no-hands balance: what makes a bike squirrely?

Didn't really know what to title this thread...but I've got a 58cm 5.2 Madone that I put about 350 miles/month on and have owned it for 3 yrs after upgrading from my trusty steelie. Smooth ride but always been curious what part of the bike geometry makes a bike unstable when you sit up/no hands for a break.

This came to light when I rode a Ridley Damocles yesterday and really noticed the difference. I used to be comfortable *bombing* down hills on the steelie with my mitts on the handlebar stem...would never attempt same on Trek. The Ridley brought back that same confidence...very stable. It was also a great deal firmer of a ride and seemed to be much stiffer in the headset area (virtually zero give in the forks under braking...my Trek forks track aft at least 1cm at the tip under braking).

Geometry specs- Trek top tube about an inch longer thus I'm just guessing that this places the center of gravity farther aft and the potential root cause of a less stable platform. Thoughts???

I knew I shouldn't have ridden that Ridely...now I know what I'm missing....

Hi


The fork rake and the rake angle are in fact he most important for this stableness ...

You might want to check out these articles:

general intro on bicycle frame geometry

the reason why is that the rake angle and the fork rake make that you have to push up the weight to make your front wheel turn. negative rake would make the bike totally unstable

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I should have known that...sounds like the same impact of caster on the front wheels of your car (making the steering wheel re-center all by itself). I'll compare the specs and see what the deltas are. Thanks!

Well...I have no idea what order of magnitude a .8 degree difference in fork angle should impact stability...but the Madone is 73.8 versus the Ridley at 73 degrees. Laying images of the bikes over one another produces the attached (we'll see how my attachment mangament works out). Wheelbase on the Trek is 6mm longer...fork angle .8 degree greater (the greater the angle, the more vertical thus less stable...right?).

If this .8 degrees is the primary contributor to the stability, it surely surprises the daylights outta me that so little delta produces such a large impact on stability.

Might have a 5.2 Madone frameset on the 'bay pretty soon....

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Believe the "trail" dimension is the critical one. That's the distance measured on the ground from the "steering axis"(an imaginary line drawn down through the steer tube) back to the center of the tire contact patch. Steeper steer tube angles and greater rake in the fork both act to reduce the trail and reduce stability in favor of quick handling response.

Agree hands-off stability is better than ultra-quick steering, at least for the riding I do. On high speed descents, will take a bike that takes a bit of steering effort to change direction and holds it's line over bumps rather than one that's darty any day. Plus, it's nice to be able to ride no-hands confidently when sitting up to stretch or take off the rain vest.

hi, it is a combination of the rake angle and the fork rake distance itself (and in fact, also the tyre section is important)

what generates the stability= the call back force on your front wheel when it deviates a bit from the straight-forward position.

This force is depending on the geometry of your fork (more rake, and more rake angle makes that the bike must move more upward to turn => more stable)

Also more weight on your front wheel gives you more stability. so in fact, the postion of your rear wheel will also have impact, but to a less extent than the rake.

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Well, let's avoid the whole "rake" debacle for now, as the proper use of "rake" in 2 wheeled industries is all over the map.

Trail is usually what dominates the re-centering force. Trail is the distance between two points:

1. Where an imaginary line drawn along the axis of the head tube hits the ground
2. The center of the tire's contact patch.

On a bike and most every two wheeled machine, the contact patch is behind the intersection of the head tube axis and the ground. Typically on a bike, trail is on the order of 5-6 cm or so, give or take some millimeters here or there. The more trail there is, the greater the re-centering force (all else being equal). Steeper head tube angles generally have shorter trail numbers, all else being equal. However, with bikes, all else isn't equal because of another dimension called fork offset or rake. That's the horizontal distance from the center of the dropouts to the head tube axis. More offset or rake gives less trail, while short offset or rake gives more trail.

Front/rear weight bias can change steering characteristics, but weight bias doesn't usually change dramatically between road bikes.

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i might be concerned that the bike is out of alignment somewhere

you can ride no hands and 'do tricks' on other bikes but on your trek no?

i would be surprised if the spec for the fork was way out of line with other bikes that share a ~74 degree head angle...

however i am not surprised that the ridley with a 73 degree head angle descends more confidently...

all's'miles

curby

Well..... All I am going to say on this topic is that bicycle stability is a complex subject. The dynamics of the particular nonholonomic dynamical system we are talking about here is so complex, in fact, that without a very solid understanding of theoretical mechanics there is little to no hope to grasp it at all. However, if you are not afraid of moment of inertia tensors you could go to Andy Ruina's website at Cornell, and look at some of the work he and some of his students have done. Be warned, this is mostly serious stuff, and definitely not for the faint of heart, mathematically speaking. On the other hand, he has a number of links to more easily digestable material as well. Alienator might also be interested in the scientific motorcycle bibliography offered there

Finally, there's the JBike software that can do stability calculations for bicycles, but again, this is really only usable by people with some understanding of theoretical mechanics.

i'm gonna go with it doesn't matter whatsoever the bike geometry, not in any way shape or form but rather the bike handler's skill. you either have it or you don't and should get some. 350 miles a month isn't gonna cut it either. and ride with your feet, not your hands fulltime.

too much namby pamby baby talk about calculations and not enough just dominate in the cycling world.

sorry if that sounds rude, not trying to be it's just how i am.

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and core strength and tone. not one thing you do on a bike will ever neglect that you should have some soild abs for any kind of balance or produced control on a ride. you can have a big old beer gut and the legs the size of redwoods and it won't make a difference as to your skills, you'll suck anyways.

lose the gut, gain control.

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Seek enlightenment and understanding, while you have the motivation to do so.

My old steel AMF (like Raleigh Record) was beautifully behaved. The Centurion (newer, better steel) Dave Scott Triathlon Master (blah blah blah) had a nice Shimano 600 group, but a nasty oscillation if I sat up and let my weight get to far to the rear. Friends have crashed because of this type of problem.

By all means, learn what's wrong, but if the bike's not working for you, get rid of it.

Life is too short already.

Make sure your wheels/rims are true, if a rim is out-of-round it can make the bike unstable, and in extreme cases can also cause shimmy.

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Another obvious difference in handling feel is the weight of the fork and front wheel on the Madone vs the old steelies, probably 50% less mass up front. When I first got my current bike in late '03, noticed the same quick steering and difficulty in riding no-hands vs the all-steel bike I was used to riding. The first few times I casually took my hands off the bars were a surprise. It's wobble-free and secure on fast descents, but darty when riding no-hands. Only time I do this now is when necessary to remove or put on a rain vest.

has anyone mentioned the headset ? if it is overtighten you get this balance problem