Calling Geometry Geeks! . Discuss here!

[Dr S]

The head angle is the same too- 140mm fork sagged 20% when I took the measurement with an angle finder. I don't just make this shit up.

Also a head angle will not be reduced by much when sagged. If you go onto the Cotic website and look in the 'Geek' section there is a nifty demonstation of the affect fork length has on geometry. The difference between a 100 mm fork and a 160 mm fork isn't as great as you suggest at all. 20% sag will only reduce the angle by a fraction.

Yes it's just basic schoolboy geometry that every cm by which the fork gets longer slackens the head angle by c0.55 degree, factionally less the longer the wheelbase. So the difference between a 100 and a 160 fork is c3.3 degrees - although obviously if you assume as much as 25% sag, that becomes c2.4degrees.

The Soul is stated to have a 70 degree angle with a 100mm fork sagged 25%, so if you have a 140mm fork sagged 20%, that's 37mm longer, reducing head angle by c2 degrees to 68 - which you counteract by fitting a stem at the bottom end of their range.

Is the even shorter stem on the Schwinn part of their original design though?

Regarding the Schwinn stem, the one fitted is an Ashtabula BMX stem which shared the same dimensions as the original (and very weak) Schwinn item. They were the stronger alternative.

We could argue all day on theory and what and who says what. So I dug out the angle finder this morning, and took the Cotic and the Schwinn from the garage. To support my original claim that contempory frame sizing and geometry are the same as those early Schwinn Excelsiors, I recorded the following measurements

Measured using a angle finder on a level floor (checked). Both bikes with 40 psi in both wheels.

Head Angles

Cotic with 140mm fork (axle to crown of 520mm, regular external headset, tyre wall height of 60mm)

Cotic static head angle 65.9 degrees
Sagged 28mm (20%) 67.3 degrees

Schwinn 67.5 degrees

Difference = 0.2 degrees

Seat tube angle

Cotic 71.8 degrees static
73.0 degress sagged 20%

Schwinn 72.8 degrees

Difference 0.2 degrees

Wheelbase

Cotic 1100mm

Schwinn 1100mm- 1130mm depending on wheel position in sliding dropout.

Difference 0-30mm

I could continue with bar height, BB height etc, both I think I've made my point.

Si

 

[Russell]

I can't help but think though there is some "psuedo science" going on with Geoff Apps theories. It's a well known fact that a lower BB on a road bike will help descending and cornering at speed. Of course there is a practical limit of pedalling around corners for a given crank length. The moto-bike industry seems to take a similar approach. Why this notion would not apply to a "off-road" machine seems more to do with other practicalties rather than handling dynamics (for example going over logs and what not).

Balancing a broom handle in the palm of a hand would be - at best - similar to a uni-cycle rather than a two wheeled machine that has two contact patches with the ground that can essentially fall to only the left or only the right. I think there is a case of comparing apples with pears going on.

I totally agree. I'm not entirely sure that the broom handle analogy is appropriate or indeed correct, given the dynamics of a bike and rider vs a stick.

A rider on a unicycle is articulated at the centre and any movement to correct balance is made at the top of the unicyle, whereas Apps 'stick' analogy states that movement is made at the bottom of the object.

Likewise, a rider performing a trackstand on a bicycle will balance by moving the top of the bike left and right (remember, you balance a broom by moving its base) If someone can explain to me how you hold a trackstand by moving the base of your wheels left and right, I'd be quite grateful.

In my head (and I'm happy to be proved wrong by someone who can demonstrate their argument with maths rather than balancing a stick on their hand) I'm quite sure that a high CoG does not help balance.

 

[Russell]

As for the broom and pen thing, a broom doesn't have the centrifugal force of the wheel acting as a gyroscopic effect.

Hasn't the gyroscopic effect of wheels in balancing bikes been pretty much discounted now?

Seem to remember reading somewhere that the effect was negligable?

Might be wrong.

 

[Woz]

Dr S. I find that very interesting you are getting into measurements with real world "sensation" and "perceiption" of the whole ride. I do pretty much the same when I've found a bike that comes close to nirvana - I try to understand why it feels so right with hard facts.

Could you also map your position on the two bikes and state the differences?

Distance from BB center to tip of saddle.
Distance from tip of saddle to front of handlebars at the stem joint.
Distance from BB center to front of handlebars at the stem joint.
If you have the tools, and estimate of saddle vs bar height is interesting too.

Also state if you delibrately made any differences based on intended purpose.

You seem to be proving a point that the rider position on a given bike regardless of the bikes intended capability is possibly the most important consideration and a dominating factor of things feeling right - or am I off here?

 

[Dr S]

Distance from BB center to tip of saddle.
Distance from tip of saddle to front of handlebars at the stem joint.
Distance from BB center to front of handlebars at the stem joint.
If you have the tools, and estimate of saddle vs bar height is interesting too.

There isn't much point in measuring these areas because bike set up is down to the individual. Saddle height, angle, position on the the rails, bar angle and rotation are all down to personal preferences and not fixed.

Of course these factors are vitally important to making ones own bike fit and feel 'just right', but in this debate they are more or less unimportant.

