Chainring and cassette choices on TT bike

Having dabbled in triathlon (and cycling in general) for many years on my (cheap and aging) road bike, I did the n+1 thing recently and picked up a second-hand Stealth. I’m still wobbling about trying to get used to TT bars and the super deep front rim but am assuming that will become less terrifying with practice.
So, drive train is all fairly well-worn and I’m thinking about replacing parts. Currently, the Stealth is 55/44 with 12-25, 10 speed cassette. My road bike is 52/39 with 14-28, 7-speed freewheel so that’s my only comparison. Have always got on okay with that - occasionally spin out on gentle downhill with a tailwind. Cycling is my weakest discipline - average just over 30 km/h in races.
I’m thinking about putting 53/39 chainrings on the Stealth (there’s a new 105 5700 53T and Ultegra 6700 39T on cheap as chips sale atm and I think I can put them together on the FSA carbon crank).
Thinking about something like 12-27 cassette.
I think I’ve got 105 (5600) front mech and Dura-Ace RD (7800???) rear mech. I’m a bit vague about total capacity but the rear mech cage looks short to me.
I also think if I’ve got a 12T smallest sprocket on 10 speed that I can’t put a cassette with an 11T sprocket on because the splines on the freehub won’t be right. I don’t particularly want 11T but there are lots of them going cheap on eBay.
That is the sum total of my knowledge. Have I missed any compatibility issues? Does any of that sound daft? Many thanks for any advice.

Wow, that’s a big chainset on it. Personally, I’ve downsized my chainrings over the years, and now I’m on a 50/36 ‘hybrid’ I made up (50 is a Q Ring). I’m still riding the same sort of bike splits. Smaller chanrings means you can use a smaller cassette, and achieve the same gearing range…for less weight as an added bonus. Plus you don’t need to worry about changing/adapting your Rear Mech for added range.

If you look at gearing charts on sites like Sheldon Brown, you’ll see there’s no need for huge chainrings, and to spin-out something like a 50/12 gear, you’re going to need to be going fookin’ fast! Faster than pretty much any AG triathlete needs.

So I would definitely consider ‘downsizing’ your whole drive chain if you’re in that position. My TT bike is: 50/36 and 12-25 (12,13,14,15,16,17,19,21,23,25) 10sp. The 50T gives me a good chain-line, which is also another thing to consider for both longevity of parts, and friction loss.

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Perfect for Cheshire.
So it depends what courses you’re doing.

That 55 front goes in a perfect chain line to a 14/15T rear.
At ~100rpm, 265W, that’s ~1hr for a 25TT.

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It’s been discussed on another thread recently, but I’m a big fan of 52/36. With a decent sized cassette on the back it gives you pretty much the best of both worlds.
I’d also recommend Sheldon Brown for gear calculators and general compatibility info if you’re new to this.

Granted, but how many (triathlon) events are drag-strips in this country? Plus, riding 265w for an hour is unlikely for most AG triathletes; certainly in a triathlon!

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2019s 113 :rofl::rofl::rofl:

I must have changed gear four times.
Once at the end of each lap.

55-14 nearly all the way.

Haha, that did spring immediately to mind! But I did manage on my granny gears :smile: That course was way too flat for me. I was going to do Outlaw Bowood this year, but now 2021 I guess!

This, especially when the OP says he generally rides 30kph … 40kph for an hour 25TT is a different ball game.

Personally, I’d agree with @JaRok2300. This is what we advised @Poet on another thread. I have 52/36, and can switch between rear cassettes as a course suits. I’ll go 12-25 for something like Bahrain 70.3, whereas I switched to 12-30 for Nice 70.3. For something in between I’d probably have the standard 11-28.

Ultimately, at the speeds the OP is talking about, you could even go to a traditional compact for ease…that will work on any course for almost everyone. Only cycling powerhouses might find it too small, and that’s not the OP from what he has said.


Ha ha ha :see_no_evil::rofl:


I’ve now got;

105 50/34
Ultegra 52/36
Dura Ace 53/39
Dura Ace 55/42

So I’m covered :+1:t3:

I would go with a 50-34 chainring and 11-28 cassette

Your DA7800 rear mech has a max cassette size of 28

Given your average speed, I think that you will appreciate lower gears on the climbs. It is better to have extra low gears than extra high gears.

What depth is your front wheel? As you are learning to ride your TT bike, may be worth practicing on a shallower front

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Too much choice on race day

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If it’s an 80mm front rim, you’ll find that very twitchy in crosswinds if you’re not used to it.

Thanks for all the suggestions.

Yes, it is massive. Before I picked it up I didn’t think I’d be able to actually see the difference between 52T and 55T but it’s like a dinner plate strapped to the crank!

Yes, Sheldon Brown calculator is really useful.

I suppose the thing I need to get my head round is that having a 10 speed cassette that goes down to 12T is quite a big change from a 7-speed that only goes to 14T. Two teeth at the back has a much bigger effect than two at the front. In my head, I wouldn’t want anything less than a 52T on my road bike but, actually, could go smaller on the TT because of the 10 speed cassette. My theory is the road bike biggest gear is 98" and I probably want one more gear, so 50 - 13 would give me about 101" and 50 - 12 about 109", which is more than enough. Sounds like a plan if I can find a worthy 50T going cheap (perhaps obvious given my main bike has a 7-speed freewheel but I’m somewhat tight about spending on sport).

Hadn’t thought about 34T inner. 34 with 28, or even 27 feels like a very low bottom gear. I rarely use the 39 with 28 I’ve got on the road bike, but I guess the thing about having so many more sprockets is that it’s not so crucial to use them all. Will consider.

Agree that my speed needs are modest. Certainly wouldn’t hold 55-14 on the flat for very long :frowning: Entered #51Fiver this year in the vague hope of getting under 2:30 by managing to average 33 km/h on a flat course, but that’s about my limit.

Yes, front is something like 80mm (it’s the older Stealth with the Planet X carbon deep section tubular wheels). More freaked by thought of hitting a bump on the aerobars than anything else but think I can run my road wheel on the front if I change the brake pads over. That might just feel more familiar as well as less twitchy in the wind. Good call.

You don’t seem to have a 44T, though :wink: When I take it off, do you want it?

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Ah - don’t think 34T will fly as I’ve got 130mm BCD cranks, and looks like they’re all 110mm below 38T. Good thought though, if I was prepared to contemplate a whole new crankset. Looking like 50/39, which I can do with new parts for £40. Then a 12-28 cassette. That’s still as low as I have at the moment so should be okay.

Found a sensible thread for this. New bike has 53/39 and 11-28. Road bike is 50/34 and 11-28 which I’m pretty happy with, it’s got me up 18% gradients before.

I’m thinking that rather than change the crank which is in good nick and pretty expensive to change, I could get something like a 14-32 cassette, Miche seem to do one. It gives nice close ratios through from 14 to 20 and the 39/32 would give me the same bottom gear as I’ve currently got 34/28. Any downsides? Or have I missed something.

Will your rear mech handle a 32, if not there are ways round it like a wolf tooth rather than replace it?

I assume the smallest chainring you can use on that spider is a 52? (130bcd). That being the case, not worth changing it.

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36 is quite a bit different to 39 though?

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I thought all Shimano cranks are now 110BCD making rings are fully swappable. I’ve just looked and the Ultegra rings will cost me around £115 and so it would probably be better to just buy a compact crank and sell this one. Different in cost would be less than that (much less if I could find a second hand one).