Frame dynamics

[Edition 507]

Does anyone know how the relevant frame dynamics determine your bicycles performance, handling and comfort?

 

In particular:

1. Wheel Base

2. Seat Angle

3. Head Angle

4. Fork Rake (would probably affect wheel base as well)

5. Anything else I may have overlooked

 

For example, I would imagine that a longer wheel base means a steadier ride while a shorter wheel base makes a frame more twitchy, but does a 1cm difference between any two frames actually make a noticeable difference?

 

JB, this would be particularly up your alley.

 

Thanks

[AndreZA]

Don't forget that 11 speeds are way faster then 10 speed.

I don't think it's got anything to do with performance but more with handling and comfort. But then again, if you are comfortable and and handling is better, you will perform better.

 

 

[Christie]

I've never come accross any info that relates geomerty to handling. I suppose that custom frame builders will know a lot, but not sure if they will give out their info. There are probably as many different frame geometries as there are bike makers.

[Johan Bornman]

Frame geometry is largely BS. No one frame maker has a secret concoction of angles and lengths that makes a noticeable difference or gives them a secret advantage. Their brochures make say otherwise.<?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

Through more than 100 years of development on the safety bicycle, we have arrived at frame angles that work - these are the angles you'll find on everyday bicycles made for athletic riding.

 

Chainstay length is nowadays determined by the size of the rear wheel or in the case of MTBs, the maximum width tyre to be fitted to the bike. Chainstays are so short nowadays that even a large tyre ensures that the wheel doesn't fit into the frame. The reason for this minimum length has everything to do with weight and nothing with handling. Shorter chainstays save frame and chain weight and in a world where everything gets weighed on a drug dealer's electronic scale, framebuilders won't come out with longer chainstays.

 

BB drop (distance from BB to road) is standard for road bikes and slightly higher for MTBs. It cannot be made any higher or we won't be able to put our feet out at traffic lights. Fiddling with this is silly and won?t let you clear larger obstacles or as the touring bike myth goes, give you more stability at lower speeds.

 

Top tube length used to be customisable according to the rider but today's one-size-fits-all doesn't give you a choice. Take it or leave it. Again, it is as short as possible because of weight.

 

Fork angles (or more accurately, head tube angles) are around 73 Degrees. Head tube angles were arrived at from the relatively smooth roads we ride on and the idea is to take road shock axially into the fork blades and over the years a balance was arrived at so that steel forkes failed equally forward or rearwards from the continuous flex from the rider?s weight and the road shock that forces it backwards. It was a balancing act arrived at experimentally. Today?s forks don?t fail in the same mode and there is room for making the fork angle steeper if we can increase the length of the top?tube for some foot clearance at the front wheel. This won?t happen for obvious reasons.

 

 Those factors thus take care of the wheelbase. I have built a frame with an extra long wheelbase. I made the chainstays as long as the tubeset permitted so that I can fit panniers and not have them touch my feet as I pedal. I also have a bicycle with the shortest possible wheelbase and I cannot tell you which one I'm on unless I look down. I cannot feel a difference in comfort, stability, climbing...nada.

 

 

Seat angles aren't experimented with much. Some cruisers and dikwiele have slack angles and that only determines your posture on the bike and again has nothing to do with handling.

 

You ask about performance. In the average bike performance from the bicycle's perspective is not improved by any frame geometry or material. All frames made from steel, ti, carbon or alu transfer 99.9% of input to the chain. Chains rob 6% of your power through the articulation process. Frames are very, very efficient.

 

There used to be arguments about curved vs straight forks but the long and the short of it is that it doesn't matter how you arrive at the front hub's position. It's the ultimate position that matters, not the curves and straight lines. Pinarello proved that with its deliciously wicked zig-zag fork.

 

Fork rake is just another word for head tube angle. We've already covered that. You probably want to know about offset. Offset or fork offset, is created on a road bike by curving the fork forwards and on a motorcycle or mountain bike, by mounting the stanchions forward of the head tube or ?offsetting them forwards.

 

Offset is necessary in order to reduce trail, the latter which makes the bicycle twitchy in the standing position. The right term for this side-to-side veering of the bicycle from too large a trail is called ?lean steer?.

 Trail is the distance between the centre of the contact patch on the wheel and the imaginary line drawn straight down the centre of the head tube. The larger this distance the more the bicycle veers from side-to-side when climbing.

 

Many years ago the editor of Bicycling magazine supposedly tested four or five similar frames for performance and handling and came out with some surprising BS. When I challenged him he couldn?t describe the effect of trail on a frame and blustered his way out of the debate by using barrels of ink.

