Considering cassette change ... advice please!


New Member
May 9, 2016
Would like some advice whether I should consider changing out the rear cassette on my bike, in anticipation of an upcoming triathlon (Ironman 70.3) which is about a month away. The Ironman course is known to have some rolling hills...I think a few may be steep, but not a huge amount of total climb -- approx 1000m elevation gain over 90km.

I currently own 2 bikes - my road bike is geared with a compact (50/34) chainring and a 12-30 rear cassette (Shimano 105 components). My tri bike is geared with a standard (54/39) chain-ring and a 12-25 rear cassette (Shimano ultegra components). My current plan is to ride the Tri bike for the race but I have noticed that every time I go to do a training ride that involves hills, I instinctively reach for the road bike. Its easier to ride both on the gearing (I can get up most hills, albeit slowly) and also on the handlebar locations (shifters & brakes in same spot). On the road bike, I lose my pedal power at around 50-55km/h (pretty easy to hit esp with a little hill help, but not sustain). My top speed on the Tri bike is higher & I don't think I really ever hit a point where I can't pedal any more.

I am absolutely a weak climber (and faster on the descents) vs the rest of the group that I ride with where I am pretty well matched otherwise. Thanks to body weight for that!

This has got me wondering whether I should consider putting a different rear cassette on the Tri bike to give me one or two more low gears on those hills. I have looked at a few gear ratio charts online but I don't really know how to interpret the numbers.

Thx for any guidance on this!
Different people prefer different ways to relate to gear ratios.
I like Sheldon Brown's online gear calculator set to kmh@100 RPM.
But for your particular use, maybe gear inches, or meters development might work better.
Simply use a gear calc to determine the gear inches for your road bike, then try a couple of different cassettes to see what cassette you'd need to get the same values.
I might be off base here, but don't most riders climb better on a road bike with drop bars than on a tri or TT bike with full blow aero bars (gearing being the same for purposes of comparing bike types)?

If it's the lower time your looking for you can calculate the time spent climbing quicker on road bike vs. rolling faster and more aero on your tri bike.

Adding a 28 to your tri bike will 'probably' (not knowing your climbing style, endurance, etc.) get you over most of the climbing. It's a big change down from a 25. Adding a 28 and a 30 for a bailout gear will definitely get you some climbing gears with your 39 ring.

Does the organization putting on the race publish the course profile? If so, compare the length and steepness of the climbs to what gearing you use on similar training climbs. This will help with your gearing needs. Also, study your best climbing positions on both bikes. Do you prefer climbing on the flats? The hoods? Do you like being up out of the saddle and stomping the climbs for as long as you can? And which for the shallow, long grades and which for the short, steeper stuff? Which types of hills will the Ironman course present the most of? Which types of climbing on the course will slow you the most?

3300 feet of climbing in what?...56 miles is a fair amount of going up. That's definitely rolling terrain I'm my book.

If you really think you are a not so hot climber the larger cassette is probably a good long term investment for the tri bike. Another option is fitting a set of aero bars to your road bike. Dialing in your position to match you tri bike may or may not be possible, but you can probably get it close enough to keep some speed on the descents and flat parts of the course. And you get to keep your hand position on the hoods for climbing and still use some of the flat portion of the bars maybe.

As you can see there are a lot of variables and gear ratios are only one of them. Good luck in your Ironman.
Cassette changes are cheap, quick and reversible wouldn't hurt to try. I second CB's alternative of slapping a pair of clip on aerobars to your road bike and adjust the seat and handlebar positions to a more tri-like fit.