Calling Geometry Geeks! . Discuss here!

[Woz]

It's late and I've had a few but there is a constant battlle of modern is better than retro. Personally cocking my leg over a retro bike brings a a happy smiile; but not everytihggis created equal. I have a 19995 Marinn Eldrige Grade that steers like a drunk pig with a stick, a 1994 Parkpre Pro Elite that goes where I think and a 1998 Clockwork Orange that goes in between the two.

Why!!!!!!!!!!!!!!!!!!!!!!!!!!!!

I would like here some proper real world analytitcis of what is optimumum for the intended purposse ot uutilisation. Not some fancy crap from a MTB mag review. I'm half-arsed with ideas has it is, but more inpuut the better.

What maketh a bike handle like a wet dreeam cum tru and why and give proof!

Discuss!!!!!!!!!!!!!

 

[Raging_Bulls]

Tough one. One person's ideal geometry is another person's nightmare.

I think it has a lot to do with the CG. The height and horizontal position has a major influence on how the bike behaves in a corner and how stable it is.
The optimal geometry probably depends on your own height and weight. A friend of mine is 5" taller and 70lbs heavier than me, and whenever he gets on my Copperhead, I can see that the bike is a lot more nervous than when I ride it myself. You can really see how the position of the weight affects the bike's behaviour.

Also, the frame flex can make or break the way a bike handles. Too stiff, and it's unbearable and gives little feedback. Too much flex on the other hand can cause unpredictable behaviour.

My Copperhead has a more agressive headtube angle and shorter rake than my Sbike, but is in fact the most relaxed ride due to having a longer wheelbase and chainstay length.
The lower CG also makes the Sbike change direction (left corner to right corner and vice versa) a lot quicker and while feeling more planted.

Which one I prefer? The old bike is always nervous, the modern one is excellent at touring but always ready to play along if you feel the need to do stupid stuff. The modern one is also a lot more comfortable due to the front suspension and how that puts the handlebars at the optimal height (same height as the saddle).
So in a nutshell, I prefer modern geometry.

Then again I've ridden old bikes that had such slow geometry that you really had to wrestle it into a corner.

There's no exact formula. You need to find out what works best for you, and stick to that.

 

[jax13]

the dark art of geometry & how it has changed is interesting.

the one thing that shocks me is the angle of some crossbars & the headtube angle, some moderns sit more like something off american chopper as compared with a rigid retro (or a suspension corrected retro with appropriate length fork).

what handling benefits have 'they' decided slack headtube angles etc. give you over a near vertical one?

 

[Andy R]

what handling benefits have 'they' decided slack headtube angles etc. give you over a near vertical one?

Headtube angles on mountain bikes only really vary within a range of say 71° to 65°, so not that great a variation. Bikes with slacker head angles tend to be better high speed descenders but there's a lot more to steering geometry than just head angle. For every change in head angle there is a resulting change in the amount of trail, and how much trail you arrive at for a given head angle depends on the fork offset and the wheel diameter.

A bit more about it here.

Back in my trials riding days I spent more hours than I care to remember machining fork yokes with custom offsets to reduce or increase trail for different steering response - most commonly yokes with more offset to reduce the trail for trials (sidecar) outfits.

Your other point about top tube angles - this, of course has no effect on steering geometry but does give you more standover clearance and makes for a frame that's easier to move around under you, when compared to a big garden gate of a thing.

 

[merckx]

my 1995 zaskar and my 2007 trek 8500 are worlds apart.

and in short, the zaskar is a basket case, and the 8500 is a dream.

the basic differences, trek has a much longer top tube, steeper head angle and seat angle. frame is designed around a 100mm fork. the front end is a lot higher than the zaskar, but i can run the stem flat and slammed and it is perfect.

the zaskar runs a 140mm stem, the trek a 110. overall the trek is still longer....

i think the big problem with the zaskar geom is:

top tube way too short
head tube is waaay to slack
seat angle is bordeline, but still a bit lazy
very low front end, designed around a 65mm or so fork. it puts too much body weight too far over the front axle.

but i still love it of course...

 

[Dr S]

what handling benefits have 'they' decided slack headtube angles etc. give you over a near vertical one?

'We' the consumers, have decided that slack angles are beneficial, not the manufacturers.

As suspension as got better, longer and lighter, and brakes have improved, it has become possible to really push the limits of just what is possible on a mountain bike. Tall, short and steep frames were alright for cross country bimbles and messing around in the woods once upon a time, but with the advent of these better technologies, riders of all abilities can tackle ever more challenging terrain. Try riding flat out down a rock filled, washed out one in three gulley on an old steep angled XC bike with 63mm travel and canti brakes and more often than not you will end up over the bars. With the new breed of long and low, slack angled frames, coupled with said modern long travel forks, you can ride must faster with greater control.


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

 

[Anthony]

I have a 19995 Marinn Eldrige Grade that steers like a drunk pig with a stick, a 1994 Parkpre Pro Elite that goes where I think and a 1998 Clockwork Orange that goes in between the two.
Why!!!!!!!!!!!!!!!!!!!!!!!!!!!!

