Calling Geometry Geeks! . Discuss here!

[MikeD]

My take on bottom bracket heights is this. A tall BB means that the bike takes longer to fall over for a given input -- the broom handle. The corollary of that is that you need a bigger input to make it fall over, as you need to do to turn a corner. A low BB -- the biro -- is less "stable" in the sense that it'll tip more quickly for a given input, but it also means that it needs smaller inputs to correct. If you don't want it to fall over it's easy to pull back. So the rider's perception is of a "more stable" bike -- it's more controllable at speed, because it takes less rider input to make it go where you want.

As soon as you stick a human rider on a bike (which is after all the point of them ), physics stops telling the whole story

 

[GrahamJohnWallace]

I love hill climbing, the steeper the better as far as I am concerned. I can't be the only rider that likes the challenge of a steep hill.

As a result I have tuned my bikes and riding techniques with climbing in mind. Locally I have a hill whose final section is 44percent of chalky grass downland. I use this to test the climbing ability of my bikes. Some can't climb it at all. Some will manage it occaisionaly but only two bikes can climb up reliably, one being my 1983 Cleland Aventura. This bike doesn't look like the ideal of a good climber and weighs 36lbs, so is hard work. However this bike's slow but consistent climbing ability often surprises fellow riders and onlookers, it frequently out climbs modern machines. A friend of mine rides a Rollhoff geared Singular Swift 29er which is also an impressive climber and has similar steep frame angles and short wheelbase.

Perhaps we should start another thread where people can record the geometries and features of bikes that climb well?

 

[hamster]

Woz, your experience of the Marin is the same as mine (I owned a 1994 Pine Mountain from new).
It was really improved with suspension. I think that they effectively fitted the wrong fork with the 390mm a/c one as standard - certainly putting a 63mm suspension fork or 410mm a/c P2 transformed the handling from slow and vague to sharper and more positive.

 

[bm0p700f]

All bike design is a trade of. I am not luckly enough to have ever riden a Cleland, I would dearly like to though. A Cleland has a wonderful riding position for what I would term woodland/grass riding up and down hills. It is not what would think is a race bike in that something that can be riden very qucikly across trails. I imagine it is more of a bike that will get you from A to B at a pace and you will have a lot of fun doing it but I cannot imagine racing one in a XC race at Thetford. I do agree that a higher COG leads to a more stable bike in that it maintains its balance better (you have to agree with Physics) but I still think that a lower COG allows the bike and rider to change direction a bit quicker but the rider will have to make more frequent (sometimes large) corrections to maintain stability (unless someone can demonstrate to me I am wrong). Such a bike may feel a bit twitchy and require more skill to ride it.

However I am not sure therfore how recumberants balance so well, maybe its is becuae the COM is much closer to front wheel and there is a significant mass (legs) above it.

It's all a trade off with no prefect geometry.

For those who like bicyle science ed 3 may like this link http://ruina.tam.cornell.edu/research/t ... papers.php

Reading the chapter on stability in Bicycle Science David Wilson notes are pertenant, in that there is little agreement among experts on the physics of stabiity of bicycles.

I would love to ride a Cleland one day I think it would suit my style of riding wuite well.

 

[GrahamJohnWallace]

My take on bottom bracket heights is this. A tall BB means that the bike takes longer to fall over for a given input -- the broom handle. The corollary of that is that you need a bigger input to make it fall over, as you need to do to turn a corner. A low BB -- the biro -- is less "stable" in the sense that it'll tip more quickly for a given input, but it also means that it needs smaller inputs to correct. If you don't want it to fall over it's easy to pull back. So the rider's perception is of a "more stable" bike -- it's more controllable at speed, because it takes less rider input to make it go where you want.

As soon as you stick a human rider on a bike (which is after all the point of them ), physics stops telling the whole story

Thanks for that mike. Your comments have been thought provoking and helped me to connect various bits of information I have come across whilst researching this topic.

Here is a question from the Berkely University article:
"Can you explain how my six-year old daughter LuLu can ride her bike in a straight line without ever exceeding 0.5 mph? Don't you have to have some kind of speed just to keep from falling over? Maybe she's a freak of nature".

Answer:
"Suppose you've tilted over three inches. To get the wheels back underneath you, you have to steer three inches over. If you are riding fast, you only need to turn the front wheel slightly to get over quickly. If you are riding slowly, however, you have to make a very sharp turn to get over quickly. Slight turns are easier than sharp turns to execute and control, so riding fast is easier than riding slow".

So the amount of front wheel movement required to balance is the same for any bike and raising the bottom bracket height and so the centre of gravity will not change this distance or amplitude. What a higher centre of gravity actually changes is the frequency of the correction required. This is because according to pendulum theory the higher the COG, the slower a bike will fall and so less corrections are required over a given time.

Wikipedia States...
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".

