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I’ve just bought a Planet X tempest, the reach in medium is 390mm
Im coming from a Croix de fer, size small, with reach of 375 and a 90 mm stem
Is it as simple as I need to get an 80 mm stem to have similar fit (75mm ideally), or are there other factors to consider?
seat angle is half a degree steeped on the tempest, and stack is around 15mm lower
ta
Is that a road bike? You probably need to give effective top tube a little more weight than reach.
It’s a gravel bike but I’ll be seated most of the time so effective tt makes more sense! In which case the tempest is only 5 mm longer
Which is good news as I should be able to continue to use my existing redshift stem, as I wouldn’t want it any shorter than it currently is. Feels spot on right now
Put the geometry of both bikes into https://www.bikegeocalc.com/
Then you can play with stuff like stem angle, rise, and length to duplicate position. Don’t ignore the reach of the bars as well.
The Tempest will put your C of G a bit further fwd of the BB than the Genesis, I'd stick to the same stem for now and see how it feels - a lot will depend on the wheelbase and how balanced you feel when sat on it.
TBH unless they're trad geo road bikes I think trying to carry one fit over between bikes creates problems as often as solutions.
Don't forget bar reach as well. Most drop bars have similar-ish numbers, but 73mm to the hoods vs 80mm to the hoods will make a difference too.
Seat tube angle is irrelevant because saddle to BB location will be the same. Measure nose vertical distance behind BB spindle (I’d guess 7cm) and replicate that and BB to saddle top height. Then just measure tip of saddle to handlebar clamp on the stem and replicate with another stem. Also replicate saddle height to bar height drop. A lower stack height might need spacers to get the same drop from saddle. But the distance from saddle nose to stem clamp axis should be the same for the correct stem. Also watch the shifters and bar reach this can change your on the hoods position by up to 2cm either way with longer shifters and deeper/rounder drops!
Handling might change due to weight distribution over the front wheel, but fit should not.