Trigger Wheels And Sensors (What worked what did not.)

[Dean924s]

I am currently running a 72-2 3" diameter spur gear that is using a sensor from a RX7 eccentric shaft position sensor.

The good: It runs

The bad it is very sensitive to spacing between the gear and the sensor. To the point that temperature seems to affect it. Cool temps = signal breakdown due to things contracting. Push the sensor close and it hits the wheel when things heat up. The biggest issue there Si that I have it mounted to an aluminum cam gear cover and the coefficient of expansion of aluminum is quite high. Combine this with the tight tolerances required for this sensor to work and problems have become apparent. (backfired so bad that I exploded a cat (not the 4 legged kind)

Things I am considering.

Making the existing gear exactly round by milling it in place.

Installing a larger gear. I have thought of going to a 6" gear. This would make mounting the sensor an issue but not insurmountable.
Now what to use as a gear as I need a 72-2
A larger spur gear. Found on line. IT will need to be machined down (thinner) to reduce the weight

Other possibilities: I was looking at what are called cogs. Like those on your 10 speed bike. The good is that they are light. The bad is that they are thin. The good is that I think the local bike shop will have them

Does anyone know what the smallest size trigger wheel that a stock ford sensor has used? What is the minimum width of the trigger wheel that has been used?

I am also hoping that this could be a place where others could post there success's and things that did not work so well. and most important why.

Anyway more to come as I researched this. I think I need to set up a bench tester for this.

[alfajay]

I am really not an expert on this. But my sense is that the narrower the trigger wheel, the closer the sensor would need to be placed to get a reliable reading. So, if you are having difficulties with your current set-up, then switching the a bicycle sprocket will only aggreviate the problem. My reasoning is that a narrow wheel puts less ferrous material near the sensor, so the sensor needs to be closer to get a good reading. A wider sensor - say the width of the metalic part of your sensor - would possibly be more tolerant of a wider gap.

But, what do the experts say?

[alexander]

hi dean,
just re recap on previous discussions, this is my installation using that sensor:


that trigger wheel is 35mm across ie about 1.5", so there is a benchmark for smallness.
there is a little runout, but <1mm: enough to see as varying signal strength on an oscilloscope
but it works perfectly.

re your installation:
*how much runout do you actually have?
*i dont think you need to go to a bigger wheel; if the <1mm teeth on mine will trigger the VR
there is more than enough metal on your wheel to do the same. further, as you know, a bigger
diameter wheel will give you a problem clearing the extension part of the VR you have.
*a bigger wheel with a bigger problem with thermal expansion

alexander

[Dean924s]

Alexander,

Do you have a photo looking down on your setup? how close is the sensor to the piece of metal that comes through the coil. What is the size of the trigger wheel? what RPMs have you run it up to?

I may have actually damage my sensor. I had it running and pushed on it and it hit the trigger wheel. I am sure that this ground a little of the metal end of that protrudes through the coil. My problems did not start until I did this. One of those things where I wanted to see just how close I really had it. I may go to the other setup I have already made up and compare.

Here are my setups

Working at the moment but I think I am going to swap out the sensor.

In the car and running



The one that I am considering swapping in

This one you can see the metal tang sticking through the center of the pickup




The two side by side


I did not consider the width issue of the trigger wheel but I think you are on to something. I am going to see if I can get a 1/2" thick wheel of the same diameter. There are some great on line catalogs for gears on line.

What about the stock Ford sensors. The actual pickup on them looks to be about 1/4 inch wide or so (narrower than the RX7 unit I am using). What is the smallest trigger wheel that has been used with one of these?

[alexander]

hi dean..
*i will find a photo looking directly down, a bit later on today
*the trigger wheel is 35mm across ie about 1.5"
*not sure which bit of metal you are referring to.... can you clarify?
*i think that extension on the sensor, which includes the magnet, is just to raise the intensity of the field around the sensor, to compensate for its smallness. i think you should be able to clamp that part in a vice, and bend it back to give more clearance for you trigger wheel (or even a bigger one).
*re the thickness... actually, i wasnt referring to that, but note that the face of the sensor is about 10mm, so a thicker wheel could well be the answer to the problem.

note that i didnt actually buy a spur gear exactly. i bought a piece of spur gear bar, and cut it to 10mm thickness. you will find the same supplier almost certainly sells gear stock.

i wil put up a photo later.

alexander.

[alexander]

dean, a couple of photos. first, you can see the thickness of my wheel. it is in fact thicker
than the VR sensor by a good margin. perhaps you could try buying a second, identical
spur gear, and gluing, screwing or tacking them together to increase the thickness of yours,
so the total thickness is at least the same as the metal strip on the sensore (about 10mm)?

the other photo is almost directly above. there is only about 1mm between the sensor
and wheel, with <1mm runout. it works fine. i have never revved the motor much over
6000rpm.

thinking about it, you might have trouble bending the bracket, as i suggested in my earlier post.
and that prevents you going to a bigger trigger wheel.

obviously i am WELL aware that there is a lot of fiddling involved, but here is another from-scratch
suggestion if you run up against a wall: replicate what i did ie gut the distributor ie remove everything
but the shaft. buy a gear stock piece, have a piece cut off it, and drilled for an interference fit on the
dist shaft. i put it on with a press. you already have the VR, and we know you have a few spares
at least you KNOW this arrangement works. cheers.

