digital tachometer

metalliatic

metalliatic

New Member
i was looking for a tach for my bike and found some on ebay for "buy it now" $20 with free shipping. or bid on them with sarting bids at .99 cents. search "hour tachometer" and you will see lots of them. easy 2 wire hook up. wrap one wire around the spark plug wire and ground the other. i havent got mine yet but let you know how it works out. does anyone else run a tach? if so what are your highest rpms?
 
I've seen just a tad over 7K with my 2 smoke. Most cruising speed is in the 5200-5900 rpm though. I imagine it also has a lot to do with your weight, altitude, road, tires, etc as to what the small engines will really pull to when out on the road.
 
Cool find. Wish they had an analog one though.

I'm planning on using the dash from my Tomos Targa. Anyone know of an analog tach that would fit in nicely next to the speedo in the blank space?

I know these things aren't incredibly useful, but I think it'l just be something more for looks.
 

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My buddy just gave me one called a Pulse Engine Tach.,( pet-1100) that he gets with the specialty tools at Polaris/KTM/Victory where he works at....It`s digital tach. and seems to be working pretty good....
I just Googled it and they have several to choose from.....
 
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got my tach today but need to know how many times the spark plug sparks in 1 rpm? i think the answer in 1 time per revolution because its a 2 stroke motor. and 2 times per revolution for a 4 stroke? right?
 
I too purchased one of those "buy it now" 20 dollar tachs on fleabay and it is on its way! Ill make sure to keep everyone posted about how it works.

Question though... I have heard that the speedo and the tach sometimes get interference from the cdi and cause them to malfunction. has this happened to anyone else?
 
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metalliatic said:
got my tach today but need to know how many times the spark plug sparks in 1 rpm? i think the answer in 1 time per revolution because its a 2 stroke motor. and 2 times per revolution for a 4 stroke? right?
Click to expand...
1 rpm=revolution per min.... So I don`t think your motor only sparks 1 time in a min....
There`s got to be a formula somewhere.....
 
Set it to one. It's asking for a number to multiply the RPM by. For instance, if you had a 4 stroke motor then the spark plug would only fire every OTHER time the engine turns over, therefore it would have to multiply the fire rate by 2 in order to show up properly.

It would also be different if you had more than one cylinder, like a V-Twin motor or something. But for a simple 2-stroke, just use the straight fire rate (multiply by 1 = no change).
 
For single speed riders, any bike speedo can be used as a tacho.

Road speed equals engine speed.

Just input the correct figure when setting up the unit.

Use a spreadsheet to calculate the values.
 
Mr Moronic Kaos,

Most small single cylinder 4 strokes fire the plug every revolution, the same as a 2 stroke, the spark during the exhaust stroke is "wasted".

Some cars and most motorcycles do this as well, unless the ignition is on the cam timing.
 
Mr Moronic Kaos posted

"Not when it's idling."

Hold the back wheel up to check idle, Its not a Harley.

A HT idles at around 2000, the issue is road speed revs not idle.
 
Wildemere don't forget that he also has to take into account his overall gear ratio to use his speedo for a tach. Our HT engines are not direct drive meaning the engine doesn't make 1 revolution for 1 tire revolution.

This is gonna take some math. There's a simple formula to calculate your rpm for a given road speed where you just plop in your numbers and ta-da but you still need to know your tire diameter and your overall drive or gear ratio. I don't have the formula in front of me but I'll walk you through the steps to calculate the rpm based on the road speed anyhow without the use of a formula. At least this way you'll know where all the values come from and how to logically apply the same steps to any vehicle.

Step 1. First, you need your tire size. If you're using 26" rims do not input 26 because you want the diameter of the tire and rim not just the rim since you're not running just on rims alone. This will vary depending on what tire you're using. Measure the diameter of the tire on rim. You might end up with say 29". Then use the pieXdiameter formula which will tell you the circumference of the tire or in other words what distance your tire is covering in one revolution. So for one revolution of a 29" tire your tire covers (3.14)X(29")=91.06".

Step 2. Figure out your 1st drive ratio. You have to count the teeth on your gears. Let's say you're using a 44 tooth rear sprocket at the rear wheel and the little drive sprocket at the engine has 11 teeth. That's a ratio of 4:1.

Step 3. Then you figure out the 2nd gear ratio inside where the clutch is. I don't have my engine in front of me so I can't give you the numbers but let's say the big gear where the clutch pucks are has 60 teeth and the small gear that drives it has 10 gears. That's a 6:1 ratio.

