Stem Length Steering Geometry

LowPlainsDrifter · Post by **LowPlainsDrifter** » 12 Jul 2007, 10:55am

Hi, I haven't ridden a Galaxy so dont know how they feel but I have got a Sadar (with the steel frame) and I noticed it feels a bit twitchy too.
I didn't know if thats normal or what seeing this is my first "modern" bike.
26 inch wheels with Conti Top Touring tyres.

PW · Post by PW » 12 Jul 2007, 11:06am

Pigman's shimmy problem was probably down to weight distribution altering the harmonics of the frame - I had something similar with a tourer which wouldn't tolerate front low rider panniers - fine with non low rider, barbag, nothing at all but fit the bags on a lowrider & it was uncontrollable at 17 - 19 mph.
Deliquium, I'd take a very close look not just at the forks & steerer but also at the frame around the headtube in case anything's coming adrift. Failing that I'm tempted to suggest a manufacturing cock-up, something like they've fitted tandem forks to a solo? It really needs an expert with framebuilding knowhow to sort this one out.

stephen · Post by **stephen** » 15 Jul 2007, 10:24pm

I've a Dawes discovery (a Hybrid) which I commute with. This doesn’t feel twitchy in normal use but cannot be ridden without hands on the handle bars, unlike every other bike I’ve owned. Yes, the top tube is to short, presumably to encourage a more upright ridding position. It also tends to steer to one side or the other if the wheel dishing is not absolutely perfect I don't know if this is normal or not.

Stephen

lb1dej · Post by **lb1dej** » 18 Jul 2007, 9:34am

I wonder if there's a psychological element in this.

I experienced shimmy for the first time on a frame that I've ridden for over twenty years on and off (73 deg parallel Holdsworth with a shortish top tube) - on a combination of strong wind and fast twisty downhill run. I've since become nervous about this road and also experienced the begining of a shimmy on another bike - a tourer with standard 72 degree geometry - though elsewhere this rides fine. Could nervousness or lack of confidence be a factor?

A factor for me in handling is that I sit high on a bike - being 6 foot tall - so my centre of gravity is higher - the more so since I've long legs and a shortish reach .

My best handling bike is my Kona Cinder Cone MTB with fat slicks - my other bikes feel less stable in comparison.

Another mental element may be that one gets used to the way in which a particular bike handles and changing to another feels quite different.

As mentioned above it may come down to just the particular rider bike and any baggage combination which may work against stability.

Dave

Post by **Graham** » 18 Jul 2007, 10:39am

lb1dej wrote:I wonder if there's a psychological element in this.

For me there is. . . . .

Before my first shimmy I would ride down hills with gusto and full confidence. I felt like the bike & I were moving in harmony.

The shimmy happened at around 40 mph and lasted for a distance of around 400 metres, with the handlebars oscillating vigourously. I was convinced that I would be horribly injured whether I stayed on the bike or deliberately threw myself off. Fortunately, I stayed on and came to a semi-controlled stop on the wrong side of the road.
( Descending from Ranmore into Dorking - for those who know it. )

Since then I have learnt about the knee against the top-tube technique, to damp the oscillation . . . . which I have had to use on several occasions.

I had lost confidence on downhills. . . . grabbing the bars ever tighter . . . . sometimes even shaking. The shaking actually initiated the shimmy on a couple of occasions. :shock:

Several years later I have trained myself out of this self-perpertuating spiral.
I deliberately relax my grip on the bars and concentrate on staying relaxed and confident during the descent.
However, I remain a cautious descender.

glueman · Post by **glueman** » 18 Jul 2007, 11:14am

One of my bikes - and a custom one at that - has a very shallow head tube angle but the forks only have a slight curve. It looks a peach but I hate going down hills on it. It's a battle to point the bike where you want it to go. It may be due to my instructions at the time that I wanted a bike that was undemanding on long rides. My guess is the builder thought a slack head angle was the way to do this. Sometimes I wonder if having a new, much curvier set of forks would recover the handling?

PW · Post by PW » 18 Jul 2007, 11:20am

Sharpness of steering comes from a combination of head angle, fork trail & length of wheelbase. You can't do much about the first or last but reducing the trail of the forks - ie increasing the offset, should quicken it up a good deal.

deliquium · Post by **deliquium** » 18 Jul 2007, 11:29am

Thanks all.

I too wonder how much is psychological. I haven't cycled for 6 years until the beginning of the year, and it may well be a confidence thing.

The dreaded shimmy hasn't occured as yet. Just that the steering feels very light and quick. Most steering appears to be done by leaning, but this bike/me seems to turn in too quick and I have to use the handlebars to constantly adjust, especially round tight bends.

Maybe my old trike head has come out of the mists of time!

glueman · Post by **glueman** » 18 Jul 2007, 11:29am

PW wrote:Sharpness of steering comes from a combination of head angle, fork trail & length of wheelbase. You can't do much about the first or last but reducing the trail of the forks - ie increasing the offset, should quicken it up a good deal.

Er, so you think a curvier, more offset fork might improve things? By that I mean one with a bigger distance between a line projected down the headtube and the front dropouts. I'm toying with it as I like the bike and the kit on it and I could take the opportunity to put some light brazings on.

PW · Post by PW » 18 Jul 2007, 11:35am

If you know a framebuilder go and have a chat, they know the exact relationships between head angle and trail. Alternatively there's a graph of the interaction between them in a book on touring frame design by Tony Oliver - if you can locate a copy.

glueman · Post by **glueman** » 18 Jul 2007, 11:37am

I have a copy of that book. Will have a chat to the original builder. Ta.

Punk_shore · Post by **Punk_shore** » 26 Jan 2008, 10:12am

Hi bicycle scientists,

To me, a torque is a couple comprising force multiplied by distance. Torque acts about a point or axis. For a bicycle, suitable planes of reference might be the front and rear wheels. A suitable frame (not a pun) of reference might be the nearest actual or potential riding surface.

With regard to tyre drag, I would consider lateral (shear) resistance to be different from rolling resistance. What happens between the two tyre contact-patches is constrained by frame design.

Many people concerned with shimmy consider it to be a front wheel event. However, the bicycle illustrated under the publications tab at my profile website (see profile) indicates that the back wheel is a good place to start. For anyone interested in gyroscopic phenomena for bicycles, please contact me in the usual way.

Many thanks, per Bicycle Renewal Programme

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Stem Length Steering Geometry

shimmy shimmy (after the Undertones)