Installing FOBO Tire Pressure Monitors

RV-12 owners (and probably owners of most Van’s aircraft) know once wheel pants are installed, it becomes a real pain to check tire pressures. The wheel pants cover the majority of the tires, leaving absolutely no access to the air valves … thus requiring the removal of the wheel pants just to check tire pressures. Sure, some have drilled holes in the wheel pants for access … but then trying to perfectly align the air valve stem with the hole is a pain … plus, it makes it very difficult to use the protective valve stem caps ... so the caps are typically left off which allows crud to get into the valve stem. Hindsight being 20/20, I should have taken a little more time with the wheel pants and installed a hinged access door with a Camlock fastener or two.


One way to get around removing the wheel pants just to check tire pressures, is by installing remote tire pressure monitors … akin to the tire pressure monitors used in most modern cars. I have read positive things about remote tire pressure monitors on the forums so decided to give them a go. FOBO makes just such a product that transmits via Bluetooth to their free App … FOBO’s free App supports both Android and Apple products. FOBO manufactures tire pressor monitors for cars, bikes, motorcycles and trikes. I purchased a trike kit from a company in California called SlingMods which sells the latest sensor 2 version.

The FOBO trike tire pressure monitor kit with the latest version 2 sensors purchased from SlingMods.

The FOBO monitoring system for trikes includes three sensors which screw onto the tire’s valve stem. Included are three lock nuts that can (if desired) be screwed onto the valve stem and cinched up to the back of the sensors to lock them in place … a convenient special molded wrench is supplied for tightening the lock nut up against the backside of the sensors. Also included in the kit are three backup batteries (standard CR1632 coin/button batteries) and three short valve stems for use with tubeless tires.

As can be seen in this photo, the FOBO sensors are actually quite small, about the size of a penny.

The FOBO sensors themselves are quite small (as can be seen in the above photo) and only weigh 7.6 grams … probably not  enough weight to worry about rebalancing the wheel, but I'll be sure to be on the lookout for take off or landing vibrations. The life span of the replaceable CR1632 battery is 1 year. Battery replacement is easy … the top screws off the sensor and the old battery is slid out of a holder and new battery slid in, piece of cake. The sensors support both Bluetooth versions 4 & 5, which is how they report to the free FOBO App. The free FOBO App supports Apple’s iOS 9.3 operating system and Android version 5.0 or later. I installed the FOBO App on my smart phone (which is running Android version 10) without running into any issues. The FOBO App is full featured allowing the user to select wheel configurations, set pressure high and low alarm points, altitude compensation, etc. and share the settings with another device or user. As a deterrent for theft, the FOBO App registers the sensors with FOBO … so if the sensors are stolen, they will not work on another vehicle.


First time setup is easy, after the desired wheel configuration is selected, the App will tell you when to install each sensor so it can be paired with the FOBO App and registered online with FOBO. This part of the process takes a few moments as the App scans, pairs, then registers the sensor. After the sensors are paired, they immediately begin updating the FOBO App with each tire’s pressure reading,  current temperature and sensor battery status. As previously mentioned above, entering the settings page will allow editing the settings for optimum tire pressure and to set high and low pressure alarm points.

Photo of a FOBO tire pressure monitoring sensor installed on one of the main gear’s wheels.

There is plenty of clearance to install the sensors on the main landing gear’s wheel … however, I did discover a clearance issue when installing the FOBO sensor on the nose wheel. The DOG Aviation RV-12 has the new thicker and much stronger WD-01230-1 nose wheel fork installed. The new nose wheel fork also requires a new mounting bracket for the nose wheel pant. The new mounting method involves riveting a mounting bracket directly onto the nose wheel fork. The clearance issue I discovered involved the shop heads of the two aftmost rivets used to attach the U-00006E-L-1 mounting bracket onto the wheel fork. The FOBO sensor cleared the nose wheel fork just fine, but made contact with the two aforementioned rivets. ( This should not be an issue for RV-12 owners who have not installed the new style nose wheel fork because the rivets that caused me grief are not present on the old style nose wheel fork). I used a file to remove a little material from the shop heads of the offending rivets so a little clearance could be obtained.

After filing down the shop heads of the two rivets a little, the sensor cleared the rivets as can be seen here if looking closely …  but just barely. Clicking on this photo should bring it up to full size making it easier to see.


