I have a couple of ride on my FBN with 65mm carbon rims. The difference is quite noticeable. The bike actually accelerates when you pedal and turns in with less effort. But it still has great traction (probably 90% of the 5" bulldozer tire) with a lot less rolling resistance. It floats over roots and rocks is almost as soft as the 5". The only issue is the BB is slightly lower so I get pedal strikes a little more often while pedaling thru a corner. Definitely a worth while upgrade that has now made my 29" HT unneeded
A big thanks goes out to Jim V at Hilltop for building these for me
I had the same feeling going from Nextie carbon 90mm rims/Dillinger5's in the winter to Nextie carbon 65mm rims with Surly Larry (F) and 45North HuskerDu (R) as a summer setup. The Fatboy seems to accelerate similarly to what I remember on my 29er (although it's getting harder to confidently remember what it felt like on the 29er as more time passes...). I recently took it to the Orange "Twisties" at Six Mile Run and it cornered very well at speed with no wash-outs (the tires make an unmistakable "buzzing" sound when you get it leaned over on the side knobs...). My only concern has been the lower BB clearance, reflected in a few more pedal strikes, but also more frequent chainring contact going over boulders (especially those with narrow, relatively pointy tops) and larger logovers (I've yet to master launching the rear wheel up and over big logs -- need some Ben instruction, I think). I thought I was going to destroy my chainring and/or chain at Dickerson Mine Preserve a couple of weeks ago on some of the fun rock gardens that they've created on the new Blue Trail -- some of the rocks have the tall peaks that I mention above.
I'm considering trying a pair of Vee H-Billie 4.25 tires to address the lower bottom bracket issue. It seems like the H-Billies are the only "tweener" sized tire that might be a good compromise between 4" and 5" tires. Some guys have posted actual measurements of these tires vs the 4"ers currently being sold and they're ~1/3" -2/3" inch taller and slightly larger in volume. Yet, the weight (user reported 1248, 1263, 1280, and 1320 gm) is just slightly higher than my Larry's (manufacturer claimed 1255 gm) and HuskerDu's (manufacturer claimed 1240 gm), depending on which of the H-Billie measured values reflects a good "average." I don't know (1) if the added height/volume of the H-Billies would
noticeably help with pedal strikes and rock contacts, and (2) if the added weight would make them
noticeably less lively/responsive (in comparison, the Dillinger5's are about 1520 gm, I believe), but they may be worth a try.
One other possible concern I've seen regarding this tire is that the tread width is said to be more narrow than most, which some posters expressed concern over, especially if mounted to wide rims (say, 82mm and up). I'm hoping that the more rounded tire profile of tires mounted on smaller, 65mm rims would make the knob width concern a non-issue. As with everything, there are always trade-offs -- I'll post-up more info if I take the plunge.
