[Closed] Bike geometry maths check

Yes, reach,  the horizontal distance from BB centre to head tube centre.  I can't even begin to understand what effect all of these dimensions and angles have when you put them all together so I just have to ride a bike and think 'do I like it?'

looking at a new bike. my current bike has a top tube length of 630mm and I run a 70mm stem, it fits perfectly.

I've said all my last 4 bikes have fitted perfectly, each one has been slightly longer than the last one!! Worth flinging a leg over the bike or at least something with similar modern geo to confim how it feels

top Tube length only really matters when you're seated - you need to compare stack and reach to understand what the differences are really.

I did this recently between two bikes and found it to be a far better comparison tool than TT length by itself.

Also be aware that a significantly slacker head angle will make a big difference to how a bike feels too.

Yes, reach is what you want to compare as the top tube length is affected significantly by the set angle as well as the length of the front end.

Well ett and seat angle have a major effect on how a bike feels when sitting down climbing, it depends what you are interested in.

as said, if the seat angles on the two frames are exactly the same, then yes 630+70 is equivalent to 650+50.

But a better measure these days is reach, as seat angles are often all over the place with curved tubes etc. Reach is from the BB forwards, so it basically insensitive to seat angles issues.

It's not always mentioned but backsweep on the bars also affects reach. If you run a lot of backsweep then you'll need a longer stem to compensate, and vice versa.