Calling Geometry Geeks! . Discuss here!

[GrahamJohnWallace]

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 believe your maths to be correct but the equation is incomplete. It states that a given mass will take longer to fall the higher up it is by a factor of 2:1. So a 1m broomstick will fall in half the time of a 2m one. However with a bicycle the broomstick has a 12 stone mass (approx) at its top, and the equation given in Bicycling Science does not factor in the considerable inertia of this mass.

For anyone wanting the the complete picture in terms of the physics of inverted pendulums, try this...

http://en.wikipedia.org/wiki/Inverted_p ... ivot_point

The quote below is from a Berkeley University document and relates to a project to make self balancing robotic bicycles.
"Have you ever noticed that it is easier to balance a long stick in your palm than a short one? This is because the stick tends to fall "faster." Likewise, a short bicycle falls over more quickly than a tall one. So a kid's bike is actually more difficult to balance than an adult bike. Even if the bike is being balanced by some active mechanism, the frequency of oscillations around equilibrium will be slower for a tall bike. And slower is easier to control. So try moving your center of gravity up as high as possible. I know this sounds counterintuitive, but it should help. For instance, mount the heavy gyro above the seat, not near the ground".

 

[Russell]

A rider on a unicycle is articulated at the centre and any movement to correct balance is made at the top of the unicyle.

Im wrong here.

 

[Coleman]

Historically, this 'newer' geometry isn't new at all. The head angle, seat angle, bar height and wheelbase of my Cotic running a 140mm fork is nigh on identical to my klunkered Schwinn Excelsior. Things have just gone full circle. As someone once said, there is rarely anything new in cycling.

Si

I did a similar comparison the other day - I parked my on One 456 Ti against my 1985 Saracen Conquest and (taking into account suspension sag and the slightly altered trail on the Saracen ) the wheel base, head angle and position of bars/saddle are pretty much the same!

Apart from the obvious differences they 'feel' very similar to ride too

 

[Woz]

I'm already starting to feel mathematically challenged with the wikipedia links on the theory of an inverted pendalum......while interesting it is I'm still having an hard time in the application of this when riding (ie. moving).

"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?"

There seems to be a giant leap being made here from Balance theory to Control (interection of these two is not fully known yet). A sit up and beg bike when ridden hardly gives any sensation of being "racey", "nimble" and totally "controllable" in a large variaty of situations. We can leave aerodynamics out too. In my own experience the handling is something more akin to riding a docile Blackpool beach pony - they perfectly fit the bill for short loaded city tootling duties in the low-mid speed range. Clearly, the designers of frame geometry are doing something to fit this to the target joe public, but it is not obvious to me what parameters they are tweaking and what trade offs they are making.

Now, Keith Bontrager in his well known de-mistification of KOPS suggested a weight distribution of around 60% rear vs 40% front for an optimal bike fit / set-up covering the seated and unseated position. Just adding this to see if it helps the discuss along.

 

[Woz]

Historically, this 'newer' geometry isn't new at all. The head angle, seat angle, bar height and wheelbase of my Cotic running a 140mm fork is nigh on identical to my klunkered Schwinn Excelsior. Things have just gone full circle. As someone once said, there is rarely anything new in cycling.

Si

I did a similar comparison the other day - I parked my on One 456 Ti against my 1985 Saracen Conquest and (taking into account suspension sag and the slightly altered trail on the Saracen ) the wheel base, head angle and position of bars/saddle are pretty much the same!

Apart from the obvious differences they 'feel' very similar to ride too

I'm sitting tight with the "re-invention" of the golden mid 90s XC ere, but I fear it's already happened with current vogue of "Trekking Bikes".

 

[GrahamJohnWallace]

"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?"

There seems to be a giant leap being made here from Balance theory to Control (interection of these two is not fully known yet)....

I may be starting to get a grip of this issue?

