Massive fork trail variation between popular race bikes

[DaveS]

@wooger. Your hindsight seems to be clear. If you make a change to the fork rake it should be on the safe side to create a little more trail, not less. Now you know.

[apr46]

I find the notion of a low BB being more stable a bit funny. Most of the mass on the bike is the human body and the higher the body is relative to the ground, the greater the distance the center of mass has to travel in order to lean the bike. Look at crazy tallbikes like Stoopidtall and Stoopidtaller. Those bikes want to stay upright and leaning over meter still leaves the bike relatively upright.

I mean it depends on the frame of reference, relative to the rider a lower BB feels more "stable" because the system is more often in equilibirum vs. trying to get there; but yeah you are correct a lower COM is going to be more responsive. Longer wheelbase and lower BB are countering each other

The fork is important. My fitter, who is also my custom frame's designer, won't produce the drawing / proceed with the design until I actually purchased the fork with the necessary dimensions. The fork rake will also dictates the front center and wheelbase, both affect the handling.

Designers of mass-produced framesets are great at designing / making compromises of rideable bikes for the masses.

Bike geometry is very important to how people feel the bike but it's too complex for marketing department to market it. There is no linear dimension to allow consumers to quickly compare and diffentiate, unlike weight or aero drag. Lower weight and drag are good, but is lower trail always good? How to market this figure / dimension?

I wont argue too hard on this. When I say trail isnt a primary driver of the model and fork rake / offset is a tool, I am completely ignoring the elephant in the room which is the simple fact that for carbon forks you cant get the exact fork you want and need to pick between a selection of sizes, so you absolutely need to refine the design based on the fork you can get or make.

The one argument I will make in terms of what is wrong with bike geometries is that I do beleive that trail figures often end up going the wrong way. Shorter riders should have less trail and not more on their bikes as they are likely riding narrower bars and shorter stems. A shorter trail also makes the bike more resistant to crosswinds with deep carbon wheels. There are obviously a lot of tradeoffs and I think its honestly a good idea to avoid the complexity and just ride the bike with the right fork.

[hannawald]

I find the notion of a low BB being more stable a bit funny. Most of the mass on the bike is the human body and the higher the body is relative to the ground, the greater the distance the center of mass has to travel in order to lean the bike. Look at crazy tallbikes like Stoopidtall and Stoopidtaller. Those bikes want to stay upright and leaning over meter still leaves the bike relatively upright.

I mean it depends on the frame of reference, relative to the rider a lower BB feels more "stable" because the system is more often in equilibirum vs. trying to get there; but yeah you are correct a lower COM is going to be more responsive. Longer wheelbase and lower BB are countering each other

Lower BB should be more stable because of lowering the center of gravity. That was the recipy of Cervelo when they launched Caledonia 5 endurance bike - lower BB and lower HT angle. It is also good for mtb but you have to keep in mind the crank clearance... traditionally BB drop on road bikes has been 68mm, the reason being you need crank clearance when leaning the bike like pros but with bigger tires it allows for higher BB drop. It is now fashionable to go to 72mm, Caledonia 55 has 74mm and Trek Domane 78mm (sizes 56).

[DaveS]

I have two bikes with 67mm of trail and one with 68mm. The two have 71.5 HTA and 43mm fork offset. One has a 71 degree STA and 45mm offset. These are all next to smallest size. They all handle great. I use 38cm bars and 110mm stems. Smallest sizes usually have 0.5 less HTA for more trail and to minimize toe overlap.

[apr46]

I find the notion of a low BB being more stable a bit funny. Most of the mass on the bike is the human body and the higher the body is relative to the ground, the greater the distance the center of mass has to travel in order to lean the bike. Look at crazy tallbikes like Stoopidtall and Stoopidtaller. Those bikes want to stay upright and leaning over meter still leaves the bike relatively upright.

