What Causes Wheel Flop?

[Peter P.]

I know my post may answer it's own question, but humor me here.

I have an old Trek 660 road frame. It's a lugged steel, Reynolds 531 racing frame.

When the headset died, I replaced it with a Stronglight roller bearing headset. The bike had a subtle case of wheel flop which I correctly attributed to the taller lower stack height of the Stronglight headset vs. the original. The flop was tolerable but after a couple years I tired of it and installed a headset whose lower stack height was a few millimeters shorter, more akin to the original headset, and the problem was solved.

So today I had to work on a friend's Trek 520. Here's an example:

http://www.ironmaz.com/images/trek520.jpg

After I worked on the bike I took it for a test ride. Out of the saddle, the wheel flop was just terrible; far worse than the Trek example above. If it were my bike I would have returned it to the dealer after the first ride and found something else. I don't remember how bad it was while seated but I certainly remember it while standing.

What mystifies me is this bike is entirely stock so I can't blame the headset or the 32mm tires or the stock fork. I would ASSUME the frame and fork were designed properly. The bike steered okay and didn't feel like it was either high or low trail-it certainly wasn't low trail because I've ridden such bikes before.

So what do you think is causing such crap handling?

[ultraman6970]

Wheel Flop, Defined
(Some readers may recognize this from recent editing work I did on Wikipedia)

Wheel flop refers to steering behavior in which a bicycle or motorcycle tends to turn more than expected due to the front wheel "flopping" over when the handlebars are rotated. Wheel flop is caused by the lowering of the front end of a bicycle or motorcycle as the handlebars are rotated away from the "straight ahead" position.

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Well by this definition I think that what you are saying is just the opposite, or not?

As for the green trek, I wouldnt tihnk too much about it, that's a bike to go to the trails and errands so doubt they thought in something else more than "it looks good... we can sell it for 800 and get 500 for ourselves".

BTW, flip the stem the handling might get better in that _____

[Mark McM]

Ultraman gives the cause of flop. It is a mostly a function of geometry, and the biggest factor is usually head angle (the shallower the angle, the greater the tendency to suffer wheel flop). I'll guess that your friends bike is a relatively small size. The Trek web site shows that the smallest size of the current 520 (size 48 cm) has a head angle of 69.5 degrees, which is low even for an MTB, so it is not surprising that it suffers from flop.

One possible way to reduce the flop is to replace the fork with one with more offset (also reduces trail). But the current 520 fork is spec'ed with 52 mm of offset, so it would be difficult to find one with more. I think you're friend will either have to learn to live with it, or find a different bike.

[Louis]

After I worked on the bike I took it for a test ride. Out of the saddle, the wheel flop was just terrible; far worse than the Trek example above.

What exactly happens? Do small motions unconsciously initiated by your hands suddenly become much larger motions?

I've never heard of this phenomenon.

[ultraman6970]

Even more trail?? IMO what could be a fast handling bike is a problem for other person and that translates in flop apparently... based in the definition of flop because i never heard of that ever, that i had to google it to figure it out what the OP was talking about.

Flip it! Flip it good!....

One possible way to reduce the flop is to replace the fork with one with more offset (also reduces trail). But the current 520 fork is spec'ed with 52 mm of offset, so it would be difficult to find one with more. I think you're friend will either have to learn to live with it, or find a different bike.

[4Rings6Stars]

I have no idea what you guys are talking about. What the hell is "wheel flop"?

Pretend you're explaining this to a fourth grader... I'm burnt out from studying for my CPA exams and have a handful of tall Sam Adams Octoberfests in my system...

[ultraman6970]

From what I understand from the definition, you turn the stem to the right or left just a tiny bit and the bike just turns more than you thought, like when cars tend to oversteer apparently. Wonder if its something I felt like 30 years ago riding my friends bike hmm.... You turn and then the bike just go all the way to the side you told it like itself, in my case the frame was way to big and was made for a guy way taller than me. the fit obviously too, I attributed the issue to the fit because the bike was not mine, he never got problems ever anyways.

What exactly happens? Do small motions unconsciously initiated by your hands suddenly become much larger motions?

I've never heard of this phenomenon.

[David Kirk]

Typically flop is cased by having too much trail and it is most common on smaller bikes with more shallow head angles and a 'one size fits all' stock fork.

I lot of guys never feel flop unless they have a girl in their live who rides a smaller bike with its shallow head angle. It's easy to prevent with the proper matching of the head angle and rake to give a reasonable trail.

Flop sucks eh? Climbing with a bike with bad flop is not much fun.

Dave

[Chance]

Another good reason for using smaller wheels on smaller bikes.:rolleyes:

[christian]

Wheel flop is easy to diagnose. Ride no-hands. Initiate a turn. If the front wheel wants to continue to steer/fall into the turn and you need to "catch" it, you have wheel flop. If you continue to let the wheel turn in, you'll end up jack-knifing the front wheel and going over the bars.

My son's 12" bicycle has a terrible case of it. Raleigh DL-1s had it some, too (but not Raleigh Sports, which steered great). Nighthawk CB700SCs if the tire pressure was too low, too.