I doubt whether they were all that different when they left the factory, but the current setups may not be original and the non-original elements could be creating big differences. Why don't you show us pictures?

A common mistake is to focus too much on the head angle. That's just one of the factors that determines a bike's steering characteristics. A variant of this is the huge number of people who think their frame has a particular head angle. This shows they're not interested. If they had ever thought about it, they would have seen that fitting a longer fork will reduce the head angle. Nevertheless, most makers quote a head angle without saying what length of fork gives that head angle (or whether they have taken it sagged or un-sagged). This is meaningless and presumably indicates that they think their customers are stupid and/or not interested.

Essentially, the factors that will make the steering faster or more lively are:

a steeper head angle,
more fork offset,
a shorter stem,
more swept-back bars,
shorter wheelbase,
smaller wheels and
narrower tyres.

Designers juggle with all of these factors to bring about the desired result, which is generally felt to be fast but not so fast as to be twitchy - so a sweet spot between the two.

The reason why 26er head angles are lower than they were is suspension. If you have a modern fork with say 120mm travel and a frame that gives a 71 degree head angle, static, with that fork, then the head angle at full fork compression would rise to c78 degrees. Compound that with the reality that max compression is most likely going fast downhill when the bike is nose down anyway and you've got a dangerous bike.

So when you have such wide variations in potential head angle, you need to start from a lower figure, say 68. But to give lively steering with a 68 degree angle, you need a shorter stem, say 70mm will be way lively. But then to give the correct length you need to lengthen the top tube, which with the slacker head angle will give a longer wheelbase. So designers juggle all these factors until they arrive at the right balance for 'the average rider'.

But you may not be average of course, so you may want to re-juggle some of them to give you what you want.

 

[Rio]

what handling benefits have 'they' decided slack headtube angles etc. give you over a near vertical one?

'We' the consumers, have decided that slack angles are beneficial, not the manufacturers.

As suspension as got better, longer and lighter, and brakes have improved, it has become possible to really push the limits of just what is possible on a mountain bike. Tall, short and steep frames were alright for cross country bimbles and messing around in the woods once upon a time, but with the advent of these better technologies, riders of all abilities can tackle ever more challenging terrain. Try riding flat out down a rock filled, washed out one in three gulley on an old steep angled XC bike with 63mm travel and canti brakes and more often than not you will end up over the bars. With the new breed of long and low, slack angled frames, coupled with said modern long travel forks, you can ride must faster with greater control.


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

Excactly!!
I would like to add, that in discussions about geometry and handling, people tend to overlook the effects of for instance handlebar width and stem lenght. Personally, I don't use stem length to correct the frame to my dimensions. I just buy frames (if I intend to ride them a lot) with a certain length of eff. top tube so I know what stem will fit me. But that's just me off course...

 

[bm0p700f]

Bicycling Science edition three is a very good read if you interested in the Physics of bicycle stability. A recent paper by one of the autors of this book looked at how trail affect the stability of a bike and the maths in the supporting documents got very complicated. The result though was even a bike with no trail and no gyroscopic effect of the wheel (a counter rortaing wheel) was still able to self correct it steering.

In this paper or was it the book a bike built without trail was done by someone at some point and it was very difficult to ride.

One of the bisgest changes in rcent years as has only been mentioned once in this thread is stem length, handlebar width and rise. These have a massice impact on the way the steers and hense the way it feels. Also changing the riding postion by changing fork length, stem position on the steerer tube and stem rise + handle bar rise affect the way we feel changes in the bike as the terrain changes.

For relaxed wood land/XC/long distance riding with I would want a bike with a steerer angle of 70/71 degrees and shorter top tube + enough rise in the stemm and handle bars to give me a more up right riding position as this is more comfortable.

If I am out on a faster XC ride then I want want a longer top tube narrower bars less rise in the stem and handle bars to move my torso and COG forward and bring it lower. I would also want shorter travel fork to bring the down the front end of the bike and lower COG.

The lower the centre of gravity the quicker you can change direction. Hence the new P7's with the frame set up for 120mm travel forks looks like the front end is too high. No longer an XC bike like the earlier ones with shorter rigid forks keep me lowr to the ground. Old Kona's do the same but mine a bit too small for me but my old Marin Pallisades feels great on a relaxed woodland/XC ride. My Orange EVO2 decends well enough due to a slack head tube angle and rding postion puts weight over the back wheel and is a decent comprimise for XC riding too but it is no XC racer.

Does this make sense?

EDit I have just relaised DrS has made some of these already. Also if you read the book I mentioned it really illustrates his point well there is nothing new in cycling.

 

[Anthony]

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.

As I explained, these a just a few of the factors involved, so the fact that a Cotic Soul and a Schwinn Excelsior might have head angle, seat angle, bar height and wheelbase in common doesn't mean that they they handle the same.

I don't think it's correct to say that things have come full circle. MTBs were designed for a completely different purpose twenty years ago than they are now, so it's not surprising that the designs are so different. Long travel forks and short stems obviously weren't even in designers minds twenty years ago, and the kind of off-road cycling that people did back then was also completely different.