What is max safe RPM?

datz510

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May 9, 2008
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204
Ok, so I finally got my tachometer on and have been watching my RPMs for a few days while riding around.

I generally cruise at around 20mph give or take. I've been finding that 22 has been the sweet spot lately.

Now, watching my RPMs, it looks like that at 22mph, i'm turning around 6,000 RPM. Is this safe long term for my 70cc HT engine or should I drop down a few sprocket sizes to get the rpms lower? I have no idea what safe redline RPM is for this engine. On a couple brief blasts, I've touched 6400rpm with it, which seemed way too high to be running the engine at.

I'm currently running a 48T drive sprocket. I have 44T and 36T on hand if needed.
 
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Large Filipino

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Try the 44. I like to keep mine about 1/2 throttle. I don't have an rpm gauge so i do it like this.

Listen to your lawnmower the next time you cut your lawn. That's the cruising rpm you want. There's a way to raise your lawnmower's RPM. If you can redline your lawnmower you'll find the manufacturer set it to run just a bit over half throttle if not right at it.
 

datz510

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May 9, 2008
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204
Thanks! I'll give that a try tonight and see how it works.

Acceleration with the 48T is quick, but I need to get the engine into a safe RPM for cruising at 20.
 
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duivendyk

Guest
Listening may be rather deceiving,it depends on the type of engine.For the same rpm,the number of explosions for a 4 stroke engine is half of what it would be for a 2 stroke so it appears to be going slower than it really does ,when judging by the exhaust note.Lawnmowers are typically 4 stroke,Chinese engines usually (but not allways) 2 stroke.Half throttle is not necessarily indicative of rpm,it depends on engine load,but it is a good guide for normal cruising.The gearing has a lot to do with climbing ability.I would use the highest gearing (smallest rear sprocket) you can use in your area of operation.This gets to be a real problem in hilly areas,where you really need a gearbox.Lugging an engine, that is barely being able to maintain speed at full throttle is not a good idea esp. with a 2 stroke.They usually have decreasing torque at lower rpm,so it starts to die on you,pedal power to the rescue !
 
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duivendyk

Guest
Come to think of it there is of course, a direct relationship between engine rpm and road speed, if you know the wheel size and the overall reduction.For a 26" wheel the following holds:
rpm= 13.4* S*R , S is the roadspeed (mph), R is the OVERALL reduction (the product of the internal engine reduction and the ratio between rear (large) and front (small sprockets), * denotes multiplication.For instance: We want to run at about 5500 rpm at 20 mph ,what reduction do we need?, we get 13.4*20*R= 5500 ,
R=20.5, assume the internal engine reduction is 5.0 then the sprocket ratio needed is 4.1,or 12T front and 48T rear. Of course this formula can be used to compute the rpm for a particular reduction, say R= 15 (5* 36/12). At 20 mph we get :
rpm=13.4*20*15 or, rpm =4020 . It's not hard,all you need is a calculator.For a 20" wheel the factor 13.4 changes to 17.4.
 

datz510

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Joined
May 9, 2008
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204
Well, I got my 44T swapped on. Definitely improved the top end. Cruises nicely at 20-22mph. I rapped it up for a quick blast down a back road and it topped out around 27mph. Very nice!

Now, the down side. I think I bent my rear wheel somehow when I changed out the sprocket. Im gonna have to upgrade to a better/stronger rear wheel I think. Its definitely got a bit of a wobble in it now. :(
 
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andyinchville1

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HI Datz510,

Glad to see a fellow tach owner....I have touched 6400 RPM on my dax 70 and didn't want to keep it there long....sounded...well not good so I backed off for fear of blowing it....Cruising around 5000 RPM feels good tho...at least on my engine...

Andrew
 

datz510

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Joined
May 9, 2008
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204
Got my wheel trued up again with the trusty old spoke wrench. Also found that the center hole on my 44T sprocket was too small, so was pulling the wheel over a bit.

Yep, I have a tiny-tach. Works great. I really like being able to see what kinda revs the thing is turning.

Mine seems smoothest around 5800-5900 rpm. The new sprocket puts my cruising rpm around 5300 or so. Seems good to me!
 
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alex

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If you guys want to hear something really scary, I've had my kings 80cc with 3,550 miles on it up to 6900-7100 rpm (with a tuned pipe) while I was on it. And also, where would I buy a good tach that I can mount on my bike?
 

datz510

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Joined
May 9, 2008
Messages
204
Here is where I got mine:
http://www.tinytach.com/tinytach/gasoline.php

I got the TT226NR-2C model. Installation is very easy. 2 wires. One wraps around the spark plug wire, the other goes to ground. These tachs update about every 2 seconds, which works fine for seeing what kinda rpms you are turning while cruising. When the engine is off, they display total hours of engine run time.
 
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