This is what I do for the Convoy L21B for use on a rifle for hunting. Install a spring inverted since the pad is too small for it to be installed correctly. You’re then limited to only flat top 21700’s
When you installed the TIR - did you put a spacer between LED-board and shelf? If not, and if the TIR is shorter than the reflector, you increased the distance between positive and negative battery contact points which obviously results in less spring tension.
If you don’t want to mount a spacer (recommended solution) you could try a protected cell instead of an unprotected one - they are longer.
Thank you all for your amazing answers! I’ve already learned so much! :+1: :+1: 
But I am still not sure what to do. :P After I have written half of the post below, my brain is cooking like a D4 on turbo mode just from all the thinking. 
I think the easiest and fastest solution would be: I ask Simon for a S21B with a driver with a spring and a suitable LED. Hopefully pre-built with a bicycle TIR.
Or I buy from another brand!
I think this would solve the problems. It’s not as elegant as fixing the S21A and it’s more expensive… buying two lights to get one. At least I could use the S21A to keep my neighbours awake at night. :smiling_imp: But since my neighbours are friendly people and this whole thread is already a technical nerdy brain festival with all your lovely comments :heart_eyes: :partying_face: I don’t want to end this thread so fast. I love technical nerdy brain festivals :innocent: and we can solve this problem for all times. Maybe not… Eventually… So let’s move on:
My driver has a red board, temperature regulation and 20 mm (not 22 mm). That’s the S21A I ordered: Page Not Found - Aliexpress.com On the product page is also a picture of the driver - it looks the same as the driver in my S21A.
As far as I understand: S21A has a 20mm driver because the pill takes some space around the driver. S21B has a 22mm driver. Right?
I use a Fenix ALB-10 as a mount. It holds the light very firm, I don’t want it to move and bounce around. I also added a selfmade shield on the head of the light to get a better dark-light-separation. Should prevent blinding anybody on local country lanes/woods. (I know, here in Germany that’s still no authorized light and is not allowed on German roads and most country lanes.) 
Thank you for this explanation. Now I understand how both problems are (probably) caused by one problem: The battery losing contact and knocking against the solid contact of the driver. This causes…
… 1. the change of modes.
… 2. the pill to unscrew over a longer period of time.
Having a spring on the driver should possibly solve both my problems, because…
… 1. the spring keeps the contact between driver and battery.
… 2. the spring dampens the “knocks”/the impacts of the battery.
But I’m not 100% sure, because “dampening” means still a constant movement of the spring… and this is transferred to the pill?
I think my original post was not detailed enough or poorly written at this point. :person_facepalming: But I don’t want to speculate too much… as far as I understand: Simon sees the solution in a driver with a spring - like spoonrobot explained. I assume that’s the reason a S21B would not automatically solve all problems: A driver without a spring could still cause a short loss of contact between driver and battery - which changes the modes. But please don’t quote me on that… Probably it’s more my miss-understanding of the answers I got. :weary:
In the youtube-video, linked in my first post, Weerapat K. says the same! As soon as the flashlight is not only steady (or in very slow and smooth motion), but is used for running or cycling, the integrated shelf looks like the better technical design (at least to me…).
And that’s still my question about my S21A: Adding a driver with a spring sounds simple and logical, BUT the spring is still a moving part… will this moving part still cause the pill to unscrew?
To be honest: I don’t know. If anybody knows something - please let us know.
In spoonrobots S2+ it seems to work.
After writing this post I will write WK from the youtube-video, if he has positive or negative experience with the S21A with brass pill and with spring. :student:
Thanks for your research! :+1: 
I think SFT40 and CULPM1 have too much throw. SST40 with bicycle TIR looks like the best combination?
I don’t really know enough about that. That’s one of the things I was hoping to learn later and for now just buy a reliable bicycle light from Convoy… I read so many comments about people who asked Simon and then got an amazing light delivered. Or got at least easy support after sending a message. :cry: Of course it was me who bought the wrong light. But let’s jump into the vast ocean of details: Stabilized output means the output does not drop in a slow curve? I think I have read that in the reviews of zeroair or 1lumen. But I never understood drivers: boost driver, 6 Ampere driver or 6x7135 Amplification driver? Has anybody a link to read basic information about drivers? :person_facepalming: I assume that most drivers are ok in a bicycle light (of course when matching the led, the tube diameter, …)?
Something like this? 2.45€ |26mm PCB board für serie verbindung. Verbinden zwei flache kopf 26650 batterien in serie. Erhöhen die länge zu füllen die lücke|Tragbare Beleuchtung Zubehör| - AliExpress
The question: Will the spacer solve the “unscrewing-of-pill-issue”? If it does that, it would be just the best and easiest way!