Not feeling comfortable with so little clearance, I decided to add a 1/16" shim (washer) between the nose wheel fork and the U-01210B-1 axle spacer, essentially moving the wheel 1/16" to the right … so fabricated a 1" washer from .062" aluminum scrap. I remembered when first installing the nose wheel on the new WD-01230-1 fork, the fork pulled in a little as the axle bolt was tightened. So I felt pretty confident there would be room for the 1/16" washer to slip in between the U-01210B-1 spacer and the WD-01230-1 wheel fork without much of a fight. Sure enough, as the axle bolt was loosened, a gap appeared between the U-01210B-1 spacer and the WD-01230-1 wheel fork … so I inserted my newly minted washer in the gap and reinstalled the axle bolt. After tightening the axle bolt, there is now acceptable clearance between the FOBO sensor and the rivets. I can now flex the valve stem and clear the rivets …whereas before, any flexing of the valve stem would create interference between the FOBO sensor and the rivets.

Looking very closely, one can see loosening the axle bolt created about a 1/16" gap between the U-01210B-1 spacer and the WD-01230-1 wheel fork … just what I needed, the spacing washer was inserted into that gap.

1"x 1/16" washer fabricated to offset the nose wheel 1/16" to the right.

Photo of the nose wheel assembly with the washer in place. Now the wheel assembly is offset 1/16" to the right creating clearance between the FOBO sensor and the rivets that secure the U-00006E-L-1 mounting bracket.


The FOBO sensors appear to be working nicely and sensitive enough to track pressure differences caused by temperature changes …. in that, reported tire pressures are slightly higher during a hot afternoon compared to the cooler mornings. The FOBO App works great and if selected in the setup menu, can also sound an alarm on the device you are using should any tire pressure go beyond the user assigned normal operating range. Below is a photo showing what the FOBO App display looks like after a front wheel trike configuration was selected, sensors installed and pressure limits configured. If a tire’s pressure is out of range, the FOBO App will display a red background for the offending tire.

Photo of the FOBO App on my smart phone after installing the sensors and configuring desired parameters in the setup menu.


Thus far, I’m very pleased with the FOBO sensors … they appear to be accurate and the FOBO App is easy to install and configure. Time will tell if the sensor battery lasts to the one year point … hopefully it will so it can be routinely changed during the yearly condition inspection. At this point the FOBO sensor system appears to be a good option for those pilots who don’t want to remove the wheel pants just to make a tire pressure check. Moving forward, I’ll be sure to update this post if any issues develop with the FOBO sensors. Oh, at the time of this writing, the FOBO sensors cost around $49 per wheel.

Return from the future: 

The clearance that was created between the FOBO tire pressure sensor and the nose wheel fork proved to be not enough. As previously mentioned above, I had to press quite hard on the sensor to deflect the tire’s valve stem enough to contact the nose wheel fork assembly. After completing some high speed taxi testing and checking it appeared all was well.

Not so! Apparently, the centrifugal force created by the 5” nose wheel instantly ramping up from 0 to 25 or 30 MPH must be very significant. After shooting a couple of touch and go landings, I returned to the hangar and inspected the sensor and found, much to my amazement, there was evidence of contact with the nose wheel fork assembly … so the nose wheel sensor  was removed from the nose wheel.

Guess I will have to check the nose wheel pressure the old way from now on. At least the sensors are working well on the main gear and the main gear wheel fairings are much more time consuming to remove.

Service Bulletin 18-03-06 Carburetor Throttle Return Spring Replacement

While reviewing some DOG Aviation photos for a fellow builder, I ran across some photos regarding replacement of the Rotax 912ULS throttle springs that I forgot to add to the Blog last fall.

Van’s Aircraft issued a service bulletin about two years ago switching to a newly designed throttle return spring for the Rotax 912ULS which will hopefully solve the throttle return spring issues that have been a nuisance for the RV-12 fleet.

Before diving into the latest throttle spring service bulletin (I think there have been at least two prior) … first a little back story regarding the throttle return springs. Rotax has designed the 912ULS engine’s carburetors go to full power in the event of a throttle cable failure. On older RV-12 aircraft the throttle return springs supplied by Rotax for the 912ULS engine were VERY strong … so strong, in fact, the throttle would constantly need to be adjusted and readjusted during flight because the strong springs would cause the throttle lever inside the cockpit to constantly creep towards full power. Another issue plaguing the Rotax 912ULS throttle lever return springs is, over time the throttle return springs were also prone to breaking.

