Notes on mtb frame geometry

I’ve been riding frame #5 for awhile now as my primary bike and have changed forks to a ‘free-to-me’ 140 carbon Cannondale Lefty set up for a 29er.  It’s a totally different experience. I have to say, i don’t like how it looks but it’s a nice fork going downhill–it’s pretty darn smooth. Very little stiction but I hate that there’s no lockout. Seems weird for an XC fork to lack lockout. But anyways…

The frame was built with an already slack at 69HT angle and 72 ST angle with 432mm chainstays and 685mm front-center.  It was set up for a Fox F29 with a 501mm axle to crown and 51mm offset which gave it around 84mm of trail. When I switched to the Lefty (and bartered the fork off with the rest of the Trek 69er to my friend Big Daddy) it changed the geometry pretty drastically.  The new HT angle is 68.6deg and the new ST angle is 71.6deg. The fork has a 508-ish axle to crown and 46mm of offset changing the trail to a whopping 94mm! That’s a DH bike’s trail figure. The BB raised up a few mm’s too and the setback increased even if the front-center decreased a few mm’s.  The one thing that y’all should know if you don’t already is that millimeters matter.  Honestly…you can feel the difference when just a few mm’s change on trail, setback, wheelbase, front-center, etc. The bike handles way differently with this fork than with what it was designed for.

So needless to say, climbing on this bike with the Lefty is challenging.  It goes downhill pretty well though. The front wheel wants to wheelie without me wanting it to. Steep uphills are particularly annoying.  The Lefty also has this stupid habit of always pulling to the right…so riding without hands is pretty much not a good idea.  Not to mention that the front wheel is so ‘light’ with 94mm of trail and such a slack HT angle that you get the wobbles almost instantly.  So basically, this has been a fun ‘experiment’ but I really want to get this fork off my bike!

The only thing I have to replace it with right now is a me-built rigid fork I made for a similar geometry bike. The fork is 468mm axle to crown with 51mm of offset.  This will change the bike’s geometry to around 69.8 HT angle and 72.8 deg ST angle with 81mm of trail. This is only ~3mm less than what I had with the Fox fork for trail, but the HT angle is steeper and the BB is lower at 11.8″…pretty low for a MTB IMO. These changes will be almost preferable especially for the new riding terrain in norcal…not as many rocks to bang the pedals on (lower BB is ok!), and less setback, steeper HT angle, and less trail for a more forward/XC riding position (quicker steering and front-wheel weighted better for XC riding).  But of course, no suspension.  No matter, I’ve always preferred rigid forks anyways.

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