TT bike or road bike for a hilly 70.3? The hills take less off it than you’d think3 October 2026 · reading it nowOn the flattest 70.3 in the library a TT bike saves our rider 16:25. On the hilliest it still saves 15:30, or 11:28 if they sit up on every steep descent. What a hilly course really changes.
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Blog · 3 October 2026 · 3 min read

TT bike or road bike for a hilly 70.3? The hills take less off it than you’d think

Ask around before a hilly race and you’ll hear the same advice: leave the TT bike at home, the hills will cost you more than the aero saves. It sounds right. The numbers say it’s mostly wrong.

Three courses, one rider

Here’s one rider on three IRONMAN 70.3 courses from the site’s library. Eagleman is the flattest of the 123 IRONMAN 70.3 courses in the library, with 3 m of climbing. Goa rolls. San Salvador is the hilliest of them all, with 1,341 m in 90 km. The rider weighs 75 kg, rides a 9 kg bike and holds 200 W normalised, harder up the climbs and easier down them. On the TT bike they sit up on anything steeper than 5%.

How much faster is a TT bike on a hilly course?

On Eagleman the TT bike is worth 16:25. On San Salvador it’s worth 15:30. The hills took about 55 seconds off it.

Why so little? A climb is slow, and at climbing speed the air hardly matters on any bike, so that’s not where an aero position earns its keep. It earns it on the fast road, and even San Salvador has plenty of that: less than a tenth of it is steeper than 5%.

Sitting up on the descents

There’s a catch, and it’s on the way down. The predictor has no brakes. Left alone, it keeps the rider on the extensions down every descent, as fast as the road lets them, and nobody rides a mountain like that. So we asked it again with the rider sitting up on anything steeper than 3% downhill and never going faster than 55 km/h, on both bikes. Now the TT bike is worth 11:28 on San Salvador. That’s what the hills really take: about a third of what the TT bike is worth on the flat.

What the numbers can’t weigh

What the predictor can’t tell you is whether you can hold the position for the whole bike leg, whether your back agrees, and how you handle a TT bike on a twisty descent. Those are real, and they’re yours to weigh. What it can tell you is the size of the prize: on a 70.3, somewhere between 11 and 16 minutes, whatever the course.

Each time rests on a drag area taken off a list, not measured on you. If your own position is better or worse than typical, the times move together and the difference between them moves much less. The table says by how much, under each figure.

Check your own course

Every course page in the library already answers road bike or TT bike for this rider, and takes your own watts and weight. In the predictor you can set your position for each kind of road.

What the predictor said, set-up by set-up

What the TT bike saves this rider, course by course

Eagleman, flat16:25 faster

Goa, rolling16:14 faster

San Salvador, mountain15:30 faster

San Salvador, sitting up on the steep descents11:28 faster

A model, not a measurement. What it can show depends on the numbers above (what you typed, and what your files recorded) and on assumptions this page states; anything it cannot stand behind says so rather than drawing a tidier line.

Road bike or TT bike, three courses

	Eagleman · flat, 91 km, 3 m up	Goa · rolling, 92 km, 334 m up	San Salvador · mountain, 90 km, 1,341 m up
Road bike	2:44:38	2:55:10	3:19:11
TT bike, sitting up above 5%	2:28:13 · 16:25 faster, ± 0:48	2:38:56 · 16:14 faster, ± 0:48	3:03:41 · 15:30 faster, ± 0:50
Road bike, kept under 55 km/h	2:44:38	2:55:15	3:19:23
TT bike, sitting up below −3% too, kept under 55 km/h	2:28:13 · 16:25 faster, ± 0:48	2:39:48 · 15:27 faster, ± 0:46	3:07:55 · 11:28 faster, ± 0:33

For a 75 kg rider on a 9 kg bike holding 200 W normalised, harder up the climbs and easier down them, in still air and with no stops: the predictor, asked on 3 October 2026. The give or take on a saving is what a drag area taken off a list rather than measured could put it out by.

Try the TT bike on San Salvador