In an effort to eliminate the throttle creep, new weaker springs were developed … but they did not totally solve the issue with throttle creep plus spring breakage remained an issue. While the DOG Aviation RV-12 was under construction, Van’s began supplying a vernier-assist throttle lever manufactured by McFarlane (a nice throttle unit), which became the standard offering. The McFarlane vernier-assist throttle is accompanied by weaker throttle springs supplied by McFarlane …. a step in the right direction, however, throttle return spring breakage remained an issue.

Van’s has now totally redesigned the throttle return spring and made it a helical torsion spring as opposed to the typical stretch spring. From a design aspect, I think this is a much better approach and should totally eliminate throttle lever return spring breakage.

Van’s Aircraft issued service bulletin 18-03-06 which covers removing of the old style throttle return stretch spring from the Rotax 912ULS engine’s carburetors and replacing the springs with the newly designed helical torsion springs. The service bulletin refers the installer of the springs to follow the procedure laid out in Section 50 of the plans. The Van’s part number for the new throttle return spring kit is SPRING-00002-1 2 PACK. That part number will provide two springs, one for the left carburetor and one for the right carburetor. Note: The spring for the left carburetor has an ink marking to denote it from the right spring.  Below is a photo of the old style throttle spring compared to the new style helical torsion spring.

The spring on the left is the old style throttle return spring … the spring on the right is the newly designed helical torsion throttle return spring.


As one can see in the following photo, the standard Rotax 912ULS throttle lever return spring is stretched between a hole in the throttle lever and a bracket attached to the body of the carburetor. I suspect, being stretched between two points and under constant engine vibrations, the throttle return springs are more susceptible to fatigue cracking.

My finger is pointing to the old design throttle return spring. The upper portion of the spring is connected to a hole in the throttle lever and the lower portion of the spring is connected to a hole in a bracket attached to the body of the carburetor.


Instillation of the new throttle return helical torsion springs is quite easy. First the old style throttle return spring is removed from the throttle lever. Then the hex nut and spring washer that secures the throttle lever and throttle stop onto the throttle shaft is removed. Probably unnecessary, but I used a red sharpie pen to mark the position of the throttle lever prior to removing the throttle shaft hex nut. Use caution when removing the throttle lever … I placed a wrench on the throttle shaft and another on the nut then twisted the throttle shaft to make sure the throttle shaft was in the center of its normal range of movement … then proceeded to remove the hex nut. Making sure the throttle shaft is in the center if its range of motion assures that the force applied to remove the nut will not be applied to the stops … possibly bending metal.

This photo shows the red sharpie marks placed on the throttle lever (actually not necessary). At this point, the throttle shaft hex nut and spring washer have been removed from throttle shaft. As a note, the stop lever can be seen quite well in this photo, it sits on the throttle shaft directly behind the throttle lever … it will also be removed from the throttle shaft.


After removing the throttle shaft hex nut, the throttle lever is carefully slid off the throttle shaft. There is no need to loosen or remove the throttle cable to get the throttle lever off the throttle shaft. Behind the throttle lever resides the throttle stop, it also needs to be slid off the throttle shaft as can be seen in the next photo.

Here one can see the throttle lever and throttle stop have been slid off the throttle shaft. Once the throttle stop is removed one can see two Philips screws … my finger is pointing to the upper Philips screw that will capture one end of the throttle return helical torsion spring.


Instillation of the new throttle return spring is quick, simple and easy to accomplish procedure. The new spring slides over the carburetor’s throttle shaft … the inboard end of the spring will sit under the head of the upper Philips screw (the screw I’m pointing to in the above photo) and the outboard end of the spring will rest on the throttle stop. Instillation of the new spring, task wise, is not difficult. That said, however, finding the right tool for the job proved difficult. I tried a couple of varieties of spring tools I had in the shop, but they all seemed to have clearance issues. I did not want to use a small screwdriver to push on the spring (as most probably do) for fear of creating small scratches that, over time and vibrations, may possibly create stress fractures in the spring. After lots of pondering and playing around with various tools an idea occurred to me …. perhaps a piece of waxed string will work to tension the spring. That idea worked like a charm!!! I slid the throttle return spring partway onto the throttle shaft and slid the throttle stop onto the throttle shaft positioning the outboard end of the spring so it is captured by the throttle stop. Next I looped a piece of waxed cord over the inboard end of the spring and slid the assembly further onto the throttle shaft. As the assembly got close to the Philips head screw, I pulled on the waxed cord to tension the spring enough so the inboard portion of the spring could be positioned under the head of the Philips head screw. Worked slick … as documented in the following three photos.

In this photo, one can see how the inboard end of the new throttle return spring will be captured under the head of the upper Philips head screw when the new spring is in its final inboard position.

