That was not uncommon in downhill bikes on the pro race circuit, or the freeride scene. You may want the smaller rear wheel for improved gear ratio in climbs and overall stiffness in the shorter suspension frame, but the larger front wheel to smooth out the transitions over roots and logs. The lowered rear also helped flatten the profile of the bike on descents, which meant you didn't have to hang so far off the rear, upsetting the center of gravity and front wheel handling.
It's not as big a deal anymore as there are so many customized frame geometries now that you can spec anything you want, but there is no reason it couldn't be done with a disc brake bike (obviously rim brakes don't play well with non-standard configurations). The biggest consideration on a 'flat' terrain bike is that too much change will change the angles of the headset tube, and more rake means slower steering and suspension rebound (while taking big hits like curbs a bit better). 2 inches change may not even be noticeable on a casual bike, but would on a high speed race bike.
Now if you went smaller on the front without careful consideration, you could be in a world of pain. As a more upright headset makes the bike turn fast and twitchy, and the suspension is overstressed because it's always being torqued to the rear on impact. Obviously if you didn't compensate for the forward center of gravity with a shorter handlebar stem and seat offset you, would be far more likely to end up in front of your ride after an unforeseen pothole or impact.
All good thoughts. Thing is it looks like when speaking in general, the outside diameter 26x3 vs. something like a 27.5x2.3 are not that different. As measured a few minutes ago-
26x3"=27.5"
27.5x1.9=27.2"
26x2" (Marathon)=26"
So there's that. I had seen mention of this previously, so no surprise.
The surprise happened when I follow the fork angle and measure from the ground to the bottom of the head race. Here there's a pretty significant difference when comparing 26 fatty w/135mm drop out vs. the 27.5" fork w/100mm drop out. I had to measure this 3 times, because it doesn't sound right when seeing the bikes sitting side by side. Made me wonder if I hadn't become a little befuddled?
The fatty measures 34", while the 27.5" measures 32.5". So the fatty, though you would think the head race LOOKS lower, is actually 1.5" higher!
So I measured from the drop outs to the bottom of the head race to confirms this.
27.5" fork measures 19.5"
26" fatty measures 21"
So, to summarize, when a 26" fatty rolls over a root, the big sidewall is going to deflect more, but the 27.5, because it's NOT deflecting more, should roll over it easier - and will likely do that with more of a jolt - hopefully to be minimized by a decent set of forks.
The head angle is going to change making this swap, to a very small degree I think, but it will change. The question might be will the change be noticeable to Joe average hybrid bike rider? There's also the question that wonders if all forks are created equally, Will the new air forks measure similarly to the pogo sticks I have now?
Noteworthy is I made no attempt at measuring accurately to the nth degree, and the measurements were pulled from bikes sitting in the garage. -Al
