Why a stable bike weaves when climbing or at low speed

Understanding wheel flop and other bicycle geometry details

Dear Lennard,

This summer I got a new Ibis Ripley AF trail bike. I was upgrading from my 30-year-old Gary Fisher KaiTai hardtail with cantilever brakes, and I can go on and on about how much better it brakes and shifts and has a wider gear range for my advanced age. I had not ridden mountain bikes in so long, and now that I’m retired, I’m getting back into it.

The Ibis is a lot of fun when riding fast, but I am not nearly as fast as I used to be, and I’m wondering if I got too much bike. I love it for riding the flowy singletrack trails in Fruita when there’s not much climbing. And I went on some chairlift-assisted runs at Winter Park with my son and daughter in law, and I had a grin on my face all day. No way could I have ridden like that on my Fisher.

I have put on quite a bit of weight, and I would say that I climb at a snail’s pace, but I think some snails might be faster. The Ibis seems like it’s similar weight to my Fisher, but I used to climb pretty well on that bike, and I feel like a dork climbing anything steep on the Ibis, especially if its rocky or there are tree roots. My front wheel is all over the place, and I constantly am putting my foot down to keep from falling over when I suddenly veer so sharply toward the edge of the trail. The same local trails that I used to be proud that I could ride without dabbing I have to walk up.

My LBS sold me on the new trail geometry and steep seat angle and lax head angle of the Ibis. It looks so different when I stand it up next to my Fisher. With the gearing on the Fisher, I couldn’t get up those hills anymore anyway, but I still think that even at my slower speed, I wouldn’t be weaving around like this on that bike.

Does any of this make sense? Do other people have this problem, or is it just me?

Arvid

Dear Arvid,

See below, under the next question. I will answer both of you at once to keep this shorter and not repeat much of the same stuff.

― Lennard

Dear Lennard,

I recently got a custom titanium bike (I won’t say who made it) with a carbon fork and disc brakes and electronic shifting that I paid a lot of money for and had planned on being my last bike. It was my 80^th birthday gift to myself.

The bike does fit me very well, so I have no complaints there. I can’t ride bent over anymore on a drop bar like my old bikes have, and the builder made it for a straight bar that’s really high and much closer to the saddle than on any of my other bikes. And it has big tires—smooth ones, 50mm. They really soften the cracks in the road and concrete bike trails.

I was so uncomfortable on my other bikes that I just couldn’t ride more than twenty minutes or so, and I want to be able to do more than that, since I think riding keeps me young. But I don’t want to fall, since that would not keep me young, and I worry about that on this bike.

I can ride this bike without pain, but I have to work so hard to keep it going straight that I tense up, and that isn’t comfortable. I am only going on flat bike trails and neighborhood loops with no traffic on them, and I just can’t keep it going in a straight line. For reasons I don’t want to go into, I can’t return the bike. Is there anything I can do so I don’t have to fight this bike so much on my slow, puttery rides?

Henry

Dear Henry and Arvid,

I think you are both talking about the same thing, so I will answer you both at once.

Arvid, your Ripley AF has a 65.5-degree head-tube angle and a Fox 36 fork with 44mm of offset (fork rake). If you still have the 29x2.4” tires it says it comes with, your “fork trail” is 121mm, which is huge (I’ll explain fork trail in a moment; it’s not to be confused with your “trail bike”). The geometry chart for the Ripley AF shows 120mm of trail, so we’re on the same page. More importantly, you have a “wheel flop factor” of 46mm. You never see wheel flop factor on geometry charts, and I’ll explain that momentarily as well, since I think that best describes what is happening with both of you.

Henry, I suspect your titanium builder did similar things with your bike’s front end geometry as on Arvid’s Ibis, but for different reasons. You being 80 and him wanting you to be safe, I assume that he would have made the bike with no toe overlap of the front wheel. To do that, he needed to do something to get your front wheel further away from the bottom bracket or your toe would hit that fat front tire on slow-speed corners, especially with flat pedals and street shoes.

Your carbon fork probably has an offset (rake) of maybe 47mm (most gravel forks have a rake around 47mm; carbon road forks usually run 43mm-50mm of rake). I would guess that your head angle is well under 70 degrees to avoid toe overlap with 700x50C tires with a short top tube (52cm?). If it is, say, a 68-degree head angle, with 47mm of rake, your fork trail is 95mm (that’s a lot), and your wheel flop factor is 33mm (also a lot).