johngwheeler

I've noticed that my Giant TCX feels significantly less "under control" in the front wheel than my Trek Crossrip, especially when riding out of the saddle. It's not exactly unstable, but I feel I have to concentrate more to keep the wheel pointing straight.

I was wondering what aspects of the bike geometry determine steering "stiffness".

The Giant's wheelbase is 1019mm vs 1043mm for the Trek, and the bottom bracket drop is 60mm vs 74mm on the Trek. Would this account for the twitchier feel of the Giant?

Thanks!

John.

Scooty Puff Jr

Stem length and the rake of the fork will make a difference in handliing. How much of a difference does your new bikes stem length and fork compare to your older one ?

Wildwood 

Head tube angle and fork rake.
Are the tires the same?


edit: lower BB and longer chainstays will also 'stabilize' the feel of the Trek, through the effect of "longer and lower".
On the scale of race bike vs touring bike - the Trek is your tourer.

kuroba

Understanding Bicycle Frame Geometry - CyclingAbout

This article helped me a lot to understand bike geometry and how it affects handling

Machka 

Handlebar width

Wildwood 

+1
Watch out for those wide handlebars.

Machka 

Actually, I find slightly wider handlebars to provide a more stable ride. I'm talking about road bicycles, of course.

Wildwood 

Funny, i find wide bars make it harder to track a straight line than narrow bars, but then i'm a tall guy.


Not sure about stem length, except at the extremes. Anything from 9 - 14cm should be good, if cockpit fit is good (balanced).

athrowawaynic

It seems to me that this should be correct. Wider bars = better leverage.

italktocats

if your balance (holding a straight line) isnt that great, it will be amplified with wider bars, placing your hands into the middle decreases leverage and thus less movement tru the bars into the wheel

WizardOfBoz

There's a book, "Bicycling Science" that covers fork stability (which I guess is the opposite of "twitchiness") pretty well.

Wildwood 

Precisely the point.
Macha rides long trips with racks, bags, etc.
I have a hard time locating a dropout brazed-on eyelet in my groupetto.


Relative to stem length, i would admit this 15cm is a bit pushing the limit.

rgconner

Weight distribution. More weight on the front will make the bike more twitchy, leaning back will make it carve like lazy dog.

Wildwood 

Back to the point of 'twitchiness'.


The center of BB to center of fork drop-out is called the 'front-center'. It accounts for the sum of HTA + rake. Find what works for your size frame and riding style. It does not account for type of fork (straightbladed?, stiff CF?, wimpy Al?), tire size or inflation, rider position on the bike, and a bunch of other factors.


Can ya believe a guy once complained about the crappy handling of his new bike, 'especially on the descents'. Adjusted his brakes and his problem was solved.
Headsets gotta be tight, but not too tight, too.




edit: most would agree that a track bike with steep hta and lightweight and skinny latex tubies is twtchy, and with those short chainstays there is no bailout rearward position to make 'lazy' turns. the reason there is no one 'do-it-all' bike. Bike geometry is a purpose driven science. The ride is an art of tubeset and mastercraftman.

DesmoDog

Timely question, I've just started thinking about what to do to my wife's bike to make it less twitchy. She had a low end mountain bike so we bought her a nice new hybrid (I've forgotten the brand now). It rolls so much nicer but she doesn't like that it's "twitchy" and steers so fast. I'm hoping to come up with a plan to change out some components on it, obviously the head tube angle isn't going to get raked out any time soon...

Viich

Tire size can change the handling as well - it has the effect of changing the trail, because wider tires are also taller. Bigger tires will tend to stabilize (increase trail). I saw this when switching between 622x23 and 630x32 - huge difference in stability. Probably made worse by having 622x25 on the rear and 23 on the front (slight change in head tube angle)

Interestingly, the difference between 700c and 27" (given the same width tire) is the same as 4mm in width change, assuming round profile.

79pmooney

Nah. That stem's got another inch or more to go. Seriously. I ride stems of 120, 130, 135, 140, 155 and 175 and have a 180 that is between bikes as well as a dead 180 that has served a long and good life. The 120 in on a custom bike designed so I could use a stock stem. Likewise the 130 and 135. The rest are to make stock bikes work. That dead 180 was a game changer. Opened my eyes to true comfort two decades ago.

tyrion

Lighter wheels/tires on the new bike might be a significant factor here. Simply getting heavier tires would stabilize the bike, but (not to second guess you or your wife) she might quickly adjust to the lighter steering of the new bike as it is.

Wildwood 

I ride my wife's 1980s stepthru Bridgestone CB-1 - 'city bike' with 26"X1.75 tires. it's fine at slow speed, very maneuverable, but not too stable at speed. The family beachcruiser with 'balloon tires' and wide sweptback handlebars certainly isn't 'agile' at any speed.

@79pmooney - personally, a stem longer than 15cm is a possible indicator that the frame isn't appropriately sized. Similarly, less than 9cm. But there are always exceptions. Again, the context is road bikes.


Ride what ya like.
YMMV

Retro Grouch 

That was my first thought. Probably the quickest thing to check and definitely the easiest and cheapest to fix.

79pmooney

Wildwood, the really long stems are on '80s horizontally dropped bikes, those bikes that a) work well as fix gears and b) handle decent sized tires and fenders; ie great rain/winter/city and commuter bikes. Since buying these frames isn't pre-planned and the need is immediate (the need arises when the previous one is destroyed in a crash), selection tends to be limited.

I could go custom, but $1000+ for a bike that's going to die the same way and looks far more attractive to steal, never mind the 6 mo wait? Nah. A $200 stem is far easier. And goes bike to bike. (Thank you, Japanese bike manufacturers, for simplifying the standards so moving parts bike to bike was easy for a 15 year run. And making a lot of frames during that run; good frames that hold up and are versatile. And doing this so effectively that it rubbed off on several other countries' bikes. That I have to put stupid long stems on those bikes? Not much of a hardship.)

Ben

johngwheeler

Good points. I think stem length is similar at about 110mm, but the Trek has a little more fork rake and trail (72mm vs 65mm).

johngwheeler

Thanks for this - that's a really good article comapring the geometry of racing and touring bikes.

I think my Trek has a more relaxed geometry that suits my lack of experience by making me feel a bit more secure on fast descents, and when riding out of the saddle. The Giant is a CX race bike and is a lot lighter and "nimbler" (if that's the right term!), but I like it too. Hopefully as my bike handling skills and confidence improve, I'll get used to it.

I'm glad I didn't launch straight into an all-out road-racer as a first bike - I'd have probably found it hard work!

John.

johngwheeler

Is this what connects the stem to the steering tube, or the fork to the frame? Could you explain what the headset tightness should be? I had assumed that it should all be done up as tightly as possible (within the torque limits of the bolt/frame), so that there is no shift between handlebar position and fork (& hence wheel).

Thanks!

John.

Retro Grouch 

If you lean your bike from side-to-side the front wheel should also flop from side to side. If it doesn't, it won't self center on it's own and you'll fight continually over correcting to make it go straight.

To loosen a too tight headset, loosen the two bolts that hold the stem onto the steer tube. Then gradually loosen the top cap as you test for wheel flop as described above. Probably won't take very much.

A too loose headset is also bad. The easiest test for a loose headset is to apply the front brake and try to push the bike forward. If you can feel a little "tick" as you push, that's too loose so you need to tighten the top cap a skosh.

When you are sure that your headset is between the too tight and too loose window, make sure your handlebar is straight and re-tighten the two bolts that hold the stem to the steer tube.