I mean it depends on the frame of reference, relative to the rider a lower BB feels more "stable" because the system is more often in equilibirum vs. trying to get there; but yeah you are correct a lower COM is going to be more responsive. Longer wheelbase and lower BB are countering each other

Lower BB should be more stable because of lowering the center of gravity. That was the recipy of Cervelo when they launched Caledonia 5 endurance bike - lower BB and lower HT angle. It is also good for mtb but you have to keep in mind the crank clearance... traditionally BB drop on road bikes has been 68mm, the reason being you need crank clearance when leaning the bike like pros but with bigger tires it allows for higher BB drop. It is now fashionable to go to 72mm, Caledonia 55 has 74mm and Trek Domane 78mm (sizes 56).

If your frame of reference is external to the system, physics says you are wrong. A bike can be abstracted as an inverted pendulum. The longer the inverted pendulum the slower the response is to input. A higher BB has the effect of lengthening the inverted pendulum.

Think of it as trading effort for time. Higher your COM is the longer your effective lever so moving off equilibrium requires less force. It also requires less force to shift the bike by a defined amount of cm. However, acheiving the desired lean angle always requires the same amount of work / energy to achieve, as the COM height in the energy calculation cancels out. While you might be able to throw the bike faster under you in terms of COM moving a defined amount of CM you are looking for lean angle and gravity is a constant so you end up with a system that responds slower the higher up you are.

[Nohands83]

The one argument I will make in terms of what is wrong with bike geometries is that I do believe that trail figures often end up going the wrong way. Shorter riders should have less trail and not more on their bikes as they are likely riding narrower bars and shorter stems. A shorter trail also makes the bike more resistant to crosswinds with deep carbon wheels.

As a larger rider (80kg 184cm) I've noticed this, always felt counter-intuitive that larger bikes (58cm+) get shorter trail numbers and proportionally short front-center when we're the riders with more mass that gets 'thrown' forward. Whenever I ride a short trail race bike, I always feel like I have too much weight over the front of the bike on a descent - even with a good fit and weight distribution.

Appreciate, this is a personal preference but as a result I've always felt more comfortable on bikes with a longer-front centre and higher trail BMC, colango etc. They still feel nimble enough but without the overly quick steering, which is probably unnecessary for most riders.

Really interesting discussion.

[MarkMcM]

If your frame of reference is external to the system, physics says you are wrong. A bike can be abstracted as an inverted pendulum. The longer the inverted pendulum the slower the response is to input. A higher BB has the effect of lengthening the inverted pendulum.

Think of it as trading effort for time. Higher your COM is the longer your effective lever so moving off equilibrium requires less force. It also requires less force to shift the bike by a defined amount of cm. However, acheiving the desired lean angle always requires the same amount of work / energy to achieve, as the COM height in the energy calculation cancels out. While you might be able to throw the bike faster under you in terms of COM moving a defined amount of CM you are looking for lean angle and gravity is a constant so you end up with a system that responds slower the higher up you are.

Yes, but ...

The "inverted pendulum" of the bicycle is inherently unstable, and can't continue to travel in a straight line without constant steering inputs. So the "stability" of a bike is more than just how much external force/energy is required to upset it, but also how much internal force/energy is required to right it. A simple example is that a bike may feel unstable at high speed, because it takes little external purturbation to knock off line. But a bike may also feel unstable at very low speed, as it may take exaggerated steering inputs to right it under even a small purturbation. An example of the latter is a rider rolling very slowly, who has to make wild steering actions, swinging the handlebars/front wheel back and forth in order to stay in balance. For the slow speed case, the slower response of a high center of gravity can make the bike even less stable.

In the end, the bicycle + rider is a system, with continuous feedback and reactions between bike and rider, so stability can't be measured simply by a few dimensions like trail and BB height.

[wooger]

The one argument I will make in terms of what is wrong with bike geometries is that I do beleive that trail figures often end up going the wrong way. Shorter riders should have less trail and not more on their bikes as they are likely riding narrower bars and shorter stems. A shorter trail also makes the bike more resistant to crosswinds with deep carbon wheels. There are obviously a lot of tradeoffs and I think its honestly a good idea to avoid the complexity and just ride the bike with the right fork.

