Maybe not hard for some non-US spec'd 200s, but the US ones have a subwoofer in that location. So, again, US owners would have to sacrifice that subwoofer to install in the passenger quarter panel.
Locating ARB compressor in the rear (and other rabbit holes related to on board air)
- Thread starter kcjaz
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Maybe not hard for some non-US spec'd 200s, but the US ones have a subwoofer in that location. So, again, US owners would have to sacrifice that subwoofer to install in the passenger quarter panel.
bloc
SILVER Star
The difference in TPMS reading can be understood by looking into absolute vs gauge pressure concept. Keep in mind the TPMS transmitters live completely inside the pressurized tire, whereas a mechanical gauge is surrounded by atmosphere that is significantly lower pressure when in a place like Ouray.
Some digital gauges have pressure cells that operate closer to the way our transmitters do, but technically it is the pressure differential between the inside of the tire and atmosphere that determines how the tire behaves. So really, pressure set with a mechanical gauge is actually a better way to do it.
The good thing is we’re only talking a few psi difference so this probably won’t lead to tire damage or handling issues.
Some digital gauges have pressure cells that operate closer to the way our transmitters do, but technically it is the pressure differential between the inside of the tire and atmosphere that determines how the tire behaves. So really, pressure set with a mechanical gauge is actually a better way to do it.
The good thing is we’re only talking a few psi difference so this probably won’t lead to tire damage or handling issues.
kcjaz
SILVER Star
- Thread starter
- #23
I understand the difference between absolute and gauge pressure and that atmospheric pressure decreases with altitude (Pa = Pg + Patm). What I was underappreciating is how much the atmosheric pressure decreases at 13,000 ft relative to sea level or even KC's 900' . At sea level, Patm = 14.7 psia and at 13,000' its 9 psia. If you fill a tire to 40 psig at sea level and then drive to 13,000' and measure the pressure with a tire gauge, it will read 45.7 psig because of the 5.7 psi reduction in atmospheric pressure. That is a lot for anal people like me.
Still, the hand held gauge and the TPMS have the same atmospheric pressure acting on them when you take a reading so I don't understand why the difference between the two would increase at altitude. Unless... the TPMs sensor/transmitter was an absolute pressure transducer that used a hard wired or constant value of 14.7 to correct to gauge pressure.
I would have not thought that TPMS sensors were absolute pressure transducers but Googling it, it appears that at least some (maybe all?) are indeed absolute transducers/transmitters. The fact that my dash TPMS readings are closer to my hand held gauge nearer sea level seems to suggest that there is a constant value being used to convert to gauge pressure. That seems like an odd engineering decision to me except that the real purpose of TPMS is really just meant to alert the driver of an unsafe tire inflation level, not really to provide precise tire pressure measurement. Most vehicles don't really hang out at 13,000 feet either and 5 psi over or under inflated isn't a safety concern. Absolute pressure transducers must be cheaper or smaller or something and no one wanted to add the cost of a barometer and the programing to correct to an accurate gauge reading.
RET2
SILVER Star
Cost, cost? I think the nail may have been hit on the head. Why would any profit making company undertake such an effort, well perhaps Bentley.
bloc
SILVER Star
That’s the thing. The TPMS sender lives totally within the tire so there is no atmospheric pressure reference.. it can’t really do gauge pressure, at least not like a gauge located outside the tire in atmosphere measuring the pressure from the valve stem can.
Neat. Where did you route the hose from front to back?
kcjaz
SILVER Star
- Thread starter
- #27
when cost is not considered, its not called engineering, its called science....
Yes, that is why I believe TPMS sensors have to be absolute pressure transducers. A "normal" gauge pressure would read zero inside a tire. So, in this case, it is not really a cost thing. I'm sure absolute transducers are more expensive as they have to have a vacuum sealed chamber on one side of the diaphragm. TPMS needs to use absolute transducers to work.
So, for the majority of people whose eyes roll to the backs of their heads during discussions like this, the practical thing to get out of this is that at elevation, your TPMS readings are not accurate and they will read low (by almost 6 psi @13000') relative to the real "gauge" pressure. This does not mean that as you drive up the mountain your TPMS reading will steadily drop. In theory, the TPMS reading would stay constant. It is the actual gauge pressure that would increase if you stopped to measure it. So, normal people never notice or care.
