Glossary and FAQ

Glossary · FAQ

Glossary

Position. How you sit on the bike, set by three contact points: saddle, bars and pedals. sedimi.bike describes it by the bar position.

Bike fit. A session with a bike fitter to set up your position. You leave with a fit report, and its numbers are what you enter here.

Frame geometry. The frame’s lengths and angles. Manufacturers publish them as a chart with a column for each size.

Bottom bracket. Where the cranks spin, at the bottom of the frame. Every distance here is measured from its centre.

Head tube. The short tube at the front of the frame that the fork steerer runs through.

Stack. How tall the frame is at the front: the vertical distance from the centre of the bottom bracket to the top of the head tube. More stack means the bars can sit higher.

Reach. How long the frame is at the front: the horizontal distance between the same two points. It’s not the top tube length. Stack and reach go together, so only compare reach between frames with similar stack.

Head angle. The angle of the head tube from horizontal. Also called head tube angle.

Steerer. The tube on top of the fork. It runs up through the head tube and carries the spacers and stem.

Stem. Connects the bars to the steerer. Length is measured from the centre of the steerer to the centre of the bar clamp. The angle is relative to a line perpendicular to the steerer, not to the ground, which is why a −6° stem still rises slightly on the bike.

Spacers under the stem. Rings on the steerer below the stem that set the bar height. Because the steerer leans back, 10 mm of spacers lifts the bars by almost 10 mm and brings them about 3 mm closer. The steerer length limits how many you can fit.

Headset cover (top cover). The cover over the upper headset bearing, on top of the head tube. The spacers or the stem sit on it, so it sets the lowest the stem can go. Its height varies from frame to frame. Not to be confused with the top cap, the bolted cap above the stem.

Bar drop. How far the drops sit below the tops.

Bar reach. How far the bars extend forward: the horizontal distance from the centre of the bar at the stem clamp to the centre of the tube at the front of the bend.

Bar position (X / Y). The point sedimi.bike uses to compare bikes. X is the horizontal distance and Y the vertical distance from the bottom bracket, in millimetres. It’s the centre of the bar at the stem clamp plus the bar reach: the front of the bend, where the shifters clamp on. See How it works.

Cockpit. The stem and bars together.

Integrated cockpit. A one-piece bar and stem. Changing the length means replacing the whole unit, not every length exists, and width is often tied to length.

Build. One version of a model as the manufacturer sells it. The same frame usually comes in several builds.

Stock build. The parts the bike ships with: stem, bars, seatpost, cranks. They change with frame size.

Seatpost and setback. The seatpost holds the saddle. Setback is how far the saddle clamp sits behind the centreline of the post. A post with no setback is called inline.

Seat tube (c-t, c-c). Seat tube length. C-t is measured from the centre of the bottom bracket to the top of the tube, c-c to the centre of the top tube.

Tolerance. How far the bar position on a new bike may be from yours, in millimetres. The default is ±10 mm, roughly one stem size or one spacer. It’s a distance in any direction, which is why the chart shows a circle.

Fits, borderline, doesn’t fit. The result for each size. Fits: the bars reach your position with a sensible stack of spacers and the stock stem, or one slightly longer or shorter. Borderline: it only just works. Doesn’t fit: the frame is too low, too tall, too short, too long or simply too big for your position. The exact rules are under How it works.

Archive. Older model years. I only have their frame geometry, not the build.

Trail. The distance on the ground between the point where the steering axis meets the road and the front tyre’s contact patch. More trail makes the steering more stable, less makes it quicker. I work it out from the head angle and fork offset, with a 30 mm tyre on road bikes and 45 mm on gravel bikes. A slightly narrower or wider tyre changes it by only a few millimetres.

Fork offset. How far the front axle sits ahead of the steering axis. Together with the head angle, it determines trail.

BB drop. How far the bottom bracket sits below the line between the wheel axles. More drop puts you lower between the wheels and makes the stack slightly lower.

Wheelbase. The distance between the front and rear axles. Longer is more stable, shorter is more agile.

Chainstay. The distance from the bottom bracket to the rear axle.

Standover. The height of the top tube above the ground, midway along it. It tells you whether you can straddle the bike comfortably.

Bar rise. How far the tops rise above the stem clamp. Bars with rise put the hoods higher than bars without.

FAQ

Which frame size should I get?

The one that matches your position. Start from a bike that already fits you, or from your bike-fit numbers. For each model I’ll show which size fits and with which stem. Start on the home page.

Why no sizing by height?

Because how you sit depends on leg, torso and arm length, flexibility and riding style. Two riders of the same height often need different sizes.

Can you tell me whether my position is right?

No. sedimi.bike doesn’t judge your position. It finds bikes that match it. Whether the position itself is good is a question for a bike fitter.

I don’t have a bike fit or a bike that fits me. What now?

Get a bike fit before you buy. A position can’t be worked out from your height, or over the internet. A fitter will set you up on an adjustable fit bike or on your current bike, and you’ll leave with a report whose numbers go straight in here. If you’re near Brno in the Czech Republic, that’s what I do at Bikefit Space.

Where do I find my numbers?

In your fit report, look for the bar centre (X and Y from the bottom bracket, often labelled HX and HY) and the bar reach. If the report only gives the hoods, or measures from the saddle nose, ask your fitter for the bar centre. No report? Pick the bike you ride in the search, with its size, stem and spacers as they are on your bike, not as it was sold. I’ll work out the bar position from that.

My bike isn’t in the database. What do I enter?

Stack, reach and head angle from the manufacturer’s geometry chart. The stem length is usually printed on the stem, and you can measure the spacers with a ruler.

Two sizes fit. Which should I pick?

That’s up to you. It depends on where your saddle goes, how you want the bike to handle and how your position might change. I lean towards the smaller one more often than not, but that’s not a rule. If you’re unsure, ask a bike fitter or an experienced rider.

What does “borderline” mean?

The size only works with no spacers or with the maximum, lands just outside your tolerance, or needs a stem 30 mm shorter. Before you buy, ask the shop whether the bike can take the stem and spacers shown in the result.

Will I have to change the stem?

Not necessarily. Each result says whether the stock stem works or which length you’d need. To rule out stem swaps, choose “Stock only” in the search. On bikes with hoses routed through the headset, often only the manufacturer’s own stem fits, and swapping it is a workshop job.

What about integrated cockpits?

I only use the lengths the manufacturer makes for that model. If the cockpit would have to be replaced, the result says so. Check that it’s possible before you buy.

What about the shifters?

They’re left out. You’ll set them up on the new bike as you had them on the old one. Different shifters or a different bar shape can move the hoods slightly, but rarely enough to need a different stem.

What about the saddle?

It’s left out too. The result assumes your saddle goes in the same place relative to the bottom bracket as it is now. Before you buy, check that the seat angle and seatpost setback allow that.

Where does the data come from?

Geometry and builds come from the manufacturers’ websites, and every model page links its source. Some values come from bikes I’ve measured in my fit studio. Where a manufacturer doesn’t publish a dimension, I use a typical value and mark it with an asterisk.

Does it work for second-hand bikes?

Yes, with a caveat. For older model years I only have the frame geometry, and second-hand bikes have often had parts swapped, so the stem is my suggestion. On an archive model’s page you can enter the stem and spacers actually on the bike.

Which bikes are in the database?

Road and gravel bikes, currently 1371 models from 284 brands, plus older model years in the archive. No mountain bikes.

What does it cost?

Nothing. The site is free and has no ads. No brand pays to be listed, and I earn no commission on links.

I’ve found a mistake, or a model is missing.

Email me at marcel@bikefit.space. Every correction is logged publicly under Corrections.