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Learning how to build Lotus Seven replicas...together!
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PostPosted: February 7, 2019, 2:50 am 
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This idea is for a BEC (Bike Engined Car).

I have a pretty decent hobby grade servo that can rotate about 360 degrees with the right PWM signals. It also says it has 38kg-cm torque.

The engine is a 2004 Honda CBR1000RR. Once you remove the shifter cover and shifter up-down linkage you have access directly to the shift drum. As near as I can measure each gear is 60 degrees apart with neutral halfway between 1st and 2nd.

It doesn't take much torque to shift it by hand (turn it 60 degrees to the next gear) while rocking the output shaft to get the dogs engaged. I would think that that servo would be able to turn it easily. I would also think you'd probably want a "buffer" or "servo saver" between the servo and the shift drum. Maybe just a rubber connector would work.

I think on the inside of the engine, the drum has a "star" cam with a follower that forces each gear location, so I wouldn't want that fighting against the servo, but I think each PWM point could be fine tuned to be pretty close to each notch.

I would probably want to isolate the servo from the motor case to prevent heat transfer maybe?

Then you could have shift paddles on the steering wheel. The concept works on the old Parallax Basic Stamp I have to control the servo. I would probably "upgrade" to an Arduino board or Raspberry PI or something like that. With a rubber coupler you could probably preload the shift drum too for a bit before shifting. Not sure how I'd cut the engine though. Maybe hook up a relay as a "kill switch".

Ever hear of such a thing? I don't know how reliable it would be. There would still probably need to be mechanical linkage as a backup.

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PostPosted: February 7, 2019, 11:13 am 
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It's been done before have you checked out sites like

https://www.flatshifter.com/flatshifter ... ckshifter/

Graham


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PostPosted: February 7, 2019, 4:40 pm 
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I can't tell what it is, is it an air shifter? Plus it costs more than my engine! I like the throttle blipper.

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PostPosted: February 7, 2019, 5:31 pm 
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I am not saying you have to buy there equipment but mine the site and forums for information. Pingle offers a ranger of shift kits.

https://www.pingelonline.com/content/demo-videos.asp


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PostPosted: March 3, 2019, 1:33 pm 
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I looked at that years ago and decided the hobby servos with a high enough torque rating were too slow - never bothered to actually try it. I think it would work but trying to sync letting off the throttle (whether it be manually or through the ECU) and the actual shift would be challenging. I suspect a mechanical paddle shifter with some sensors to manage the ignition cut would be quicker.

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PostPosted: March 18, 2019, 3:05 am 
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Well I did a test with a 38kg-cm hobby servo. It had no problem rotating the drum against the internal detent spring/star thingie. That particular servo is a bit slow, maybe .2-.3 second for a 60degree shift. But I would have the servo offset and driving the shift drum via a chain, so it could be easily geared up. Also that particular servo has over 300 degrees of rotation so it could be a 1:1 drive. Once the angle gets close to where it should be, it snaps right into the detent. It almost seems like a little slop in the chain would be a good thing.

I was also thinking about (and tried) controlling the throttle with a servo. The test I did used the stock cable (shortened) but it seemed very sloppy and inaccurate. I think I would hook it up with a pushrod link. That would also allow an "automatic" throttle blip on down shifts. I removed most of the throttle return springs so they wouldn't be fighting the servo. Any ideas how to design a failsafe to close the throttle if the servo fails? A kill switch obviously.

Heck, both of those could make it almost an automatic transmission, except for clutch at stops.

Then I got to thinking, if you could control the shift forks directly (with two servos and removing the shift drum) you could pick any gear at any time without having to go sequentially, I was bored. Not going to try that.

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PostPosted: March 19, 2019, 8:17 am 
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Another thing you can do is the relieve the notch on the indent cam on the gear selector. Soften up the contour to make it easier and quicker to shift.
Davew


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PostPosted: March 20, 2019, 6:51 pm 
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Johnsinski wrote:
Any ideas how to design a failsafe to close the throttle if the servo fails?


Two rotary encoders mechanically connected to the throttle plate and two forms of actuation. If either servo disagrees with an encoder, power is removed from it and the other closes the throttle. If a disagree occurs between the two encoders, both servos remain powered but go to idle.

I'd go for something better than hobby servos considering the safety nature. I haven't played with any OEM solutions but I think they'd be a better starting point.

I'd also be curious to know how DIY throttle by wire would be from an inspection standpoint (either state or track tech inspection).

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