The Numbers on Your Bike That Are Worth Knowing
Crank length, stem length, and seatpost offset — the three numbers nobody mentions at the bike shop, and why they matter more than you'd think.
Introduction
You’ve sorted your saddle height. Your handlebars feel okay. But something still isn’t quite right — maybe a hip twinge at the top of the pedal stroke, or a weird knee tracking issue that saddle adjustments alone haven’t fixed, or a sense that the bike still doesn’t feel like it was made for you.
This is where we get into the stuff that most beginner guides skip entirely. Crank length, stem dimensions, and seatpost offset are not glamorous topics. Nobody mentions them at the bike shop. But they’re the difference between a road or gravel bike that fits and one that almost fits — and on a $1,000+ bike, almost doesn’t cut it.
How to Check What You Currently Have
Before changing anything, find out what’s already on your bike. All three specs are stamped directly on the components.
Crank length: Look on the inside face of the right crank arm — the one on the same side as the chain — near the pedal thread hole. It’ll read something like 165, 170, or 172.5 (millimetres). Wipe the crank clean first if it’s dirty.
Stem length: Look on the top surface or side face of the stem body between the two clamp points. The length is stamped directly on the metal: 80, 90, 100, 110 and so on (millimetres). The stem angle (+6° or +17°) is often stamped nearby too.
Seatpost offset: Look at your seatpost from the side. If the saddle clamp sits directly above the centre of the seatpost, you have an inline (zero setback) post. If it sits noticeably behind the post centre, you have an offset post — typically 20–25mm.
Crank Length
What it is
The cranks are the arms that connect your pedals to the bottom bracket. Crank length is measured from the centre of the bottom bracket axle to the centre of the pedal axle. Common lengths on road and gravel bikes are 160mm, 163mm, 165mm, 167.5mm, 170mm, 172.5mm, and 175mm.
Why it matters especially for women
Crank length guide by height:
| Height | Recommended crank length |
|---|---|
| Under 155cm | 160–163mm |
| 155–163cm | 163–165mm |
| 163–170cm | 165–170mm |
| 170–178cm | 170–172.5mm |
| 178cm+ | 172.5–175mm |
Is it worth changing?
If you’re under 163cm and your bike came with 170mm+ cranks, yes — it’s worth exploring. Cranks aren’t a cheap swap, so it may be something to do when you replace components anyway. But if you’re experiencing persistent knee pain or hip tightness that saddle adjustments haven’t resolved, this is the next place to look.
Stem Length
What it is
The stem connects your handlebars to the steerer tube. It determines your reach — how far forward you extend to grip the bars. Stems are measured in millimetres: 60mm, 70mm, 80mm, 90mm, 100mm, 110mm, 120mm. A shorter stem reduces your reach.
How to assess if yours is right
The clearest sign your stem is too long is upper body tension — neck pain, shoulder ache, hands taking too much weight, elbows locked straight. Your elbows should have a relaxed, soft bend in your normal riding position.
A rough at-home check: sit on your bike in your normal riding position and look down at the front hub. If the hub appears behind the handlebar from your eye line, your stem may be too long. If it appears in front of the bar, it may be too short. Lined up directly behind the bar is roughly where you want it.
Stem rise and angle
Stems also have an angle — usually ±6° or ±17°. A stem angled upward raises the bars and slightly reduces effective reach. Flip it upside down and it drops the bars and extends the reach. Most stems are reversible — try flipping before spending money on a new one.
Seatpost Offset (Setback)
What it is
A seatpost isn’t always a straight tube — many have an offset, or setback, which positions the saddle further behind the centre of the bike. Standard inline (zero setback) posts sit the saddle directly above the post clamp. Offset posts position the saddle 20–25mm further back, affecting your weight distribution.
Which do you need?
The easiest way to tell is to check your saddle rail position. If your saddle is slid as far back on the rails as it will go and your knee is still tracking too far forward over the pedal, you need more setback. Conversely, if your saddle is slid as far forward as it will go and you still feel too far back, an inline post brings the saddle closer to the front.
Putting It All Together
None of these adjustments exist in isolation. Changing crank length affects saddle height. Changing stem length changes how the bike handles. Adjusting the seatpost changes weight distribution, which may make you want to revisit saddle fore-aft. Make one change at a time, ride for a few sessions, then evaluate before making another change.
Frequently asked questions
Are my cranks too long for me?
Start by finding out what length you currently have — look on the inside face of the right crank arm near the pedal thread hole. If you're under 163cm and your cranks are 170mm or longer, they're very likely too long. The symptom to look for is compression or tightness at the top of the pedal stroke — your knee coming up higher than feels natural, hip flexor tension, or an ache at the front of the knee that saddle height adjustments alone haven't resolved.
Does changing crank length affect my saddle height?
Yes — and this is the step most people skip. Shorter cranks reduce how far your foot travels at the bottom of each pedal stroke, so your saddle needs to come up slightly to compensate. The general rule is 1mm of saddle height change for every 2.5mm of crank length change. Going from 172.5mm to 165mm cranks means your saddle should go up roughly 3mm. Don't skip this — changing cranks without adjusting saddle height means you're effectively riding with the saddle too low.
Is my stem the right length?
The clearest sign your stem is too long is upper body tension — neck pain, shoulder ache, hands taking too much weight, elbows locked straight instead of softly bent. Find your current length by looking on the top surface or side face of the stem body between the two clamp points. A rough fit check: sit in your normal riding position and look down at the front hub — it should appear directly behind the handlebar from your eye line.
Do I need an offset seatpost?
Check your saddle rail position first. If your saddle is slid as far back on its rails as it will go and your knee is still tracking too far forward over the pedal, you've run out of adjustment room — an offset post would help. If your saddle sits comfortably somewhere in the middle of the rails with room to move either way, your current post is probably fine.