 

You say you imagine that length in wheelbase makes a difference by stabilising the bike more as the length increases. Imagine is the right word. Our bikes are stable, most of us can ride no-hands no-problem. My long bike doesn?t display any extra stability and within the range that we?re working you won?t see a difference. A very long bike can be tested ? go for a solo  ride on a tandem and you still won?t notice an increase in stability.

 

Chainstay length in extreme climbing does have an effect. Longer chainstays on mountain bikes are an advantage on very steep climbs in that it helps the rider keep the front wheel down. Draw a simple line diagram and you?ll see what I mean.

 

It is a myth that shorter chainstays improve climbing. There certainly is no scientific bases to this. Perhaps in astrology or numerology.

Go ride and forget about your bike's geometry.

Oh yes, when you pick up a female rider on the road, geometry comes to play. You ride up next to her, give her a good lookover and say. "I really like your geometry." It works every time.

 

 

 

 

 

[Guest Big H]

Oh yes, when you pick up a female rider on the road, geometry comes to play. You ride up next to her, give her a good lookover and say. "I really like your geometry." It works every time.

 

Jou stout blixem....... dit werk NIE as jy Tandem ry en EM is jou stoker nie!!!!!!! Sy klap te hard!!!!!!!

[Dynamix]

I wasn't comfortable with my bike's handling. I tried different stem lengths and seat laybacks, nothing helped. I went for a fitting at the Sports Science Institute in Newlands and it was suggested that I needed a frame with a longer horizontal toptube length. I got a frame that's 27mm longer, and used a 10mm shorter stem (17mm nett), and I cannot believe how great my "new" bike feels! I agree that most geometries on bikes are so similar that it doesn't make a difference. However, I'm now convinced that each person has an ideal frame length. Determine that first, then find one that comes closest.

[1kTT]

you left out frame material (carbon, steel, Ti, etc) & construction method (glued, brazed, 1 piece, etc).  In my experience this makes the biggest difference to the 'feel' of the frame. 

 

The two biggest contrasts I can think of are an old Vitus ali frame that was super comfortable but always felt 'soft' and mushy a cannondale six13 (carbon and ali) which was a harsher ride but much more lively.  

 

Carbon, in the right hands, can be tuned to give the best of everything- light, comfortable, great handling and lively- see any Parlee review.

 

[LazyTrailRider]

Johan, if I may add:

 

Changing the head tube angle can have a dramatic effect on handling. Slackening it (ie more towards 68 or 69 Degrees) is beneficial to stability, as is typically needed on a downhill bike used on fast sweeping courses. Because downhill bikes tend to be steered (if done correctly) with whole body movements more than small movements of the bar (as is typically done in a road race), a slack head angle increases the amount of movement needed, thus reducing that twitchy feeling a road bike has through the average rock garden ;-)

 

Of course, a steeper head angle (some DH bikes are adjustable) provides a much more precise ride, but with increased concentration needed from the rider because each input at the bar has a more pronounced effect.

[Christie]

you left out frame material (carbon' date=' steel, Ti, etc) & [/quote']

 

I saw a test by Tour magazine where they took 8 frames,  made from carbon, Ti,  Alu, and a 1970s steel frame. They put the same components & wheels on, added fearings to disguise the frame shape.  They then added small weights till they all weighed the same. The frames were then painted grey all over.

 

A group of riders then tested the frames over 2 weeks. Long 3 hr rides over the 1st week, and shorter rides during the 2nd week, changing bikes multiple times per day.

 

They found that the riders could not tell the difference between the frames, like which ones were carbon and which were alu. They could not even id the 1970 steel frame.

[Edition 507]

Thank you gentlemen, I appreciate your input. So from this I conclude that the difference between available road frame geometries is neither here nor there and that worrying about a 1cm longer wheel base or a half degree steeper seat tube angle is just plain stupid.

[Johan Bornman]

Johan' date=' if I may add:

 

Changing the head tube angle can have a dramatic effect on handling. Slackening it (ie more towards 68 or 69 Degrees) is beneficial to stability, as is typically needed on a downhill bike used on fast sweeping courses. Because downhill bikes tend to be steered (if done correctly) with whole body movements more than small movements of the bar (as is typically done in a road race), a slack head angle increases the amount of movement needed, thus reducing that twitchy feeling a road bike has through the average rock garden ;-)

 

Of course, a steeper head angle (some DH bikes are adjustable) provides a much more precise ride, but with increased concentration needed from the rider because each input at the bar has a more pronounced effect.

[/quote']

 

Perhaps that should be "chganging the headtube angle DRAMATICALLY can effect the handling.

 

I would expect a downhill bike that's going to receive head-on impact to have a head tube angle to take on the impact head-on. A 68 degree head tube angle is dramatic.