Step 4. Now you multiply these ratios to give you the overall drivetrain or reduction ratio. So 6x4=24. So your overall drive ratio in your entire drivetrain is 24:1. Which means that for 1 tire revolution your engine rotates at 24rpm.

Step 4. If you use miles/hour for your speed then you have to convert the distance your tire covers in 1 revolution from inches to miles. The conversion is 1"=0.00001578 miles. So if you recall for one revolution your 29" tire covers 91.06". So (91.06)X(0.00001578)=0.001436miles.

Step 4. Let's say you're riding at 25mph. Then your tire would have to rotate a total of (25)/(0.001436)=17398.24 times in that hour. Since we measured your engine speed in rpm (revs per minute) we have to convert the bicycle speed from from mph to miles per minute so the units are the same. So dividing (17398.24)/(60 minutes in an hour)=289.97 which is the number of revs your tire does going at 25mph. Now remember that 24:1 drivetrain ratio? Now all you do is multiply the 289.97 by that ratio. So (289.97)X(24)=6959 rpm AND YOU'RE DONE!!! So for the gear ratio given and teeth numbers given at 25mph your little China Girl would be turning 6959rpm.

ARE YOU GUYS STILL AWAKE? LOL The good thing is that you only need to calculate the drive ratios once and the distance your tire covers in one revolution once as well since those all stay the same and just use Step 4 and substitute different speed values to get your rpm at that speed. Once that is done you can write the values down somewhare and you'll know that if say you're going at 25mph you're turning 6959rpm. If you want to know what you're doing at 15mph or 20mph or 30mph use Step 4 to calculate your rpm and so on and so on.
 
Far out man, you have typed a novel, and all of your steps and numbers are wrong, I have attached some spreadsheets (not my own) to show you the correct maths,

For a HT

Initial engine ratio is 4.1:1 20t drive, 82t driven

Secondary is usually 4.4:1 10t drive 44t driven

It equals around an 18:1 reduction ratio. I like an 16:1 ratio on my single speed bikes.

Normal 26" bike wheels are usually between 26 oto 27" high, the bare rims are around 23"

Measure it all up and a speedo is a tacho.

A stock 26" HT setup turns around 5800rpm at 25mph. Not 6959 or any other random number.
 

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wildemere said:
Far out man, you have typed a novel, and all of your steps and numbers are wrong, I have attached some spreadsheets (not my own) to show you the correct maths,

For a HT

Initial engine ratio is 4.1:1 20t drive, 82t driven

Secondary is usually 4.4:1 10t drive 44t driven

It equals around an 18:1 reduction ratio. I like an 16:1 ratio on my single speed bikes.

Normal 26" bike wheels are usually between 26 oto 27" high, the bare rims are around 23"

Measure it all up and a speedo is a tacho.

A stock 26" HT setup turns around 5800rpm at 25mph. Not 6959 or any other random number.
Click to expand...


LOL I did type a novel didn't I? Didn't think it would take that long to go through and explain all the steps. :eek:

HOWEVER MY MATH AND STEPS ARE 100% ACCURATE!!. I dare you to point out a step where I made a mistake. 6959prm IS NOT a random number. All my conversions are correct.

Like I mentioned in my 'novel' I didn't have my bike or engine in front of me so I guesstimated the numbers of teeth in the gear train and also the diameter of the tire/rim. Those are the only numbers that are not correct because they were estimates and they will vary on your bike based on what tires you're runnning and what sprockets you're running. 6959 is the rounded rpm you would obtain based on the figures I provided. Put the gear ratio and rim/tire #'s I provided in your spreadsheet and see what comes up. Anyone can plop numbers into a magical formula. To make things faster absolutely use the formula but I thought some might actually have enough curiosity to want to know the logic and steps in calculating the rpm so they could understand the process. I apologize for my rogue attitudes and for making anyone's head hurt from all the math. (^)
 
OK so I just installed the "20 Dollar" tach from fleabay on my bike and I am pleased to say it works great! I had a problem in the beginning as it kept resetting itself but after unwrapping it a few time from the spark plug wire it started working perfectly. I used a reflector mount on the handlebars and have the meter mounted over my handlebar clamp.

HERE is the tach I bought (Its at 10% off so hurry fast! lol). If you are interested in adding a tacho on the cheap on your bike I would DEFINITELY recommend this one

P.S. I have my tach wire wrapped around about 3 1/2 times. That seems to be the perfect number
 
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