DIY 42T Cassette Cog

[blown240]

So we all know about the new 10-42 11 speed cassette. I love the idea of having a 42t rear cog to go on my 1x8 setup.

I had some time tonight, so I made it a happen.

Heres how I did it:

My idea was to make a 34t cog that you can bolt a chainring too.

Here you can see the difference between the 34 and a 42. Pretty big jump, but not too crazy.



I went to my hardware store and got some steel washers that are the same thickness as a cog and have the right size hole for a chainring bolt:



The hardest part was trimming the cog to flush fit the washers. The steel is HARD. Here is the cog trimmed sitting on top of the chainring:

[blown240]

Next I bolted everything to an old ring and tack welded them in place:



Once it was tacked and secure, I welded the washers soled from both sides and ground the welds smooth.



I found these nylon spacers at the hardware store for cheap. They may not last, but I though I would give them a try. They are the perfect thickness for an 8 speed cassette:



I bolted the rings together and this is what I ended up with:





Time to see how it "stacks up". First I used a bottom bracket washer and an 8 speed spacer to get the rings where they should be.



Here they are on the wheel, with a 28 in the 3rd place. As you can see the chainging bolts dont interfere with the chain on the 28 at all:



Here is the whole cassette. Its an 11-42 8 speed. Ya the gaps are HUGE, but I am willing to deal with that:

[blown240]

Here it is on the bike:








I havent had a chance to ride it yet, but on the stand it shifts pretty good. The 11-34 cogs are perfect, the 42 rubs a tiny bit on the upper derailer pulley. I need a longer screw on the back of the derailer. If it was 1/8 inch longer the derailer would totally clear the pulleys. A 38 or 40 tooth ring would probably not need this.

Total weight gain was about 80 grams. Once I know it works, ill lighten it up like I did my chainring...

[randan]

Great mod! Let us know how it rides!

[customfab]

Curious to see how long it lasts.

[blown240]

Ya me too. Part of me thinks its going to be pretty flexy, but its a fun project and totally worth trying. It only cost me about $6 because I already had most of the parts.

[rydbyk]

Skills! Cool idea. How is the jump from your lowest cog to your new lowest 42 cog? Seems like a pretty large jump visually?

[bikeabuser]

Subscribed

[blown240]

It's a pretty big step up, but it still shifts ok. If I had a friction a shifter I would almost add it as a 9th gear. I think there would be room for it.

[irv_usc]

could you make use of the 38 or 42 tooth cog that's available on ebay? it seems like it might last longer than this if you can get the spacing right.

[blown240]

Hmmm. I don't know. I didn't even know about the eBay ones.

[rydbyk]

pic? link? thx

[customfab]

Ya me too. Part of me thinks its going to be pretty flexy, but its a fun project and totally worth trying. It only cost me about $6 because I already had most of the parts.

I'm guessing your washers are going to rip out at the first sign of serious torque. Since you didn't bother to grind the plating off and your ground down the weld they are pretty minimal.

[blown240]

Well I took it for a short ride today. Everything is working VERY well. The shifting onto the 42t is a little slow, maybe 1-2 seconds instead of instantly. I am using 8spd XT rapid fire plus shifter with the matching derailer.

Tomorrow I am leaving for NV to ride Bootleg Canyon and Red Rock Canyon. That will be the real test...

[gmats]

Love it, great ingenuity. Good luck, hope it all works out.

[blown240]

Just got back from a loop around Blue Diamond in NV. It was GREAT!! This setup works even better than I thought it would. Shifting onto the 42t was perfect, shifting off if it was a little slow, but doable...

I had no issues with mis shifts, or chain suck. The gearing of the 32-42 is about as perfect as it could be. I had plenty of granny gear.

All in all, I am 99.5% satisfied!!

[blown240]

OK, So I want to address the issue with this setup not liking to come off of the 42 onto the 34.

I am thinking of shaving the top off of a couple teeth in 2 spots on the 42t cog. Hopefully that will give me a gate for the chain to drop thru.

Right now if I want a fast shift off of the 42 to a higher gear then I need to shift down to the 28t. Other than that this setup if GREAT. I rode Bootleg Canyon on it Sunday and it really performed good.

[customfab]

You might try twisting the teeth first. Shift ramps and tooth profiles seem to be a cryptic science but with the cassettes of yester-decade they would take the straight cut teeth and rotate some of them so the unloaded side of the tooth pointed down the cassette. Might be worth a shot before you grind on them, as you could always do that as phase two.

[blown240]

Ya thats a good idea!