My road bike has elderly 7800 Dura Ace 10 speed gearing on it. Double chainring 52- 39, 28 – 11 cassette, short rear mech. I intend to have a go at Everesting a hill right outside my front door. It’s short (will take 150 ascents) and pretty steep. Too steep for me to use that gearing for 15 hours plus. So I want to lower it obvs. However, this will be a one off occasion so it will be a temporary change and I can’t justify chucking loads of money at it.

Any reason why this wouldn’t work: use a cheap old triple Shimano road crank off eBay. Chain on 22 chainring and bypass the front mech. Get a narrow wide chainring to keep chain on, although this shouldn’t be a big problem I hope. It’ll be one gear going up, turn and freewheel back down.

What am missing?

If you want cheap, I’d get an old road triple, use the middle or small ring as you prefer and keep the derailleur on as a chainguide.

Edit, oh hang on, you’ve already done the road triple bit, but my front mech suggestion stands.

22t narrow wide chainring with the same bcd as your triple probably doesn’t exist. You’d need to shorten the chain to keep some tension on the rear mech. If you were on 22-28 I think the chain would still touch the front mech so better to remove the mech altogether if you aren’t going to use it.

Is duraace 7800 HT2? Old Shimano triple might be taper or if it is HT2 you frame might not have clearance for the space 3 rings take up

use a 10sp mtb cassette (you may have one but easily could borrow one) plus maybe one of those roadlink things for the mech ?

10 speed MTB cassette (11-34) and 9 speed MTB rear mech will work with what you have. I don’t know if that’s low enough for you.

The reason I was going to bypass front mech was it busy set up for 39t double. To get it in far enough and low enough for a 22t inner would be difficult I think.

Can’t use a different cassette in it as 7800 used a unique freehub (of course) 🙄 and cassettes are like hens teeth and I’ve got the lowest I can get.

Don’t want to change rear mech as it is only a temporary change. If I was doing that I’d have to change mech, cassette and rear wheel y’see.

It is HT2. Don’t mind shortening chain.

Borrow an alternative rear wheel with a 34 or 36 cassette and put your B screw in from the other side to give clearance.

Assume you won’t be using more than a few gears, if so you might not even need a longer chain.

How steep is the hill? On the beacon i was in a 34-30 (10% incline) the whole day and it was spot on.