 

[Woz]

I agree, but on one side you have proved the point that two frames and forks are virtually identical down to mm and 0.x of a degree and handle very similar, but we don't know how radically different or similar your own personal position is on each of them.

Hope you see the point, because I think you are onto something and so far all we know is that two "bikes" are similar but no confirmation you are set-up on them similar. Surely this a large part of the puzzle to crack?

 

[Dr S]

I agree, but on one side you have proved the point that two frames and forks are virtually identical down to mm and 0.x of a degree and handle very similar, but we don't know how radically different or similar your own personal position is on each of them.

Hope you see the point, because I think you are onto something and so far all we know is that two "bikes" are similar but no confirmation you are set-up on them similar. Surely this a large part of the puzzle to crack?

I can see where you are going with this, but like saddle and bar position, the rider is not fixed in any one position. We are constantly moving around, moving weight around the bike for and aft and side to side. Saddle height, angle and bar rotation are factors that determine personal comfort on those rare occasions we get to sit down. On a road bike these factors may be more relevent as more time is spent in a fixed position. Even then a rider moves forward on the saddle in climbs for example, but largly the road rider tends not to have to move around the bike as much.

 

[GrahamJohnWallace]

As for the broom and pen thing, a broom doesn't have the centrifugal force of the wheel acting as a gyroscopic effect.

Hasn't the gyroscopic effect of wheels in balancing bikes been pretty much discounted now?

Seem to remember reading somewhere that the effect was negligable?

Might be wrong.

Yes! in bicycle physics circles the idea that wheel gyroscopics has anything more than a miniscule influence on balance has been old hat for decades.

To use the broom handle analogy but think 4ft long broom handle (lever) with a 12stone weight at top end and the fulcrum at the bottom end.

With all that weight top and the force of any movement being magnified about fourfold by the lever by the time it gets to gyroscopic input, (i.e. the 13" above the ground where a wheel would be connected) the force generated by a gyroscope would be puny in comparison. Even from a very heavy and fast spinning wheel.

Page 266 of Bicycle Science (third edition) has a fuller explanation including Geoff Apps' "pseudoscience" broomstick analogy. Apparently it's because "...the time it takes an object to fall is proportional to [yCM/g]1/2, where yCM is the height of the Centre of Mass above the support..." (Support = fulcrum of the lever).

It's a braver man than me who dismisses the ancient wisdom's of Geoff Apps. Geoff's understanding of all things bicycles is intuitive and he notices things going on when he rides, in a way that most riders don't. He then looks for explanations. When no one can explain whats going on he experiments to get to the bottom of things.

 

[Woz]

I can see that "things" higher up will take proportionally longer to fall. If you use the formular and find the speed for something to fall at, say 1m high and then 2m high what do we get?

If I did this right I get 0.052 seconds and 0.102 seconds. I then went here http://www.humanbenchmark.com/tests/reactiontime/ to get an indication of human reaction time. I came out with 0.228 seconds on the second attempt - note that this is clicking on a mouse from a visual input rather than sensing a bike starting to toppling and initiate a counter reaction to straighten it again.

Now, I can only judge this would be ever so marginally helpfull when going extremely slow, for example starting to pedal, but the above suggests (if I got it right) I don't have any chance to sufficiently react before it falls over.

How the above relates to speed increasing and when the bike is actually going forward in a straight line and going around corners I'm not sure; the reaction time and falling speed suggest there are other more dominant factors at play to how a bike handles.

 

[GrahamJohnWallace]

I can't help but think though there is some "psuedo science" going on with Geoff Apps theories. It's a well known fact that a lower BB on a road bike will help descending and cornering at speed. Of course there is a practical limit of pedalling around corners for a given crank length.....
.....Balancing a broom handle in the palm of a hand would be - at best - similar to a uni-cycle rather than a two wheeled machine that has two contact patches with the ground that can essentially fall to only the left or only the right. I think there is a case of comparing apples with pears going on.

I totally agree. I'm not entirely sure that the broom handle analogy is appropriate or indeed correct, given the dynamics of a bike and rider vs a stick.

A rider on a unicycle is articulated at the centre and any movement to correct balance is made at the top of the unicyle, whereas Apps 'stick' analogy states that movement is made at the bottom of the object.

Likewise, a rider performing a trackstand on a bicycle will balance by moving the top of the bike left and right (remember, you balance a broom by moving its base) If someone can explain to me how you hold a trackstand by moving the base of your wheels left and right, I'd be quite grateful.

In my head (and I'm happy to be proved wrong by someone who can demonstrate their argument with maths rather than balancing a stick on their hand) I'm quite sure that a high CoG does not help balance.

I agree that the broomstick analogy is highly simplistic even in terms of modelling a unicycle. My own analogy would be that of a rigid vertical squarish board with two bottom corners touching the ground. Between these points, high up near the middle of the board is a large weight of 12 stone (approx).

One point of ground contact is fixed (the fulcrum) and the other is moved from side to side in order to counterbalance the weight above.

The two points of ground contact also form a second horizontal lever.
The further back the centre of mass is towards the rear wheel (point of contact) the more there is over leverage over the centre of mass and so more control.

SHOCK/HORROR! If a high centre of gravity improves control over the riders mass, then by the same logic sit-up and beg riders with their weight closer to the rear wheel should have more control than those of racing bikes?