A wise test engineer once told me, while we were discussing redundant instrument readings, "A man with a watch knows what time it is. A man with two watches is confused."
kcjaz
SILVER Star
- Thread starter
- #28
Thanks! Good info. I'll have to read through your build thread closer, seems like a lot of cool things. What's funny is that I initially missed your reply because I got caught up in my own hi-jack of my own compressor location thread, going down the TPMS rabbit hole. Several good concepts here and I'm going to grab a bit from each. Ordered the ARB twin kit from Slee and am going to put the compressor under the hood using the Slee bracket. From there I plan on making a 4 tire inflation system that is permanently tubed up with pig tails or just quick connects at the corners.
As for TPMS accuracy, I haven't done enough digging to find an actual spec but some Google hits seems to say manufactures claim +/- 1 psi. While that is probably adequate, a good hand held gauge should be accurate to 1% of the reading which is better than 1 psi. The thing is that there is more going on that the accuracy of the TPMS devise going on.
Sounds like you intend to mount under hood now, so no big deal, but noticed you were considering using the trailer plug to power the compressor. An ARB twin requires 25-50amps at 12v. You'll need a bigger wire than the trailer plug can provide if you put the compressor in the rear of the truck.
RET2
SILVER Star
Just a thought, while the idea of filling all 4 tires at once seems to be time saving I would offer that perhaps it isn't a real time saver not to mention deploying and re-stowing 4 air lines. Your compressor is only going to provide X amount of CFM. A single connection in front and back may prove to be more efficient in the long run. It's only 8 feet around to the other side of vehicle to switch fill sites. Beauty of this endeavor is one size doesn't fit all and some of us have the need for a "project" to fill the primal drive and it seems like 200 owners aren't satiated by the old standby the fire thing. Ha ha
RobW0
GOLD Star
There are some out there that think the reason for early compressor failure is because of back pressure on the compressor when only filling one tire at a time. It is felt that the limiting factor is the tire valve. It can only take a small potion of what a dual compressor can deliver. By filling four tires at once you reduce the back pressure on the compressor and extend its life and also fill four tires faster. Not sure if this is valid or not, but it is an interesting theory.
RET2
SILVER Star
Well it is Friday so I'll respond. Filling all 4 at once being faster is IMHO bunk. You need a certain volume of air to raise pressure to"X" from"Y" and the compressor provides a certain CFM at a specified pressure. Most compressors cycle off at approximately 100 to 110 psi so how could running it against 40 or so PSI be harmful unless you run it for hours on end. Oh and don't even think of blaming the poor little Schrader Valve, you know the $0.75 cent piece that supports our gazillion dollar rigs. All of this said in a non confrontational way of course. My friend also wants to know if the back pressure theorists think the world is flat.
RobW0
GOLD Star
This said in a non confrontational way, I guess you have it all figured out.
To some degree you make a good point, but all of the action of filling up 4 tires isn't just in air moving from compressor to tires. There is connecting hoses, moving hoses, checking pressure. All of that stuff is simplified by doing 4 tires at once.
On the air moving front, I don't know for sure that the Schrader valve is the limiting factor of air movement in the system, but it seems a likely culprit. So pushing air against 4 valves at once vs only 1 at a time would allow more air to pass over time. Of course, I'll argue against myself here, and the counterpoint being that my previous statement would be true if the compressor provided a fixed pressure (like a regulated supply on a tanked compressor), but since the compressors we are talking about are generally providing unregulated air, the pressure will increase, therefore increasing the flow through the valve. So it may all be moot.
As a data point, I can drop drop air from my tires faster than I can put in with my 4-flator set up and a Viair 400c. 5psi/min drop and 1.8psi/min increase, so doesn't seem like the Schrader valve is limiting my setup, but that is with 4 valves. I'd have to test to see what pumping up one tire looked like with my compressor.
Last thought, my compressor is rated differently at different working pressures. I may be misinterpreting this, but if I'm running unregulated air supply from my compressor against one Schrader valve, I would assume that the system between the compressor and the Schrader valve would be greater than the 20-40psi in the tire until it equalized after the compressor shut off. I don't have any real world measurements to verify this though. If it is true though, then pushing against 4 valves instead of 1 would cause my working pressure to be lower, therefore my CFM would be greater and the work my compressor was doing would be lower.