OR, just try one of the ford sensors, as you alluded to. no harm in trying; it is not as if they are hard
to come by!

regards
alexander.

[Dean924s]

alexander,

I have a ford sensor from a 94 mustang that I was looking at and I have come to the conclusion that it may actually be a better option as the actual pickup point on it appears to be about the same width as my trigger wheel (1/4" wide). The wheel I am using is only a quarter of an inch wide where as the pickup on the RX7 VR sensor is about 10mm (3/8" +/-) This may actually be the root of my problem.

I have found several other possible trigger wheels of 3/8" and 1/2" thickness. I even found a 3/4" one but I would start to worry about the weight and the importance of balance especially since it would be putting some unusual forces on things that were not designed for it. The good side is that it is actually spinning at half the engine speed. The other issue is that they start to grow in diameter so I would have to re do the sensor mount but that is just part of the fun.

Getting back to the ford sensor. What is the smallest trigger wheel that it has been used with? If it were not for the fact that setting up a test bench is kind of a pain I would do it to run various testes. I would also need to get a Olicop or equivalent software for my laptop to "see" what is going on. This is starting to add up in to some $$$$.

[alexander]

Getting back to the ford sensor. What is the smallest trigger wheel that it has been used with? I.

ah well, that is the $64 question. i have two ford sensors - a fatter one and a thinner one. my guess is that the thinner type will work with your wheel. unfortunately, i couldnt even call that an educated guess!

OTOH it shouldnt be too hard to test. as someone cleverly suggested in another thread, mount the trigger wheel on a bolt, and put the bolt in a drill press. then set up your test rig, and simply hold the sensor near the spinning wheel. you will see soon enough if it triggers the plugs.

[Dean924s]

Ok I decided to look at how the VR sensor in the RX7 is actually spaced from the trigger wheel in its housing as well as what the size of the trigger wheel is.

Points of interest. The trigger wheel is .395" thick (.375= 3/8")
The sensor pickup is .314" wide
The sensor is approximately .010" from the trigger wheel
The diameter of the trigger wheel is 1.340"

So for starters my .250" (1/4" wide trigger wheel is two narrow. I am now sure that it is what is causing the miss and the degraded performance. I need to go to a minimum of .395" thick trigger wheel. 7/16= .4375" but reality is that I will probably have to go to .500" (1/2") as the 3/8 would be under what the Mazda engineers did. I could probably get away with it but alignment of the sensor to the trigger wheel would be critical. you would have about .061" to play with. Not a real problem but I would prefer to build in the tolerances that Mazda did to ensure that it works as designed.

Here are a bunch of photos I took of the sensors in there housing as used on the FC RX7

[Dean924s]

I spoke with the manufacturer of the the gear I used (Martin Sprocket & Gear) and they were very helpful. They would gladly make a one off custom made unit for between $400-$500. A tad over my budget for this project!

This would be the same diameter and offset but with a wider gear face.

The price goes down as the volume goes up but basically to get it in the 40-50 range they would need an order of 15 pcs.

So that leaves ordering a different/bigger/wider gear. I have to do this through a dealer as Martin is a wholesale only. So I spoke with Motion Industries (recommended by Martin Sprocket & Gear) who is one of the largest stocking dealers of of there product. They quoted me the following on the two options I am looking at.

I have two choices from stock gears as supplied by Martin Gear

1.0 MARTIN S2072 14 3/8 SPUR GEAR od-3.7" center hole = 3/8" 1.6 lbs $41 Plus shipping

1.0 MARTIN S1672 14 1/2 SPUR GEAR of 4.62" center hole = 1/2" 2.8 lbs $54 Plus shipping

It also turns out that there is a branch office of Motion Industries about 15 min from my office. Now to decide 3/8" or half inch.. . .

I am thinking of getting both as I wonder if the stock ford sensor would work with the bigger one. It would also eliminate any doubt as to if the trigger wheel is wide enough.

[brentp]

Hi Dean,

You might want to check out the offerings from McMaster-Carr:

http://www.mcmaster.com/#spur-gears

[Dean924s]

Hi Dean,

You might want to check out the offerings from McMaster-Carr:

http://www.mcmaster.com/#spur-gears

He he he.. .. . It is where I got my original part from. They are a dealer of Martin Sprocket and Gears. I think I noted this in the original build thread I did. If not I need to add that to the end of it. I am putting together a parts list and sources for my conversion.

I could get the parts through them that I am now looking at BUT I would much rather get it from a local distributor as McMaster does not stock the ones I was looking for. If you go to the Martin Sprocket and Gear site they have a great on line catalog.

[alexander]

dean, without looking it up, i dont suppose your existing gear was terribly expensive.
so why not just buy another one, put them both on a bolt, and lock them together with a couple of bolts, epoxy glue, tackweld etc.
that will double the thickness and you wont have to change your existing work.

alexander.

[Patriq]

I had a 72-2 wheel manufactured. I first tried with a stock Ford VR-sensor. But it wouldn't trigger properly. Causing only sporadic sparks.
I then tried with a RX-7 sensor that Alexander kindly provided me with. That worked fine.

Haven't really run the car much after, it turned out the engine was in very poor state. I got the car to start and run. But since the engine has been dismantled, and is being rebuilt now.

Here are images of my two versions of sensor with the same wheel.
So, it seems the wheel you see in the picture at least is to small for Ford sensor. (to rule out how small the wheels can be)

[Patriq]

second image