I’m looking at a new frame which has 30mm longer reach than my current one, but slightly shorter top tube. I currently run a 60mm stem, the new frame is designed for a 35mm stem

doesn’t this mean the new frame would feel shorter when seated, due to shorter stem and top tube?

Too many variables there… Seated depends on seattube angle and length of seatpost… But if you put your seat the same distance behind the BB as on the old bike, and you’ll probably want it in the same position relative to that because that’s what gives you the same pedalling feel when seated, then it’ll feel about the same (it’ll be effectively 5mm longer to the bars). And that’s an amount that you can lose in the rounding just with a couple fo steerer spacers or a different roll on the bars.

But if it’s got a really slack or bendy seat tube then that might be tricky to get the same balance. Otherwise it’ll be grand

I went from a MK1 Cotic Soul, large, to a current Giant Trance, medium. The shop assured me that I (5’11”) was spot on for the medium. The distance between saddle and bars is massively less with the Giant and riding seated is noticeably different, but not wrong. Quite nice. Once out of the saddle and that becomes irrelevant. I’m absolutely loving the bike and my initial doubts are gone.
Get with the new, Grandad, as lots of people often have to tell me.

I have the same query… if I were to replace my bike (which fits well) with something else with the same reach, everything has both a shorter stem and a shorter effective top tube. Could compensate 1-2 cm with bars and saddle rails, but for seated climbing I can’t see how the cockpit won’t shrink around an inch? Don’t yet know whether it matters or whether I therefore need a bike with a bigger reach, (given the shorter stem) I presume the latter, but will have to test ride?

On a drop bar bike, take note of the frame reach compared to a known reach that works for you, but also take account any stem and/or bar reach differences between the bike you have and the one you are looking at.

The above applies to flat bar bikes too, but normally, I wouldn’t expect too much reach variance in bars compared to drop bars.

Just flipping the stem and changing the amount of spacers over/under it can have a surprising difference on reach and stack figures. For example, I originally swapped the supplied 80mm +6 degree stem on my Wazoo for my 110m +35 degree, it raised the bars ~55mm and shortened the reach by ~10mm (which helped my very weak lower back at the time). I’ve had a 110mm -8 setup for 18+ months now, which is ~36mm longer reach and ~12mm lower, which has made the steering far more stable on the roads and my back is much happier than it used to be.
Handy stem calculator http://yojimg.net/bike/web_tools/stem.php

The new frame has a 2 degree steeper seat tube angle. 77 vs 75

I know a lot of people seem to think they need to be stretched out to be in an efficient pedalling position – but I have no idea why! Can anyone explain?

The more strecthed out you are, the less aero drag your body creates. This is why for the same effort in the same conditions* and with same tyres fitted, you normally travel noticeably slower on a mountain bike compared to a road bike on a flat piece of tarmac.

* Would be less difference in a direct tailwind.

The new frame has a 2 degree steeper seat tube angle. 77 vs 75

I think this is they key change in mtb geo… I have no idea whether, with my hips in a totally different position whether I’ll mind a much shorter cockpit

For so many bikes these days the traditional ways of communicating frame geometry is completely useless. For a traditional diamond frame it’s ok, but with different actual and virtual seattube angles it’s meaningless with your saddle anywhere other than level with the top of the headtube (the level at which top-tube is noramlly measured). Sorry of that doesn’t help… what frame are you considering, and which one are you comparing it too?

the new frame is designed for a 35mm stem

Frames aren’t really “designed for” a stem length. You can run whatever length stem you feel comfortable with. A test ride before buying would be the obvious advice, if it’s possible.

The00 – 2016 intense primer vs 2010 orbea occam, both large

Regarding designed for stem length, I guess I meant what the full builds come with