Calling Geometry Geeks! . Discuss here!

[Dr S]

You might be surprised Anthony, but they are not a million miles apart. Yes the Cotic weighs half as much and then some, and the suspended front gives greater grip and comfort. But allow me to post my own personal comparisons.

Both the Schwinn and the Cotic dislike climbing. The rearward weight bias on both bikes, coupled with high-ish front ends, makes the steering rather unresponsive. They both tend to wag about unless you conciously load some more body weight over the front, which in turn kills the traction. Both bikes therefore prefer to be pushed up a steep slope by a cursing rider.

When decsending, both bikes come into their own. The slack geometry puts you in the right place for really hammering down the trail and make it easy to pop the front over obsticles. The high speed steering response is similar on both bikes- easy to place with a nice blend of communication without being nervous. At speed the longer wheelbase also makes for a stable platform, making you push harder and harder.

Out of all the bikes I own they are the two that I most feel comfortable jumping. I've never been one for getting air as I tend to panic and stiffen up, but both bikes fly really well, feeling stable and controlled and without drama on landing. They both inspire great confidence.

It comes as no surprise to me. As previously mentioned, geometry and wheelbase are the same. The bars on both bikes are 750mm wide with between six and eight degrees backsweep and both run stems between 60-70mm. Remember that the Klunker pioneers used short BMX stems as long XC stems were unavailable.

I've riden both bikes down black graded DH tracks now, and can vouch that both bikes handled just the same- both bikes pushed as far as I can as a rider, me being the limiting factor in their performance. I'm sure in the right hands the Cotic would be much faster downhill- that's only natural, but to say the handling between a modern bike and those early Klunkers are poles apart is far from the truth.

To pick up on your final point about bikes being designed for different riding... Those original Klunkers were built (not really designed as such) to ride down hills as fast as possible. They did have short stems, no they didn't have long forks but found that a high front end was beneficial, hence the use of BMX riser bars in many instances.

As final proof for my arguement, if you took three bikes- a Repack style Schwinn, a mid 90's XC bike and a modern slack angled trail bike and rode all three on the same trail, then itwould be very clear to see which bikes are most alike in character.

 

[Anthony]

You're right that I'm surprised, but if they do handle similarly I would guess that it's by coincidence to be honest, rather than design. For example they can't really have the same head angle, as the Schwinn's is obviously fixed, while the Cotic's head angle will steepen by 2-3 degrees the moment you get on the bike. Also I'd be surprised if the Soul's stem isn't a lot shorter, which has a major effect on the outcome.

Still, if you find the Schwinn comparable to the Soul, even though it is so much heavier, it must be a pretty good design for its day.

 

[Dr S]

You're right that I'm surprised, but if they do handle similarly I would guess that it's by coincidence to be honest, rather than design. For example they can't really have the same head angle, as the Schwinn's is obviously fixed, while the Cotic's head angle will steepen by 2-3 degrees the moment you get on the bike. Also I'd be surprised if the Soul's stem isn't a lot shorter, which has a major effect on the outcome.

Still, if you find the Schwinn comparable to the Soul, even though it is so much heavier, it must be a pretty good design for its day.

Here is a photo of the Schwinn. The Ashtabula stem has a C to C measurement of 55mm. The Cotic has a 60mm stem.

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.

 

[Russell]

...if you find the Schwinn comparable to the Soul... it must be a pretty good design for its day.

Or that the Cotic (given that it is a mountain bike that needs to be pushed up the hills) is a pretty bad one?

 

[REKIBorter]

1994 Parkpre Pro Elite that goes where I think

Mind operated stability control was an optional extra on Parkpres'. The geometry is rather special and they do handle very well. As long as you don't ruin things with putting on a too long travel suspension fork on they corner like they are on rails. 72 degree seat tube and head tube angle. How'd a thunk it?

 

[Anthony]

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?

 

[Woz]

I'm loving this discussion!

I've always feel best on some classic XC 71 73 degree long top-tube affair with suspension corrected rigid forks. Stretched out and very open position. A bit arse-up and head-down style. Flat 5 deg bars cut to 540mm. I'm very much into the mid-90's late 90's and if I can find something right in the 00's. I'm always after something stabile but doesn't have a mind of it's own and get in panic situations on corners.

The "interesting" rides have been on a Cannondale CAAD3; long wheelbase seemed to put the bike into the "stability" at speed corner; it never felt nimble or a lively ride. Just solid and stiff but ground compliance was low - probably more to do with the material though.

Voodoo Hoodoo AL by contrast had a very short wheelbase even with 80mm REBA was nimble but the damn thing wheelied at an steepish incline - fore / aft weight balance just seemed wrong and could never find the sweet spot of saddle position and reach. I noticed it had a short tube and was disappointed it didn't have the characteristics of the Kona AA that I (wrongly) expected.

I've rode a PACE 2000 full rigid with RC31 which was essentially a rocket and bone-shacker; for me it felt a little "clunky" on anything with bumps but was fast and secure. They certainly got some things right.

The Marin Eldrige Grade is only a hack / pub bike and it feels sluggish; position wise I've got it virtually identical but I can't put my finger on why; I've put an offset seat-post to remove the "over the front" sensation and it's a little better.