Surely you have it the wrong way round for crosswinds? Longer trail is more stability and less twitchyness, so shortening the trail is far worse. I've certainly found this the case with my forks before after the replacement, running 42mm rims. I've used the same rims previously on a gravel bike and a TCR, with 65 and 59mm trail - much more, and crosswind gusts are much less of an issue.

Shorter stems also make handling faster / more twitchy vs longer ones.

[apr46]

The one argument I will make in terms of what is wrong with bike geometries is that I do beleive that trail figures often end up going the wrong way. Shorter riders should have less trail and not more on their bikes as they are likely riding narrower bars and shorter stems. A shorter trail also makes the bike more resistant to crosswinds with deep carbon wheels. There are obviously a lot of tradeoffs and I think its honestly a good idea to avoid the complexity and just ride the bike with the right fork.

Surely you have it the wrong way round for crosswinds? Longer trail is more stability and less twitchyness, so shortening the trail is far worse. I've certainly found this the case with my forks before after the replacement, running 42mm rims. I've used the same rims previously on a gravel bike and a TCR, with 65 and 59mm trail - much more, and crosswind gusts are much less of an issue.

Shorter stems also make handling faster / more twitchy vs longer ones.

Trail in this case acts as the opposing lever to the stem / bar. The longer the trail the greater the distance to the steering axis the hands need to be acheive the same leverage ratio. In the specific case of stall in crosswinds a longer trail for the same handlebar stem setup exerts more force on the rider.

If your frame of reference is external to the system, physics says you are wrong. A bike can be abstracted as an inverted pendulum. The longer the inverted pendulum the slower the response is to input. A higher BB has the effect of lengthening the inverted pendulum.

Think of it as trading effort for time. Higher your COM is the longer your effective lever so moving off equilibrium requires less force. It also requires less force to shift the bike by a defined amount of cm. However, acheiving the desired lean angle always requires the same amount of work / energy to achieve, as the COM height in the energy calculation cancels out. While you might be able to throw the bike faster under you in terms of COM moving a defined amount of CM you are looking for lean angle and gravity is a constant so you end up with a system that responds slower the higher up you are.

Yes, but ...

The "inverted pendulum" of the bicycle is inherently unstable, and can't continue to travel in a straight line without constant steering inputs. So the "stability" of a bike is more than just how much external force/energy is required to upset it, but also how much internal force/energy is required to right it. A simple example is that a bike may feel unstable at high speed, because it takes little external purturbation to knock off line. But a bike may also feel unstable at very low speed, as it may take exaggerated steering inputs to right it under even a small purturbation. An example of the latter is a rider rolling very slowly, who has to make wild steering actions, swinging the handlebars/front wheel back and forth in order to stay in balance. For the slow speed case, the slower response of a high center of gravity can make the bike even less stable.

In the end, the bicycle + rider is a system, with continuous feedback and reactions between bike and rider, so stability can't be measured simply by a few dimensions like trail and BB height.

Yeah you are diving into the human factors part of this which is where it gets way more complicated and its definitely a balancing act between the different factors. One correction though, one way to model is to model to the capsize speed of the system (i.e., the speed at which the bike becomes unstable vs. continuing to roll in a straight line. While you are right that the human control and feedback loop on top ultimately matters in these scenarios the capsize speed is lower for lower center of mass, shorter wheelbase and steeper head tube angles, so you can make the objective argument that those design elements contribute to a less stable system. You cannot make an objective argument the other way.

[MarkMcM]

Yeah you are diving into the human factors part of this which is where it gets way more complicated and its definitely a balancing act between the different factors. One correction though, one way to model is to model to the capsize speed of the system (i.e., the speed at which the bike becomes unstable vs. continuing to roll in a straight line. While you are right that the human control and feedback loop on top ultimately matters in these scenarios the capsize speed is lower for lower center of mass, shorter wheelbase and steeper head tube angles, so you can make the objective argument that those design elements contribute to a less stable system. You cannot make an objective argument the other way.

Cyclists tend to worrry about stability while riding the bike, rather than how stable the bike is when rolling on its own without a rider. So unfortunately, in discussions about bike stability, the rider will always come into the equation.