I am sensitive to wheel flop and hate it. One of the things I consider most divine about my new Zanc cx bike is that it steers steadily enough to allow me to do a u-turn inside a single parking space, yet exhibits no wheel flop. For a cyclocross bike, the former is ideal, and the latter is like magic.

[Peter P.]

Dave Kirk and christian are closest is what I felt and its likely cause.

I would turn the bike and it felt like the front wheel would want to "flop" over or just continue to turn until the bike fell over, or as christian said, it would quickly lead to the front wheel jacknifing.

The bike in the photo is not my friends but it is the same size I think, and he is not aware the frame is doing anything bad at all which is good because that's all that matters.

I suspected more rake would be needed but you would think a touring bike such as the 520 could get away with a 71 or 72 degree head angle at that small size and still use a common fork rake to get decent trail. There must be more going on than I'm assuming.

Thanks for all the replies.

[palincss]

Part of the confusion is the characterization of wheel flop as 'excessive steering.' You might find this easier to understand:


Also known as the steering axis, [head tube] angle influences stability in combination with fork offset by controlling “wheel flop”, or the tendancy for the wheel to turn when leaned.
--http://www.calfeedesign.com/tech-papers/geometry-of-bike-handling/


For those who claim to have never experienced it, try this: grab hold of the bike only by the saddle and start walking with it. When you need to turn, you lean the bike and the wheel turns. That's wheel flop in action. Every bike you've ever ridden (unless you're a practitioner of "artistic cycling") has it. Some bikes have more than others, and the "just right" amount will change depending on how the bike is loaded, etc.

[fiataccompli]

So, basically, you want a certain amount ? And perhaps it is the same geometric phenomenon that will allow you to do most of your steering with your body (vs bars) ipbto the point that it is too much? (And then labeled wheel flop)

[palincss]

So, basically, you want a certain amount ? And perhaps it is the same geometric phenomenon that will allow you to do most of your steering with your body (vs bars) up to the point that it is too much? (And then labeled wheel flop)

Yes, you do want the correct amount to make handling seem natural and intuitive. Too little and weight on the front wheel creates too much inertia, making steering difficult; too much and the bicycle will over-react to disturbances such as road irregularities and wind, and will become very hard to keep going in a straight line without veering all over the road at slow speed while climbing steep hills. The "Goldilocks amount" provides just the right amount of "power steering assist" to make handling seem natural.

[Chance]

.....
If you continue to let the wheel turn in, you'll end up jack-knifing the front wheel and going over the bars.
.......


This sounds very similar to what happened to me one time a long time ago with a bike that had shallow head tube angle and lots of trail. After standing to go up a short climb the front wheel felt like it was going to "jackknife" around and throw me over the bars. Fortunately it was momentary but it did startle me. Not something you forget.

At the time just figured it was due to being tired and or due to bike having very low bars. Still think that when bars are too low for a rider (like when testing someone else’s much smaller bike) it is easier for the bars to feel like they are going to get away from us. Probably not the cause of “flop” but may accentuate the issue.

[Chance]

For those who claim to have never experienced it, try this: grab hold of the bike only by the saddle and start walking with it. When you need to turn, you lean the bike and the wheel turns. That's wheel flop in action. Every bike you've ever ridden (unless you're a practitioner of "artistic cycling") has it. Some bikes have more than others, and the "just right" amount will change depending on how the bike is loaded, etc.

If you do that with a bike even when standing still, the front wheel will rotate around when leaned. Agree some bikes rotate front wheel around much faster and to a greater degree than others. Would add though that there seems to be a correleation to trail. On bikes with lots of trail the front wheel rotates a lot, and bikes with less trail the front wheel rotates considerably less.

Personally prefer handling of bikes with less trail. Just feel better to "me" although accept the opposite feels better to some other riders. Just saying the "just right" amount is probably rider dependent.

[David Kirk]

Flop is most common on smaller bikes (which most often come with more shallow head angles to help with toe overlap) and one-size-fits-all forks. This combo can result in trail numbers up in the 70+ mm range. This is a LOT of trail and while it works OK when you are going 80 mph it's really not good going 12 mph. For road use a trail of 55-60 mm is considered to be desirable, neutral and intuitive - and - going much below or above this range will result in some less than desirable handling traits. The ONLY reason the big guys make and sell bikes like this is that they are counting pennies and don't want to make more than one fork to fit the entire size range of the bikes they offer. I've heard some say that 'trail make the bike stable' as if that was a good thing. But one can have way too much of a good thing and suffer for it. Stability is good and more stable isn't always better. At some point if all goes bad.

A bike with too much trail and flop can of course be fixed with a different fork with the appropriate rake but it can be easier said than done as it can be pretty tough to find a fork with the right rake. A builder can of course make a fork to fit the frame and sometimes this is money very well spent - other times.......not. It depends of the situation. There are few things in life that are guaranteed but here is one of them - if you are riding a bike with 70 mm of trail and you change out the fork to give a better number it will ride and handle better.

When buying a stock bike I always advice looking through the listed geometry and seeing that the trail is (or taking the listed rake and h-angle numbers and putting them in a trail calculator to figure it out) before taking the plunge. If the numbers are outside the 55-60 mm of trail range I'd think hard about the bike before pulling the trigger.

Dave