Right, the Parkpre is a "little" different and to be honest I had reservations; 72 deg parallel with 400mm rigid is fine and a surprise. The wheelbase is short, but it still feels confident and lively but certainly not twitchy with a 120mm stem. Weight distribution seems spot on. I'm thinking of going with a 130mm stem to only increase reach but keep the same vertical differance between the saddle and bars.

I had for a short while a BP Stealth Fiberlite with full rigid. Essentially the forks were 20mm too short BUT on corners it would fly around. I suspect it had something to do with a lower BB shell meaning a lower CG. Needless to say at speed and on steep descents it was a nightmare. Another expensive disappointing XC modern it turned out to be.

A note: in every case on every bike I'm using the same pedals, same saddle and same grips meaning any influence of "different" contact points is removed.

I'm right now in the process of a DBR Axis TT build; looks like it will be good for my tastes but what is really annoying geometry specs don't REALLY tell the complete picture. I would like much more info on exactly the horizontal distance from the BB to the top of the seat-tube and head-tube and BB height in relation to horizontal wheel height. Another point is related to trail, off-set etc. Here it seems a dark art of selected the "right" fork for the frame for a given purpose. A rider has basically three contact points and mapping them in advance on frame geometry doen't answer questions like seat-post off-set, how long is the stem, what rise on the stem. There is too much guess work and trail and error IMHO.

A simple scaled drawing on a "grid" background would be much better.

 

[GrahamJohnWallace]

My experience is that tinkering with tyre width, air volume and pressure can have a far larger effect on handling than altering a few degrees of geometry. I associate frame geometry more with issues of efficiency and comfort.

There has been some mention of low centre of gravity improving control and changes of direction as would be the case with a car. But physics dictate that smaller masses can change direction quicker. Try chasing rabbits, they know a thing or two about escaping lager predators. As the rider is most if the mass of a moving bike we will have to accept that lighter riders will be able to change direction faster.

A bicycle however is totally different to a car as the rider can control the position of their body's centre of mass relative to the contact point of the tyres. The key factor in cornering is grip. A high centre of gravity will in fact improve balance and so control, but will not change the amount of grip as the downward force into the ground remains the same even on a penny farthing.
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Bellow is Geoff Apps' take on how a high centre of gravity improves balance...
"... try another experiment: find a long piece of material, such as a broom handle or length of wood about four or five feet long. Attempt to balance it vertically on your open palm. Tricky, but do-able. Now try the same thing with a pen. More or less impossible".

"This is because the centre of mass of the broom handle is further away from your hand than the c/m of the pen".

"This distance is critical; the greater the distance, the more reaction time you have to adjust the balance of the broom handle, whereas the pen has fallen over before you can react to get it into balance again."

"It's the same with a bicycle: your open palm represents the ground and the broom handle/pen represents a bicycle; your movements to maintain the balanced vertical position of the broomhandle/pen represent your use of the steering to maintain balance on a bicycle".

"Unless, I'm a complete idiot, this knowledge tells me several things:
1) A bicycle is unstable, so the idea that any design feature (apart from a sidecar) will provide any degree of stability, is nonsense.
2) Very few people understand this, or have even given the concept a moment's thought.
3) Centre of mass is the 'tool' by which a bicycle rider maintains balance when riding a bicycle.
4) The further away the c/g is from the ground, the less effort is required to maintain balance, leading to greater efficiency and less rider fatigue over a longer haul."
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This is why it is easier to do a track stand with your back arched high than when sitting in the saddle.
The only contradiction I can think of would be a fast slalom situation where the need to change the angle of lean quickly would favour a low centre of gravity. As this would require the centre of gravity to travel less distance and time between the extremities of lean. With too low a centre of mass, as with a recumbent, overbalancing would become a problem.

 

[Raging_Bulls]

I second Graham's notion of tyre width and pressure. I mostly ride on the road, and found that I can radically alter the behaviour by simply changing the tyre pressure.

The height of the CG does have a big influence on handling, but not in the same way it affects a car's behaviour.
As you mentioned near the end of your post, you need to move your weight over a smaller distance to change from left to right (or vice versa).

However in high-speed cornering a lower CG is not ideal because you have to lean at a greater angle to counter the centrifugal force.
Also, a lower CG usually indicates a lower BB, which means you need to stop pedaling at much less of an angle than you would with a higher BB.

As for the broom and pen thing, a broom doesn't have the centrifugal force of the wheel acting as a gyroscopic effect. Also, the bike's trial will improve stability. The geometry alone can have more of an effect on the way the bike balances itself while rolling than the CG does.
I have access to 2 Bullses with the same wheelbase, same BB height and same tyres. However mine has a 1° shallower head tube angle and 10mm more trail than the other one. You won't believe how much more nervous the other bike is, even though its CG is actually a bit higher (20mm riser bar vs my straight bar).

 

[Woz]

Interesting. BUT. I suppose many of us have done the case of using the same wheels, same tubes and same tyres on the same courses / trails with different frames. The noticiable difference must be then the frame & forks.

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.