PSI | CFM | A | BAR | LPM | A |
|---|---|---|---|---|---|
12-VOLT COMPRESSOR | |||||
0 | 2.62 | 16 | 0 | 74.0 | 16 |
10 | 2.39 | 18 | 1.0 | 65.0 | 20 |
20 | 2.21 | 20 | 2.0 | 60.0 | 23 |
30 | 2.11 | 22 | 3.0 | 56.0 | 25 |
40 | 2.01 | 24 | 4.0 | 53.0 | 27 |
50 | 1.93 | 25 | 5.0 | 49.0 | 28 |
60 | 1.84 | 26 | 6.0 | 46.0 | 28 |
70 | 1.73 | 27 | 7.0 | 42.0 | 28 |
80 | 1.65 | 27 | 8.0 | 38.0 | 28 |
90 | 1.52 | 27 | 9.0 | 35.0 | 27 |
100 | 1.45 | 27 | 10.0 | 31.0 | 27 |
110 | 1.35 | 27 | | | |
120 | 1.26 | 27 | | | |
130 | 1.19 | 27 | | | |
140 | 1.08 | 27 | | | |
150 | 1.01 | 26 |
All of the air flow talk is fairly academic in my mind. The biggest bonus of doing 4 tires at once is how much less cumbersome it is than to move around the truck and fill each tire. It takes me less than a minute to get the hoses connected, then all I have to do is turn it on and wait 10-12 min and I'm done. Another 1-2 min to wrap everything up and put it away.
1world1love
SILVER Star
I can say anecdotally that for me, at least, there is not a huge time savings in doing all 4 at once, but I damn sure enjoy being able to kick back and drink a beer for the 15 minutes it takes to go from 18 to 35 PSI. And, having my quick connect behind the tailgate works out well because that is where all my gear (and beer) is so I would need to open it up anyways. With the 4-way inflator, I can pop on the quick chucks on all 4 in about a minute, grab a beer from the fridge, sit on the tailgate, and stay out of the rain while the tires fill. The Morflate has the extra benefit of maintaining the same pressure across all four tires. It also works well for deflating for the same reason.
laserturbo91
GOLD Star
I like how you have it in the cover on the tailgate!
1world1love
SILVER Star
Thanks. I am pretty happy with it as well. I cannot take credit for the idea though. I lifted that and the power steering relo from @TeCKis300 .
For what it's worth, on my common manifold setup, I've found the following:
Deflate from 41psig to 20psig:
Takes me about 30seconds a wheel to connect my hose whips (includes retrieving the whips from the hatch)
Deflating takes just a touch over 60seconds.
Call it trail ready in 5 minutes.
Inflating from 20 to 41psig (ARB Single):
On average takes 11minutes, a bit less time in Denver, a bit more time at higher elevations.
While overkill, I have four 3/8" I.D. lines to each wheel. If I rework the system in the future I'll likely run 1/4" I.D. tubing.
Deflate from 41psig to 20psig:
Takes me about 30seconds a wheel to connect my hose whips (includes retrieving the whips from the hatch)
Deflating takes just a touch over 60seconds.
Call it trail ready in 5 minutes.
Inflating from 20 to 41psig (ARB Single):
On average takes 11minutes, a bit less time in Denver, a bit more time at higher elevations.
While overkill, I have four 3/8" I.D. lines to each wheel. If I rework the system in the future I'll likely run 1/4" I.D. tubing.
bloc
SILVER Star
Doesn’t the compressor sit snd cycle against its pressure limit switch in applications with air lockers? Waaayyy less duty cycle then.. but much higher pressure than tire filling.
Having thought about this only a small amount, I am inclined to believe it is more the heat from under hood installs that kills them. But this is just my gut. And why I have a process to try and cool the compressor down a bit before filling tires.
Also a single air line takes up enough room in my camping/roadtrip setup.. I wouldn’t want to add so much extra hose to do the same job.
Having thought about this only a small amount, I am inclined to believe it is more the heat from under hood installs that kills them. But this is just my gut. And why I have a process to try and cool the compressor down a bit before filling tires.
Also a single air line takes up enough room in my camping/roadtrip setup.. I wouldn’t want to add so much extra hose to do the same job.