So steering amplitude is related to speed and not the height of the COG.

However steering amplitude is also connected to the position of mass within the wheelbase as a 3" movement at the front wheel will will only produce 1.5" of movement at the COG if the COG is halfway between the front and rear wheel.

I am starting to piece together the "big picture" of how balance, COG height and COG position interact. and will post something for people to pull apart soon.

 

[GrahamJohnWallace]

All bike design is a trade of. I am not luckly enough to have ever riden a Cleland, I would dearly like to though. A Cleland has a wonderful riding position for what I would term woodland/grass riding up and down hills. It is not what would think is a race bike in that something that can be riden very qucikly across trails. I imagine it is more of a bike that will get you from A to B at a pace and you will have a lot of fun doing it but I cannot imagine racing one in a XC race at Thetford. I do agree that a higher COG leads to a more stable bike in that it maintains its balance better (you have to agree with Physics) but I still think that a lower COG allows the bike and rider to change direction a bit quicker but the rider will have to make more frequent (sometimes large) corrections to maintain stability (unless someone can demonstrate to me I am wrong). Such a bike may feel a bit twitchy and require more skill to ride it.

However I am not sure therfore how recumberants balance so well, maybe its is becuae the COM is much closer to front wheel and there is a significant mass (legs) above it.

It's all a trade off with no prefect geometry.

For those who like bicyle science ed 3 may like this link http://ruina.tam.cornell.edu/research/t ... papers.php

Reading the chapter on stability in Bicycle Science David Wilson notes are pertenant, in that there is little agreement among experts on the physics of stabiity of bicycles.

I would love to ride a Cleland one day I think it would suit my style of riding wuite well.

Hi bm0p700f.

There used to be a Cleland owner who lived in Bury St Edmunds but I have lost touch with him. I may be holidaying on the Shotley Peninsula in early June and will be taking some Clelands. It may be possible to meet up there?

Anyway, to imagine what a Cleland rides like, think big wheeled BMX bike with a proper seat position, if that makes any sense. They can be much swifter than they look especially when the terrain suits them.

I own a carbon fibre Giant Nrs which was a well reputed XC race bike in its day. It is a lot faster than a Cleland on the road or towpath but is not as fast over technical terrain. It generally downhills at a similar speed.

The next step after I work out exactly how COG and balance interact is to work out how significant these effects are to real bikes, including recumbents.

 

[Woz]

Woz, your experience of the Marin is the same as mine (I owned a 1994 Pine Mountain from new).
It was really improved with suspension. I think that they effectively fitted the wrong fork with the 390mm a/c one as standard - certainly putting a 63mm suspension fork or 410mm a/c P2 transformed the handling from slow and vague to sharper and more positive.

Interesting. I'm using the original Rocksta fork and original Marin Light whopping 120mm or 130mm stem. There are 1.75" roady type tyres on it and I'm running a very low 42 x 20 magic gear SS set-up since it's for short crappy weather commuting duties. It's got that brilliant "roughness" about it meaning it won't get stolen...Position wise, I'm set-up as close as I can get with my other bikes.

Right now I'm thinking:
a) Just wrong for what I want. I suspect it's one of those bikes that start to feel lively around the 30 MPH mark. Marin looks like they positioned it close to some sort of recreation / touring 26" bike than responsive trail race pedigree (I stand corrected by Marin fans )
b) Flog it in spring and be done with it. If I kept it for longer rides I'd probably fall asleep at the wheel and wake up just to late on a corner.
c) Don't bother with it, leave it be. I just can't justify it being a money pit.

I have noticed the wheelbase is long, and the original in-line post didn't get me over the back wheel enough. Reach is fine. I couldn't find any geometry information about them.

 

[Woz]

Some real world observations - very personal of course. Everytime I do a new build I'll have a ride on familiar ground with same wheels, tyres, saddle, grips and pedals to get a feeling of the new build. One part of this is riding with no hands at a moderate speed - it gives me a sort of quick indicator how the bike is responsive to my command.

If I can hardly ride with no hands I know it may be twitchy untill I hit some faster descents to find out, but will already know it demands my hands on the bars to exercise control - the steering is very light.

Extreme opposite is no hands and climbing an incline and the bike just keeps holding it's own straight line; here I have an idea the bike will need to be wrestled around most corners, won't be sharp, but will be stable when fast in a straight line or on long pedalling sweeping corners.

9 / 10 I'm finding the bikes that give me the most enjoyment and handle best are those in the light steering camp give me white knuckles in top gear but nonetheless behave.

Does that blurb make sense or should I start taking medication?

 

[Rio]

Does that blurb make sense or should I start taking medication?

it totally makes sense to me. Or does that mean we both need medication?

 

[bm0p700f]

Yes GrahamJohnWallace I would love to meet up and at least see on in the flesh.

Maybe we can work something out come June.