Chainring wear on a fixie

[silva]

I was browsing on a rare historical photos website and came across this:
https://rarehistoricalphotos.com/students-at-smith-colleg-massachusetts/

This is dated 1948, look at how far the chainring teeth are separated, it's like half density, aka every inch instead of every 1/2 inch.
Someone here recently wrote having removed every second tooth, without elaborating why.
Apparently, back then, chainrings were produced like that.

 

[Ian H]

Inch pitch chains were common back in the day (inner link was solid), hence the inch-pitch ring. You can run a half-inch chain on it no problem, but not vise versa.

 

[silva]

How does a "solid" inner link differ from nowadays inner link?
2 outer plates, 2 inner plates, 2 pens, 2 bushings over the pens, 2 rollers spinning over the bushings?
I found this:
http://www.american-vintage-bicycles.com/home/vintage-bicycle-parts/skip-tooth/
"skip tooth" chain it is named.

The inner link plates are longer than the outer so not equal gaps between rollers.
1 gap is big enough to accomodate a tooth, the other isn't.
The rollers are alike in pairs, so between 2 chainring (and then also rear cog I suppose) teeth, instead of 1 roller, 2 rollers.
On the 1950 dated pic I linked, not that detailed to be sure, but it does look like that, you see more white / background every second link.

 

[silva]

Well, it looks like the rear cog now also ceased wearing further.
I had grinded off the teeth tops to nearly points, as to allow the rollers to easily disengage, mounted a new chain, retensioned it now 5 times, and still no teeth broke off, alike happened before.
I have a cog here with 5 such broken teeth, the not broken ones are still quite thick (in the chain direction), but due to the hollowing out like hooks the rollers ripped the teeth off.
All 16 teeth of the current cog became thinner than that used cog, without break offs.
So it looks like it increases the life span alike it happened for the chainring.
How long, will remain to be seen.

 

[Sharky]

If the teeeef look a bit sharky there done.

What are you implying?

 

[silva]

With that mentioned rear cog, 4 months later of which at least 2 with all 16 teeth broken off, yet chain still doesn't skip, when up on a bridge, more force, it occasionally makes a noise and that's it.
Apparently teeth remainder, barely 5 mm, still suffices to not slip, jump over.
I now wonder why they produce these fixed gear application specific - cogs with such high/tall teeth. It's listed as a plus property, but since wear concentrates at the middle of their height, the formed hook shape causes a longer trajectory and opposite to force direction for the rollers to disengage, accelerating wear.
And, since teeth are thicker (plate thickness) at their bottom, the contact surface is bigger so less wear distance radially.
If over again some months (with eventually a replaced chain)

 

[silva]

4 months further - still same 16T cog, all teeth broke off in the middle, which was 3 months ago, but apparently no problem - rollers don't slip over.
I retensioned the chain more frequently, to decrease risk of chain humping off at the slackest point.
The bottom bracket eccentric tensioning module is now turned near to the end of its range, so new chain needed then, wonder what that it will do on that extremely worn cog, since with a worn chain, a set of rollers nearly simultaneously engage and thus spread force, while with a new chain, only one will.

 

[silva]

At now 40 km/day, still the same rear cog, bottom bracked based eccenter tensioner was 3 weeks ago at its end, replaced with new chain.
The circumference of the inbus heads of 1-2 of the 6 mounting bolts showed a flat section due to chain links grinding over.

I unmounted the cog to inspect and clean it, and put specific bolts back in same holes in the hubs IS disc, as to try to have the damaged / flattened sections again closest to the chain path, which succeeded.
Sometimes when pushing hard, I still did hear links grinding over bolt heads, so I decided to try something: allowing 1.5 cm instead of 1 cm vertical movement when tensioning, as to give the links more movement room over the rear cogs teeth - instead of force exerted on the bolt heads, force gets "diverted" by tensioning chain more.
The idea turned out to be working, it's now nearly silent at the rear cog.

The extra risk involved is due to the more slack chain, it may bump aside the rear sprockets teeth which can cause damage.
So I pay more attention to / avoid bumpy road parts.

The chain is a model 420 1/4" motorcycle chain, so it's quite heavier, less easy to bump off.

So, I now have some experience to judge, apparently, as proven, cog teeth for a no derailer use, don't need to be high. Yet they are produced that high, even for dedicated singlespeed/fixed gear alike former Velosolo company did. The "tall teeth" was even listed as a plus.
But the rollers of the chain hollow them out (sharkfin shape), proving that the upper parts of the teeth act more as a resistence to disengage than as a facilitate to engage.

So, couple weeks ago I prepared for a next test: I have with me on the bike a spare Velosolo chainring and a spare cog, brand new.
I took them out, went to the table grinder, grinded down the top of all teeth, then a little rounding in both directions, as to make the curves smooth.
So, whenever I DO replace current 2019 chainring and 1 of 2 cogs that already had have a service life (I replaced them after 3 teeth broke off), and are now living an unimagined next service life), the "new" drivetrain will start with those "adjusted" new ring and cog.
The now lower teeth, and abit wider valley ends, should avoid the increasing wear rate due to the hollowing out.

That is, to be experienced in a future, if I survive it...