First, here's Wikipedia's take on the fundamentals:

"Forward speed"
"The rider applies torque to the handlebars in order to turn the front wheel and so to control lean and maintain balance. At high speeds, small steering angles quickly move the ground contact points laterally; at low speeds, larger steering angles are required to achieve the same results in the same amount of time. Because of this, it is usually easier to maintain balance at high speeds".

Center of mass location

"The farther forward (closer to front wheel) the center of mass of the combined bike and rider, the less the front wheel has to move laterally in order to maintain balance. Conversely, the further back (closer to the rear wheel) the center of mass is located, the more front wheel lateral movement or bike forward motion will be required to regain balance. This can be noticeable on long-wheelbase recumbents and choppers. It can also be an issue for touring bikes with a heavy load of gear over or even behind the rear wheel.[19] Mass over the rear wheel can be more easily controlled if it is lower than mass over the front wheel".

OK, I'm fine with that. But what about low speed maneuvering over technical terrain? Here the rider wants to achieve precise turns without being forced to repeatedly veer off erratically in order to maintain balance. More weight towards the rear wheel would help in such situations. However, traveling in a straight line at low speed would require more rider skill as erratic handlebar movements would unbalance the bike. Conversely, on a bicycle with an extreme forward weight bias like when riding a tandem solo, could become twitchy as small amounts of steering movement would have a proportionately larger effect on balance?

This raises the subject of rider skill.
Within limits? a skilled rider could ride a bike with poor geometry well.
Though a rider with only basic skills would be advised to ride a bike with safe and predictable geometry.

*"Within limits"?
I recently made slight change to a prototype bike that completely messed up its hill climbing capabilities.
Before I changed the saddle it was a superb climber, afterwards the front wheel kept lifting with each pedal stroke. What I had not realised is that I has been used to sliding forward onto the nose of the saddle in order to keep my centre of gravity forward. The new saddle was extremely short and I was forced to ride out of the saddle which was too tiring for such a long steep climb.

So bike geometry is a complex and subtle subject than most manufacturers would have you believe. And good geometry and is down to so much more than the effects of steering geometry alone.

 

[Woz]

Brilliant epic post there GJW. Right now I can't study it in detail because I'm low on blood(y) suger and feeling the calming effects from the after-post-performance mandatory stout.

 

[FluffyChicken]

...
Both the Schwinn and the Cotic dislike climbing.
...

I'm not quite sure it is the bikes that dislike the climbing

 

[GrahamJohnWallace]

A cycling friend of mine has a Cotic Soul which can't handle some steep slopes that my 1983 Cleland can. This is despite the Cleland's steep geometry, upright and high riding position and centre of mass over the rear wheel. I haven't tried riding the Cotic up these slopes so can't gauge whether this is down to rider technique, tyre pressures, or inherent geometry.

It could be extremely enlightening to gather data on the performance of various designs and then look for common traits that link to their geometries.
A kind of evolutionary approach to finding an optimum geometry.

 

[Woz]

GJW you have made some good points.

If you go to google image and search on "DH bike" you get an idea. Lot's of specifically designed machines which are fairly common place. Bars higher than saddle etc. and monsterous forks. Geometry wise, characteristis fairly understandable and obvious enough to generalise.

Now do "hill climb bike". Interesting how the moto-x world sees the design with an uber long rear stay. Road bikes are there too but it looks like they tend to shave all weight off rather than design a specific frame (I stand corrected and hope a road guru can comment!)

Ironically, there is not a single specific "hill climb MTB bike". Perhaps there is an assumption that "XC MTBs" are great climbers of mountains anyway; or at the very least sufficient along with the other capabilities of spinning a 42T x 11T downhill / on the flat. The industry seems set in pushing next years XC bikes closer to last years DH bikes (BTW this is why I like Retrobike full of proper bikes )

This is perhaps the core of the discussion; the MTB industry has not matured enough and produced a sufficient spectrum of dedicated quantified designs and so it's up to us to figure this out through tinkering, empirical means and forums like this etc.