The car wash undercarriage usually does a better job. Really hard to reach into the middle of the undercarriage with a power washer. The car wash I go to has spray nozzles directly underneath the vehicle you drive over.
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I have 32’ of extensions with 120 degree bend to nozzle and 8 nozzle types with various spray patterns. Getting under the middle of the car will not be a problem 
the convenience factor is the biggest driver for me. I maybe go through car wash once in the spring. After that it’s nature or at home if I’m motivated enough…
You might want to look into something like this…
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When I was stationed in Italy the early '80s, if we were shipping a vehicle back to the states, we had to provide proof that the underside of the vehicle had been cleaned out of all insect hiding dirt and debris… The Post Service Station has a stall set up specifically for this. Large diameter water hose hooked up to a pressure pump (their version of a “pressure washer” and the attendant dressed up like the “Gordon’s Fisherman” who went under the car and blasted out every nook and cranny…
Now a days, modern vehicles have all sorts of smooth underbody panels, also known as underbody shields or trays, on the bottom of vehicles to improve gas mileage. These panels reduce aerodynamic drag and turbulence underneath the car.
But they also cover all those normally exposed components: the lower engine bay, the unibody components, the suspensions parts, etc… making your idea of a standard pressure washers near useless as it sprays forward and not directly up into the bottom of the vehicle…
Is it necessary to clean above the underbody/splash shields? They protect the body from road splash, I usually find dust up there.
BTW; I believe he was planning to clean the frame under a truck.
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Every vehicle is different, I only have recent firsthand experience with underbody shields on my 2019 Toyota and my 2020 Honda Fit, since both cars have the underbody Shields.
And if you are not shooting the water upwards on these vehicles, you will not be able to blast clean water up into the weep holes and various openings to clean out what is covered…
The Toyota and its undercarriage Shields and Trays…
he Honda and its undercarriage Shield…
And there is my 2001 Dodge Ram, 2500, Diesel, 4x4, with its body on frame with it 20-inches of ground clearance makes it a snap to clean… (Does “clean as a whistle” come to mind – remember, it’s 25-years old…)
It’s already been used to spray off the underside, quite effectively. They do sell wand ends with wheels and nozzles specifically for this purpose but I am too frugal to buy something like that. I would fabricate my own before buying such a simple tool. But what I have now works fine.
Yeah, I’m talking about rinsing the inside of a boxed frame on a full sized truck. Regardless, my truck is a real work truck and doesn’t have anything besides a small skid plate for the engine/tranny.
The salt, sand etc still finds ways up into every orifice or pocket that exists under my vehicles. Pull down an underbody panel and there is sand on top. Frame rails are constantly sprayed with water and debris from the road. It makes it’s way into the frame. I bought a farm truck once, came from Iowa. Had straw remnants throughout the bed and interior. Dust everywhere. When I opened the door panel to fix a window, I found about 4" of accumulated dirt plugging the bottom of the door. Dust, road spray always seems to find a way to get inside of any area it is around…
Thanks for that! Looks like it could work quite well. Got me thinking of making a metal version of it so it can take the pressure washer directly versus a hose. Well water pressure is OK but cycles up and down with the pump. Pressure washer is fairly consistent regardless of the supply changes.
My initial thought was to make something similar to what they use for sewer drains. A buddy worked for a company that cleans city sewer pipes. They have a nozzle on a hose with jets facing backwards. The water pressure propels the nozzle forward into the pipe while the hose uncoils. Then they jack up the pressure even further and draw the hose back toward the reel. This forces all the debris backwards in the pipe where they vacuum it up. Something like that I can push up the frame tube and draw it backward, flushing the boxed frame tube to the drain holes…
Funny story- I guess the typical rookie mistake in the sewer cleaning business is when you’re done, forgetting to unplug the downstream pipe before opening the upstream plug. Then, “someone” has to go in to retrieve the downstream plug as the junction reservoir quickly fills up. That is a mistake you likely make only once…
That is my motivation for using a car wash, though the blow dryer is inadequate. I do dry it where the vacuums are located.
Dec-Mar when the weather is nice, I occasionally use my power washer at home.
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Yeah, maybe some how attaching some 3/8" aluminum rolled line to the pressure washer, hard part is getting it through an access hole in the side of the frame and being about to push it up in there, has to be hard enough so that you aren’t trying to shove a wet noodle up a Tigers ■■■ (backside), but soft enough to be pushed through the bend/entrance hole… Just hope it doesn’t get caught on any bolts and whatnots…
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Good thoughts.
I have about 150ft of very flexible, reinforced hose that I already have to reach 3 stories up on the house. Nothing like that garbage hose that comes with the machine that is so stiff it hardly wraps up. It does get fairly stiff when water pressure is applied but still can form it. To your point about getting it snagged, I’m thinking of a nozzle shaped like a bullet with jet holes facing backwards out the sides but smooth along the entire surface so it doesn’t have any protrusions to get caught on anything. Should be a fun machining exercise…
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I hope you keep us up to date with pictures (that don’t disappear like your "I spy with my little eye thread pics did) or at the very least PM me with it…
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Possible indication you need a new pressure tank, had that issue, new pressure tank fixed it.