Hi
I am working on using a servo to change the gears on a land speed motorcycle. I have got the code working with 2 buttons. 1 Up and 1 Down with an LED that indicates the gear number. At the moment its all running on a little ATtiny85.
Now I need scale it up with a stronger servo. I have used a luggage scale to get the force I think I need. Its approx 7kg at 7 cm so thinking I need a 50kg/cm
I would like to run it on 12v if I can but prepared to run it on LIPO at 8.4v.
Looking for a recommendation on a Servo to use. I am guessing something that is used in a robot?
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I like your idea.
7kg can be estimated as 70N by multiplying by the force of gravity(9.81 rounded to ten) by the percieved “weight” of the scale (7kg). 70N at 7 cm equals approximately 4.9 Nm of torque if you push that lever either way. Calculated here because im too lazy to do it in my head -> https://www.omnicalculator.com/physics/torque
So you need a servo with probably 5 to 5.5 Nm of torque just to be safe. 5.5 is more of an industry standard so i’d just go with that.
The only issue you will have with a motor of this size is that most in that range are either expensive, or run on 230/40V ac. (from what i’ve found anyway)
By no means do I know your project, but if you do more research into servos and dont find any in your power range that arent a high voltage, to avoid putting an inverter in your bike, I might suggest two electromagnets above and below the lever that pulls a metal plate (which is bolted or welded to the lever) towards them. This would require a smaller amount of force, an 80N (extra on top of the 70) magnet. This is not unheard of in your voltage range.
In fact, heres a push/pull electromagnetic solenoid that runs on 12v as an example. With a device like this, a plate would not be needed. https://www.amazon.com.au/DealMux-Spring-Push-Pull-Solenoid-Electromagnet/dp/B01EZZIDIG
Anyway, I’m sure my essay has provided some insight.
Hope that helps!
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Abram
Thank you for replying. I was lent a solenoid based changer. In my view they have a few issues. Firstly the way they apply their force is very harsh. Secondly the max force is not till right at the end of there pull so if you do not have them correctly setup mechanically, they can fail.
I agree. I think 55kg/cm will do the job. I have seen these but have never used one. [https://banggood.app.link/O3dbwfbbZ5](56kg/cm Servo)
I was hoping someone who makes robots might be able to give me some guidance on which brand to use or not use.
I updated my code yesterday. Removed the flashing LED and replaced it with an 8 LED NeoPixel strip. Operates much faster (not having to have the LED blinking for the gear number) and only uses one output from the ATtiny 85 to run 8 LEDs
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While I had a chat about this forum question yesterday on the live stream, @Abram68481 has some much better insight. Cheers!
This was where I got to while answering it “real time” 
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Aah yes @Graham The thing you said about the worm drive was good.
@Don89191 That would be a great solution with something such like a bldc motor as displayed on the openDog project by this guy that makes robots -> https://youtu.be/Mg3Dh8D3LSI
He used threaded rods and bldc motors for his robot. You could do the same thing and hook up a rotary encoder to track the position of the motor or just use a stepper motor, however this would obviously mean you need to keep the gear lever.
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Hello Don, Interesting project! I’ve used some of Banggoods smaller M/G servos on a high speed jet boat [radio controlled] and they performed flawlessly.If the smaller ones are any indication of quality,then the larger ones should be OK.
Cheers-Ian
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Hi Don,
What you’re trying to do is pretty common in the amateur motorsports world. I strongly recommend you take a look at the Formula Student / F-SAE forums (http://www.fsae.com/forums/forum.php ). It’s an undergrad student level motorsport engineering competition. They run ~600cc bike engines in open wheelers and a quick shift system is a common project for final year students so there’s plenty of resource material available if you google “F-SAE Quick Shift” or “Formula Student Paddle Shift”.
Day 3 of MoTeC’s FSAE Seminar covers gear change ignition cut sensing which is a useful resource for this: https://www.motec.com.au/downloads/downloadseminars/
A few notes:
- I’m assuming you’re aiming for sub 100ms total shift times.
- I’m assuming you don’t care about auto-blip on down shift.
Gear shifts with a sequential box have a very non-linear force curve. With a servo you’re asking it to track a position and reject disturbances - they aren’t really designed for super high speed - so you may get some strange behaviour through the shift as it tries to compensate for the changing resistance to its motion, but it’ll probably still work well enough.
You’re probably best to start with a strain gauge on the shift lever for accurate gear shift detection and shift force feedback before you install the shift actuator. Depending on your ECU you can use this for manual shifting with gear shift ignition cut, with re-ignition only after gear engagement. This’ll get you easily into sub 200ms (total) shift times with most standard sportsbike gearboxes, maybe even down to 120ms per shift.
You’ll probably be hard pressed to find a 12V servo that can provide the necessary power (ie. torque AND speed). Solenoids are the usual option for this application, and pneumatic systems are also very popular for their simplicity - despite providing a limited number of shifts and adding an extra system to the bike. A linear actuator might work if you can find one with appropriate specs, but I suspect it’ll be prohibitively expensive for this application.
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@Graham
I have had a look at that video. And its got me thinking. I could use 12V DC Motor 122rpm w/Encoder
SKU: FIT0403
The only thing I am not sure about is the torque in Kgs. If I read it on the CE site its 8.7 kg*cm, when I go to the wiki it Stall Torque: 38 Kg.com
What is Kg.com?
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Oliver
Thanks for the info. In my application I am not trying to shift fast I am trying to move the shifting from my foot to my hand. The bike is for landspeed racing and I have a 2 mile run up so lots of time to shift up the 6 gears.
The gearbox has springs in it so will help the servo get back to the rest postiton.
Just looking for something reliable that is not $1000 that I can mount on the bike. Run from 12v if possible.
Ah I think I get it @Don89191 - this is a rider position / aero thing? Do you have a web page or something for the bike? I’m pretty interested in this project!
And lots of good suggestions in this thread!
If you don’t care for shift speed too much then a high torque servo or a small linear actuator would do the job. You could make a crude linear actuator using some well lubricated all-thread and a motor - a bit of M10x1.5mm at spun at 122RPM would get you 3mm/s with enormous mechanical advantage. I’d suggest you put a stiff spring between your actuator and the shift lever though to act like a suspension bump stop so you don’t snap something!
Kg.com is definitely a typo - it should be kg.cm (or kgf.cm, strictly speaking). They are different specifications - the rated torque (8.7 kgf.cm) is what it can deliver continuously without over heating etc. The stall torque (38 kgf.cm) is the absolute maximum it can provide in short bursts.
If you are interested here is a link to last years bike and its build. https://40mmracing.wordpress.com/ the new bike is the same but with a body.
Although I said I have time to change I guess I am targeting 500ms.
Not sure if I should be using a large RC servo or a motor with a gear box and encoder
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