Starter Motor

[Adrian]

Hi once again Brains Trust - I think it's time to replace the old - but rebuilt a couple of years ago - Bosch starter motor.
.
It's always been a bit sluggish in initially turning over the motor but has become really clunky & slow. I am thinking of replacing it with the Nippondenso which appears to be a direct replacement.
.
The battery - which is quite new & a sealed version - shows 14.2 volts with the engine off.
.
Thoughts anyone 

[Bob_Roller]

A quick test for the starter, connect a multimeter to the battery remove the ignition leads and put spare plugs on them, or run the carbs dry .
On a fully charged battery operate the starter and check the battery voltage should not go below 10 volts .

[dogshome]

I had something similar when swapping to high compression 860. I changed for the standard gear type. There was nothing wrong with the huge bosch original - after some serious cleaning - but the geared design is more efficient. It doesn't suck the life out of anything less than a large and healthy battery.

Starts are spinny spinny, rather than chug chug .....chug.. now.

[Adrian]

Hi dogs home - what do you mean by "standard gear type" - thanks

[Bob_Roller]

These new after market starters have planetary gearing, instead of direct drive gear on the original OEM starters .

[Adrian]

Hi Bob - what is Planetary Gearing & why us that important - thanks 🤔

[Bob_Roller]

Here's a short video .

https://www.youtube.com/watch?v=9bCLZsLueIM


To put it simply it gives more mechanical advantage to the starter .

[Adrian]

Thanks Bob - the Nippondenso fits your description so I have just bought one of those - thanks 

[georgesgiralt]

Hello !
Simple explanation:
An electric motor (and any motor) produce torque and power. P(power)=T(torque)XRPM (speed).
The P is the power produced and due to efficiency the power consumed is somewhat superior.
To spin a high compression engine having big jugs you need a lot of power. (and a big torque). So a big fat starter motor drawing huge amperage (300 amps max for the Bosch). But the speed is not that high.
So the idea is to have the motor produce and consume way less power by turning at high speed. And as it is a small motor it gives less torque. So Big RPM X small torque still equate to high power...  But you must reduce the speed with gears otherwise you grind the teeth of the starter motor and the flywheel with the small pinion turning that fast....
As all bike makers chase weight they decided to use smaller and smaller batteries with less and less amps and cranking power. And smaller and smaller starter motors to save a lot of weight. So you must have very fast speed to produce the power still needed to spin the engine... So a big reduction to get the proper flywheel speed ....
And of course lightweight and small. Planetary gears give that. Big ratio and small space.
I'll get my aspirin.....
Have a nice day !

[Adrian]

Thanks George - interesting stuff 

[dogshome]

You read "simple.......... aspirin" like me didn't you? 

[dogshome]

Hello !
Simple explanation:
An electric motor (and any motor) produce torque and power. P(power)=T(torque)XRPM (speed).
The P is the power produced and due to efficiency the power consumed is somewhat superior.
To spin a high compression engine having big jugs you need a lot of power. (and a big torque). So a big fat starter motor drawing huge amperage (300 amps max for the Bosch). But the speed is not that high.
So the idea is to have the motor produce and consume way less power by turning at high speed. And as it is a small motor it gives less torque. So Big RPM X small torque still equate to high power...  But you must reduce the speed with gears otherwise you grind the teeth of the starter motor and the flywheel with the small pinion turning that fast....
As all bike makers chase weight they decided to use smaller and smaller batteries with less and less amps and cranking power. And smaller and smaller starter motors to save a lot of weight. So you must have very fast speed to produce the power still needed to spin the engine... So a big reduction to get the proper flywheel speed ....
And of course lightweight and small. Planetary gears give that. Big ratio and small space.
I'll get my aspirin.....
Have a nice day !