Also, I think you have Capsize Speed backward. At speeds below the Capsize speed, the bike will tend to steer into the turn and right itself. Above the Capsize Speed, the bike will tend to fall over without self righting. So if the Capsize Speed is lower for a lower center of mass, then, the bike will tend to be remain stable (self-righting) at a lower speed.

https://en.wikipedia.org/wiki/Bicycle_a ... e_dynamics

[apr46]

Also, I think you have Capsize Speed backward. At speeds below the Capsize speed, the bike will tend to steer into the turn and right itself. Above the Capsize Speed, the bike will tend to fall over without self righting. So if the Capsize Speed is lower for a lower center of mass, then, the bike will tend to be remain stable (self-righting) at a lower speed.

https://en.wikipedia.org/wiki/Bicycle_a ... e_dynamics

There is both a low speed and high speed capsize in this kind of model. I was lazily reffering to zone of stability between the two unstable conditions. This is about road bikes so the question is whether or not you are going to be in the upper range of that zone or not. But caught red handed not being precise on this one.

[CR987]

I have an issue with all the YouTubers and journalists who excitedly use the word 'responsive'. There isn't a bike on the planet that is slow steering. Bikes are just too unstable. On a steep descent near me I can clearly see all newer bikes getting very twitchy above 65km/h whereas most older bikes are still dead solid at 80km/h.
To be frank, you don't understand how to steer a bike if you think a bike steers too slowly

[DaveS]

There's certainly some truth to the idea that some bike riders don't really know how to steer a bike. When I moved to Colorado and started riding fast mountain descents, I took a motorcycle training course so I could legally ride a motorcycle. The first thing taught about steering a motorcycle is to push on the right side bar to make the bike turn to the right. That's counter steering. If you quit pushing, the bike will straighten up all by itself. Motorcycles require a lot more force to steer than a bicycle and drop bars aren't as easy to push on as MTB bars. Some bike riders think that steering is all about leaning with your body, but it's counter steering that leans the bike, if it's done correctly. I hit 80-90 km/hr regularly. My bikes are new with disc brakes, but all have trail values of 67-68mm. They aren't the least bit twitchy at high speeds.

[Hexsense]

Lower BB should be more stable because of lowering the center of gravity. That was the recipy of Cervelo when they launched Caledonia 5 endurance bike - lower BB and lower HT angle. It is also good for mtb but you have to keep in mind the crank clearance... traditionally BB drop on road bikes has been 68mm, the reason being you need crank clearance when leaning the bike like pros but with bigger tires it allows for higher BB drop. It is now fashionable to go to 72mm, Caledonia 55 has 74mm and Trek Domane 78mm (sizes 56).

Bb isn't really lower these days.
They're just recalibrated for taller tires.

Bb drop were 68mm ever since 700c by 19-21mm are common.
Now, common tire size is 700c by 28-34mm. That roughly 10mm wider tire is also taller than tires of the past. So more bb drop just lower the bb height back to where it was.

Instead, it's the new bikes with more tire clearance and still keep 68mm bb drop that are taller than ever.

[CR987]

There's certainly some truth to the idea that some bike riders don't really know how to steer a bike. When I moved to Colorado and started riding fast mountain descents, I took a motorcycle training course so I could legally ride a motorcycle. The first thing taught about steering a motorcycle is to push on the right side bar to make the bike turn to the right. That's counter steering. If you quit pushing, the bike will straighten up all by itself. Motorcycles require a lot more force to steer than a bicycle and drop bars aren't as easy to push on as MTB bars. Some bike riders think that steering is all about leaning with your body, but it's counter steering that leans the bike, if it's done correctly. I hit 80-90 km/hr regularly. My bikes are new with disc brakes, but all have trail values of 67-68mm. They aren't the least bit twitchy at high speeds.

That's exactly right. A two wheeled machine should inherently be stable and difficult to change direction due to centrifugal forces trying to keep the thing upright. That's why motorbikes are rock steady at 300km/h.
Somehow they have engineered instability into bicycles so they can be described as 'lively' and 'responsive'. Utter nonsense. I have had MASSIVE tankslappers at 70km/h.