generic can be bought for about $15 at teh parts store. Washer pumps are just like other impellar pumps; they’re subject to the motors weakening when they get old (read: winding insulation beginning to fail and brushes/commutators wearing out) and to erosion of the impellars.

Probably right. I think the guy might have been ch…, uh financially challenged!

Regarding Wolfgang, I think we’re basically stuck. There is no email for him.
Emails to Tom, Ray, and their staff are all unanswered. Even though that’s justified due to the probable volume and cost, still I don’t see any way to facilitate a Wolfgang intervention. It’s their show, their physics consultant, and if they don’t think it’s important and don’t want to answer emails there’s little anyone can do about it! I KNOW I’m right, though.
I even suggested a Chump the Stumps followup on this, since I’m sure that if the caller did buy the aquarium clamp it would only have made things worse!
Oh well, it’s still a great show and I’m still a big fan of them despite the phlaky phluid physics.

I’m still waiting for Wolfgang on this!
I think the reason we didn’t hear this on the show is because we are right and Tom and Ray simply muffed this one.