'Magic Gear' help

I have a regular vertical dropout frame I'm considering single speed-ing. It would be nice to avoid a chain tensioner if I can find a magic gear that works. Firstly, do 'magic gears' generally work ok? Or are they usually a bit crap due to iffy chain tension or chain stretch over time? or other issues I've not foreseen?

Frame has chain stays of 430mm, I've popped it in on eehouse. I just want some advice before I buy any chainrings/cogs etc.

From experience will any of the gears within the purple strip in the middle of the chart work, or am I best sticking only to the ones bang in the middle? I presume the ones closer to the centre will be the best fit in terms of chain tension?

52x19 is looking like a decent option at the moment, particularly as it doesn't require a half link chain.

Or is it just better/easier to use a tensioner?

Just get a tensioner if you intend SS, the chain will stretch and it will feel horrible.

I’ve run an old kona kilauea ss using a magic gear 34:17 for 15 years with out any issues. As long as the chain line is good you shouldn’t have any problems but it depends on the frame, I believe konas were designed with a magic gear in mind

I believe konas were designed with a magic gear in mind

Can you explain what you mean by this?

Here's a magic gear generator: http://eehouse.org/fixin/formfmu.php

48x15 and 50x17 seem to be the other 2 magic gears besides 52x19. I wouldn't think 46x13 is a gear anyone would wanna run, but is also possible.

Funny because I ran a MG on a Cove for fixed bike polo. Cove and Kona have some design cross-over I believe.

It might be an urban myth (a really geeky cycling one) but I believe they designed the chainstay lengths on there early hard tails so people could run them singlespeed with a ratio of 34:17

If you are very lucky and you get a gearing you like with a very even chain tension that starts off tight all round (no loose spots) if would be fine.

However, that is very unlikely. Maybe just me but I am very particular with my gearing on single speed (down to fine tuning with 1 tooth increments on chainring so can't personally put up with a gearing that is not ideal just because it works for a magic gear.

You have other solutions to run ss/fixed without any chain tensioner with vertical dropout frams which are eccentric bottom bracket and excentric hub. I have the latter from White Industries (called ENO) which is also flip flop, works very well and is really really easy to set up to have a chain well tensioned. It is a bit costly but there is currently one for sale on here for a much more reasonable price by @Fabs if you want

Yeah, I agree in principle. The issue is cost, as you say. This is really designed to be a cheap build, if it comes to it I'll just use a tensioner. The other issue being that I wanted to be able to use all the wheels I already own which is self defeating if I have to build a new wheel to make it work. Appreciate the input though.

I ran a magic gear on one of my frames for a while. I used the same link that you posted to work out the gearing. I had to run it with a BMX half link chain to get the gear to work and I was totally happy with it. Two things to check are
1) that the chain is long enough if you go half link
2) that you are happy with the available gear ratio - I think my only choice was 48 / 18

Looked like this (ignore the bars - they were swapped for risers immediately afterwards)

1 Attachment

Cheers, chain tension looks fine. I guess it’s all a bit of luck really if it all just slots together nicely and works or if it’s just a bit too loose / too tight.

I’ll give it a go and see what happens!