 

But like I said, the head tube angle doesn't affect the handling much, it is the trail. Also, it is not so much that the bar has an increased effect but that the tendency to swerve is pronounced as the trail is increased.

 

I think this discussion started off on a road bike or general X-country bikes and I answered the question for that scenario. Richshaws, downhill bikes, jet skis and unicycles have different requirements and fall outside the norm.

 

I just want to point out that we don't steer with movements of the handlebar but with leaning into the corner. I can prove this by riding no-hands and steering into the corner without making movements to the bar.

 

 

[Johan Bornman]

I wasn't comfortable with my bike's handling. I tried different stem lengths and seat laybacks' date=' nothing helped. I went for a fitting at the Sports Science Institute in Newlands and it was suggested that I needed a frame with a longer horizontal toptube length. I got a frame that's 27mm longer, and used a 10mm shorter stem (17mm nett), and I cannot believe how great my "new" bike feels! I agree that most geometries on bikes are so similar that it doesn't make a difference. However, I'm now convinced that each person has an ideal frame length. Determine that first, then find one that comes closest.[/quote']

 

I agree with your general comments about bike fit but not that size affects handling. You were simply not comfortable on your bike and the SSI found you a more comfortable frame size. I agree that frame length cannot compensate for a too short a frame but today's Small, Medium or Large sizing doesn't give a fig about your comfort. Also, with the advent of carbon and aluminium bikes, the chances of getting a custom frame are nil.

 

 

[Dicky DQ]

Johan' date=' if I may add:

 

Changing the head tube angle can have a dramatic effect on handling. Slackening it (ie more towards 68 or 69 Degrees) is beneficial to stability, as is typically needed on a downhill bike used on fast sweeping courses. Because downhill bikes tend to be steered (if done correctly) with whole body movements more than small movements of the bar (as is typically done in a road race), a slack head angle increases the amount of movement needed, thus reducing that twitchy feeling a road bike has through the average rock garden ;-)

 

Of course, a steeper head angle (some DH bikes are adjustable) provides a much more precise ride, but with increased concentration needed from the rider because each input at the bar has a more pronounced effect.

[/quote']

 

I am no guru, but what i can say is that on an MTB, going from a Merida (relaxed head angle) to a mongoose. It was quite a big change. the Mongoose was fun and nippy until the singletrack got a bit long.

 

Then the Merida with the more "Relaxrd"head angle became more fun, or should I say easier to deal with. Not as eay to throw about, but if used a bit slower I could run "Faster" for longer............ or maybe this is just perception.

 

I am back on a Merida and feel way more comfortable now. Oh yes and the heas angle is the only significant difference between the two.

 

Just my 2ZAR's.

 

Not to say the one is better than the other, just that there are horses for corses.

[1kTT]

 

you left out frame material (carbon' date=' steel, Ti, etc) & [/quote']

 

They found that the riders could not tell the difference between the frames, like which ones were carbon and which were alu. They could not even id the 1970 steel frame.

 

Interesting, I would be surprised, but maybe it is like blind beer tasting- nobody gets it right. 

 

I would be very certain that I could tell the difference between my current road bike (Parlee), my previous one (Cannondale Six13) and the last bike I test rode (Cervelo Soloist).  Also between my last two mountainbikes (an Ali and a Ti).

 

[1kTT]

 

Also' date=' with the advent of carbon and aluminium bikes, the chances of getting a custom frame are nil.

 [/quote']

 

I have a full custom Carbon frame.

[Yang]

you left out frame material (carbon' date=' steel, Ti, etc) & [/quote']

 

I saw a test by Tour magazine where they took 8 frames,  made from carbon, Ti,  Alu, and a 1970s steel frame. They put the same components & wheels on, added fearings to disguise the frame shape.  They then added small weights till they all weighed the same. The frames were then painted grey all over.

 

A group of riders then tested the frames over 2 weeks. Long 3 hr rides over the 1st week, and shorter rides during the 2nd week, changing bikes multiple times per day.

 

They found that the riders could not tell the difference between the frames, like which ones were carbon and which were alu. They could not even id the 1970 steel frame.

 

Now THIS test sounds like a useful test, instead of the tests that you read about. It appears that test reports are like wine reviews- subjective and full of BS, determined by brand names and price. I'm with JB in terms of my cynicism of these things.

 

But the geometry is important. Rather than try to summarise it here, The book Serious Cycling by Edmund R Burke has a chapter "Perfect Positioning" which deals with the relationships between the various components of the bike in respect to performance. Eg, the relationship of angle of seat tube to the ground and your leg length, the length of your stem in relation to your torso length, etc. Its a must get book for any competitive cyclist.