That looks genius! How is the spring installed? Soldering? I’m not sure at the moment - my soldering experience is limited to change the led-board in my Sofirn SC31 Pro: the 6900k (according to reviews) of the cold white SST40 made it inevitable. AND those blue coldwhite Luminus seem to have the lowest of all cri-numbers when compared to other leds: in the low to mid 60s. :laughing: But if I would buy a driver with a spring - I would have to solder as well. So perhaps your solution is still something worth trying.
Thanks Johm for your comment and the magnet idea. I think I will follow Funtastics advice and not try it with this light.
As far as I now understand: 
In the S21A there is no integrated shelf - only the pill that creates kind of a “screwed-in-shelf”. I didn’t unsolder the LED-board and didn’t put anything in between the LED-Board and the pill.
My meassurments (I think they are around +/- 0,2 mm):
TIR:13,6 mm
Reflector: 11,6 mm
When installing the TIR I removed the gasket, which is approx. 1,3 mm.
—> The TIR is still longer than the reflector and gasket together. But the difference is very small.
I think, in a perfect world, there should be no need to add additional spacers. :question:
Simon asked for pictures how I installed the TIR. And then gave me the advice to also add the glass lens in my build with the TIR. Before I had removed it. But with the glass lens and TIR the measurements should be the same as the measurements with the reflector and gasket.
Should I upload the pictures here?
Yes, it’s soldered in. You could actually buy one of those spacers and solder a spring to it. If you remove the solid post on the driver you could then have the spring facing towards the driver. If a spring is used you need to make sure you use one that allows high enough current or you can’t use turbo (100%). I’m using some that take up to 18A which I bought off a BLF user from Canada. Convoy sells these and take 8A no problem, so you’d perhaps need to install a bypass, not sure what the driver needs.
I agree with the suggestion that a spring on the driver will likely solve your problem. I don’t mountain bike any more, but last year, my commute included some singletrack for a good portion of the year. I never had my S21A change modes or had the driver come lose. I’m pretty sure I have over 3k miles of commuting on that light with no problems.
Mine is the SST40 version that has a spring on both ends of the battery. It’s been rock solid.
The spacer won’t solve the unscrewing pill issue. Also, you would want the 21mm pcb version (he has 18mm, 21mm, and 26mm for each battery type)
A spring on the driver may stop the pill from unscrewing. The pill is held in place by friction, and the hard impact directly on the driver is generating enough force to overcome the friction force. A spring will spread the impulse out over a longer period of time, resulting in a lower peak force that hopefully won’t exceed the friction force.
And you have the right idea regarding the driver. A linear driver will lose output as the battery voltage drops below the voltage needed to deliver the rated current. A boost driver uses inductors to convert low voltage, high current into higher voltage, lower current. So even as the battery voltage drops, a boost driver can draw more current to produce the same output voltage.
Also, it looks like you may be in luck - Simon is now selling the S21a with the 6A driver (with spring) paired with the Nichia 519a. Won’t get the benefits of a boost driver, but it should be much nicer than the SST40. So you could see if he’s willing to sell just the driver + emitter.
Thanks for these links to all the material. For the S21A a 21mm spacer is needed. Thanks, Delta_V!
Soldering the spring on the spacer is actually a very good idea for someone with no soldering skills like me. The parts are sturdier than the driver and if I destroy something while soldering, the spare parts will be cheaper.
Yeah, I just copied your link above and didn’t realize.
There seems to be something up with the Convoy store on Aliexpress. They dont have as many products as they used to?
Are you sure you’re on the official Convoy Flashlight Store - convoy.aliexpress.com ? There are a few other imitators on Aliexpress - Convoy Online Store, Convoy Factory Store, Convoy Lighting Store, etc. But the official one is the Convoy Flashlight Store.
Yes I am. There doesn’t appear to be a metal button version of the S21A
AFAIK, the black and gray have always been rubber buttons, while the colored hosts use metal buttons.
It would appear not.
I don’t remember every seeing metal buttons on the S21A. My old purple S21A with an Osram emitter has a soft plastic switch cover.
That’s what I remember for S2+ buttons but not the S21A.
Also if you go to the Convoy Flashlight Store and click on links to “S21 series” or any other general link you get “Sorry, no matches were found for your refined search.”
Oops, was thinking of the S2+ for some reason. But the links seem to work fine.
Not for me.
Convoy store is missing parts, noticed that too, but I buy in bulk direct using PayPal, so rarely ever look at AE