Geometry question: shorter stack with same reach?

Question

I'm planning on building a new bike soon but I'm quite conflicted about choosing a frame. For now I'm riding a GT Grade AL 2015 that I use everyday to commute but since I'm planning on doing some bikepacking tours overseas, I want to have a lighter bike. Buying a carbon GT Grade might be nice but they don't sell them as frameset and the specs they are offering are... Uuuh... Undesirable.

Now then, I might buy a CX or gravel frame, both are okay as long they can fit at least 38C. The CX frame that I have my eyes on has a stack/reach of 561/374 mm and BB drop 57 mm while my Grade has stack/reach of 578/373 mm and BB drop 70 mm.

Would the differences in stack and BB drop make a noticeable difference? I would say I wouldn't mind a bit of racier position since I also have a road bike but I just need some more inputs. Also the CX frame costs like 50% less than the current gravel carbon frames that's why I'm leaning toward it. Plus higher ground clearance might mean I could fit 650B wheels there (hopefully).

Answer 1

Don't just look at the stack and BB drop, consider the steering geometry as a whole (chainstay length, BB drop, head angle, front-center [BB axis to front wheel axis]) which will affect how the bike handles; and the cockpit geometry (stack and reach) which will affect your position. Cockpit geometry and steering geometry are interrelated obviously, a bike with a long front-center will have a long reach for instance.

The Grade geometry is really long chainstays, long front-center and a slack head angle (FYI, I own one). The stack is quite high too. If the CX frame you are looking for has a very high 57mm BB drop I bet it has traditional CX race steering geometry as well: short wheelbase and steepish head angle. You'll very definitely feel a big difference between the Grade and and a trad CX frame like that.

I'd really hesitate to use a race geometry CX bike for touring. The high BB moves your center of mass higher up and the short frame and steep head angle make for quick steering - not what you want for loaded touring. I don't think you'll want a low stack that results in a leant-forward rinding position for long days in the saddle either.

There are plenty of gravel frames out there that are shorter, have 1/2 degree steeper head angle and a little less stack than the Grade. That's what I'd look for for a touring adventure/gravel bike.

BTW you don't need a high BB for 650b wheels, as the point of using those is to run larger tires (45mm+) than you can with 700c rims. The difference in whole wheel diameter between a 700c with 25mm tires and 650b with 45mm tires ends up being quite small.

Answer 2

There are three separate topics/questions blended together in the OP...

1. Fit:
   This is determined by a triangle comprised of the seat, bars and BB (assuming the same length cranks). IMO the two numbers which determine my ability to match an existing fit to a new frame are the reach and drop. If one has to use extreme stem rise and/or length then the frame (IMO anyway) is too small. If the Grade has a flat stem angle and few spacers under it then an additional 17mm of rise shouldn't be a problem. But note the headtube angle of the two bikes as this affects the stem rise. Gravel bikes typically have a more slack (smaller) headtube angle and this will naturally provide more rise.

2. Ride/Handling:
   The BB drop/height affects the ride but not the fit. Moving from 700c to 650b wheels usually results in a slightly smaller wheel diameter which drops the BB a bit and changes the steering. Generally speaking, in order the from most stable and predictable handling to quickest steering and lightest/fastest are the bikes labeled/marketed as: Touring -> Gravel -> CX -> Road Endurance -> Road Race.

Again that's just a generalization and there are lots of exceptions but there simply are no standards so it's all up to the marketing departments.

3. Touring Bikes: Pretty much any bike can be used for touring. If one is credit card touring and has support transporting most of one's gear then a road bike with a bit larger tires will be just fine. For an unsupported ride across the US on unpaved roads I'd want lots of attachment points for fenders, racks, bottles and a frame that could handle 45mm or wider tires.

Typically CX frames will have few if any attachment points beyond that for water bottles. The standard crankset for CX bikes are 46/36 or the new 1x setup - both of these are narrow by touring standards but may be fine for lightweight touring in relatively flat regions.

Components on touring bikes are generally much more robust and heavier than race parts. Most notably the tires and wheels are much heavier duty to minimize the risk of being stuck on the side of the road or worse. Drivetrains are setup with wide range gears and this adds weight to the bike.

Also consider the front-center measurement (measured center of BB to center of front axle) as this will indicate how much clearance between one's foot and front fender/tire. Adding a front fender will take up 1 to 2 cm of toe clearance.

Titanium is an excellent frame material for travel, it's tough and doesn't show signs of abuse (assuming of course it isn't painted). BUT a Ti frame will weigh a pound or two more than a light weight carbon frame.

Panniers, frame bags, handlebar bags all add A LOT of weight to a bike and eliminate any aero advantage provided by high end race parts.

There are weight limit for many race components found on the high end bikes. It's easy to exceed these weight limits if the parts are used for touring. To give an idea of weight differences, here's a test of the Schwalbe Marathon Mondial tires which are considered to be among the best for long distance touring. Spoiler alert: 722 gm for the 700x32mm tire up to 987 gm for the 48mm tire.

Schwalbe Mondial Tire Test

So my suggestion is before deciding on a frame/bike, consider the type of touring and the requirements. You could end up finding the aluminum Grade is a great starting point for a touring bike and your carbon bike purchase instead becomes an 18 pound fast gravel/road bike.