As one can easily see here, a piece of waxed cord was used to capture the inboard end of the spring so it can be tensioned by pulling on the string. Looking closely one can see how the outboard end of the throttle return spring is captured by the throttle stop. All that is left to do is pull down on the waxed cord so the inboard end of the spring clears the Philips head screw and push the assembly in the remaining 1/8” so the inboard end of the spring sits under the head of the Philips head screw.

This photo shows the final position of the new throttle return spring … the inboard portion of the spring is captured under the head of the upper Philips head screw and the outboard portion of the spring is captured by the throttle stop. Using a waxed cord to tension the spring makes this task truly a piece of cake.


Once the throttle return spring and throttle stop are fully seated on the throttle shaft, the throttle lever is positioned back onto the throttle shaft and the assembly is secured on the throttle shaft by the spring washer and hex nut …the hex nut is tightened to 44 inch pounds. I accomplished that by using a crows foot wrench attached to my torque wrench and holding onto the throttle shaft with another wrench … here again, making sure the throttle shaft was positioned in its center of motion so no force would be applied to the throttle stops.

Completed reassembly of the throttle lever on the throttle shaft. Unfortunately, once in position, the new throttle return springs are hidden from view by the throttle lever.


Service bulletin 18-03-06 is a very easy service bulletin to complete (especially if one uses my trick of using waxed cord to tension the spring) and having helical torsion throttle return springs should put an end to the broken throttle return spring issue.

Completing Service Bulletin 19-03-22 Replacement Of #2 Exhaust Pipe

Last fall during the condition inspection, I completed a few service bulletins and am just now getting around to documenting them on the DOG Aviation blog. From a safety aspect, the most important service bulletin I completed is Service Bulletin 19-03-22 ... which involves replacement of the #2 cylinder’s exhaust pipe due to some of the RV-12 fleet experiencing cracking at one of the welds and in a couple of cases, a  complete separation of the #2 exhaust pipe.


First a little history about the RV-12’s exhaust system. The exhaust system on early RV-12’s placed the muffler very close to the RV-12’s oil cooler which contributes to high oil temperatures … especially for those in really hot climates.  In an effort to move the muffler a little further aft from the oil cooler, Van’s changed the shape of the Rotax 912ULS cylinder’s exhaust pipes (which are custom welded to begin with) allowing the muffler to be positioned a little further aft. The original four exhaust pipes are part numbers EXH-1201 through EXH-1204 and are not affected by the service bulletin. As a side note, on the original RV-12 exhaust system, the muffler’s exit pipe protrudes through the lower cowling at an aft angle.


The DOG Aviation RV-12 received one of the first shipments of the redesigned exhaust system … which has exhaust pipes numbered EX-00015 through EX-00018. One of the identifying features of this exhaust system is the muffler is positioned a little further aft and the muffler’s exit pipe protrudes straight down through the lower cowling. Also of note, the #2 exhaust pipe is positioned close to the lower cowling and one of the springs is very very close to the lower cowling. The cracking #2 exhaust pipe has a part number of EX-00017.


The cracking or complete separation of the EX-00017 #2 exhaust pipe is a serious safety problem which needs to be taken seriously. In addition to the obvious threat of carbon monoxide, the #2 exhaust is so close to the lower cowling that a separated exhaust pipe could have the potential to easily start a fire.

My finger is pointing to the weld on the EX-00017 exhaust pipe that is cracking/separating.

The old EX-00017 #2 exhaust pipe is on the left and the new EX-00017-1 #2 exhaust pipe is on the right. Looking closely, one can see the new EX-00017-1 exhaust pipe has a few obvious differences …. a doubler plate is added over the weld that was cracking for added strength, the overall shape of the pipe is a little different and the welds are larger.


Not wanting to take any chances, decided it best to not rely on inspections and just go ahead and replace the #2 exhaust pipe with the newly revised EX-00017-1 exhaust pipe along with new copper flange nuts and new springs where the #2 exhaust pipe meets the muffler. Instillation of the #2 exhaust was simple and done in a few minutes … replacing the safety wire I use through the springs is another matter.


After installing the new EX-00017-1 #2 exhaust pipe and attaching the lower cowl, I immediately noticed the new shape of the EX-00017-1 exhaust pipe created more clearance between the #2 exhaust pipe and the lower cowling … which I was glad to see. Unfortunately, I did not have my camera with me the day I reinstalled the lower cowling, so did not get a photo of the completed instillation with the lower cowl in place … will have to try to remember to do that and place the photo here.