Understanding Descending Mannerism of Bikes

[DaveS]

There's a lot of misconceptions posted above. Caster is an angle, just like head tube angle. If you review the formula for trail, it's the head tube angle that produces a large amount of trail, which can be reduced with various amounts of offset. I'd never call offset rake. On motorcycles head tube angle is often called rake too, which is a poor choice of wording.

Most cars don't have offset, just caster. Caster is measured from a vertical reference instead of horizontal, so 17 degrees of caster is like a 73 degree head tube angle. Trail is the same as for a bicycle, with the offset portion at zero. An ordinary car might have 4 degrees of caster, while a corvette may have 7. My C7 steered strangely when barely moving. With lots of trail, you could feel it skidding sideways. Without power steering it would be very difficult to turn the steering wheel. Cars setup without power steering may have only 1 degree of caster. With much more, parking lot manuvers become very difficult.

One of the problems with trying to analyze bicycle handling is that many riders don't really understand how to steer a bicycle with handlebar input and think that a bicycle is steered with various applications of body weight to other parts of the bike, which is not necessary. You can negotiate high speed corners without butt or foot interference. I change stem length and saddle position to get the fit I want, not to change the bike's handling.

The formula for trail is: (R/tanH) - (offset/sinH). R is the tire radius and H is the head tube angle. This arrangement of the formula separates the trail created by the head tube angle from the reduction due to offset.

[Mr.Gib]

Going off of to changes made to a single bike over the last few years I have found position (or perhaps more accurately, weight distribution within the bike) to make the biggest difference. I've got long legs and found the further forward I move, the better the bike descends.

But how do you know if you weren't simply too far back for good weight distribution in the first place? Maybe all you did was correct a positioning problem. The question then is: if you move even further forward, does the descending get even better? I would think not. Each body has an optimal location between the wheels on a given frame. Once you have found that, then consider the impact of trail, steering geometry, etc. It is a complicated recipe for certain.

[jadedaid]

But how do you know if you weren't simply too far back for good weight distribution in the first place? Maybe all you did was correct a positioning problem. The question then is: if you move even further forward, does the descending get even better? I would think not. Each body has an optimal location between the wheels on a given frame. Once you have found that, then consider the impact of trail, steering geometry, etc. It is a complicated recipe for certain.

I think we agree in that case - in the sense that if your position (specifically weight distribution) on the bike is compfomised, then handling characteristics of the bike are off no matter what the other geometry considerations may be. I would love to see an experiment with multiple bike frames which offer identical features to one another except for one variable, thereby allowing for A/B testing.

In my experience I found the weight distribution to be more important than head angles or trail, though that may speak to how incorrect my weight distribution was in the first place. The bike fit dance is a complicated one

[MarshMellow]

But how do you know if you weren't simply too far back for good weight distribution in the first place? Maybe all you did was correct a positioning problem. The question then is: if you move even further forward, does the descending get even better? I would think not. Each body has an optimal location between the wheels on a given frame. Once you have found that, then consider the impact of trail, steering geometry, etc. It is a complicated recipe for certain.

I think we agree in that case - in the sense that if your position (specifically weight distribution) on the bike is compfomised, then handling characteristics of the bike are off no matter what the other geometry considerations may be. I would love to see an experiment with multiple bike frames which offer identical features to one another except for one variable, thereby allowing for A/B testing.

In my experience I found the weight distribution to be more important than head angles or trail, though that may speak to how incorrect my weight distribution was in the first place. The bike fit dance is a complicated one

There was a recent thread created over an 'understeering' road bike. Seriously hoping this thread is not a progression of that; if a road bike is always understeering when cornering, it's also right before crashing in flames.
There's no 'understeer' condition during proper cornering on a road bike. If there were 'power-on-oversteer' as was trying to have been inferred, the bike would instantly snap into oversteer once corrected. Bicycles just don't have this trait to them and, if it were at all happening - the bike would be entirely unrideable in the wet. Observe high-speed-wobbles on GP Motorcycles where the couple of the F & R suspsension momentarily get packed down and force the bike to want to swap ends.

It's all distribution of weight and knowing how to weight the bike. What was most likely being wrongly given sensation of understeer was the bike wanting to stand-up during cornering-progression and riding position not allowing the front and the rear of the bike to couple and rotate. So, the bottom bracket wants to rise and the handlebars find increased resistance to steer - where the other-rider was thinking the bike was plowing through all the corners.

An optimal weight distribution is a formula unique to each rider. Once there, the bike will be flickable, will be able to pop in the air over crumbled pavement and, most importantly allow the 'system weight' balance to rotate around the bottom bracket.

After railing a corner with the outside leg dropped, gathering up the bike at the crankset should provide the last bit of rotation though the corner once a predictable and balanced weight distribution is being had.

Headtube Angle + Fork Offset contribute to mechanical grip and, bottom bracket height is the next area paired to handling traits.
Seattube angle + stack height can be subtly altered to compensate for having static numbers across the discipline.

[Alexbn921]

I love to go fast downhill. What makes a bike fast is the rider.

The most important thing is GRIP. The tires are the suspension unless you have passive frame flex or a shock like the Roubiax. Having a suspension adds a substantial amount of grip on rough roads or when transitioning from sliding to gripping.

Tire selection and pressure matter more than geo and frame.

Next is weight balance. Geo, bike size and ride effect this.

Low BB height helps a bike feel planted and a longer slacker bike reacts slower, BUT this speed of reactions has NO bearing on actual descending. You simply compensate your input timing. This is why mountain bikers can run crazy long slack bikes.

A lot of stability comes from the stiffness of the frame and way it reacts to forces. Too stiff is just as bad as too soft. Rider weight and power are a factor in how a frame feels. Your deminer on the bike and your ability to stay relaxed effects grip and stability.

Brakes are another area that matter. Being able to repeatably shed just the right amount of speed in the smallest distance makes you faster.

If you want to go faster get some 30/32 grippy tires and run them at 60-70psi.

[Mr.Gib]

Never thought of understeer on a bike, perhaps what people mean by understeer is that their bike is running wide from the intended radius. What I have experienced is particular bikes that require what feels like too much lean in certain radii at certain speeds. It creates the sensation of choosing between sliding out and running wide. But same corner/curve, same speed, same rider, same fit, same technique, same wheels, same tires, different bike, and I have no problem making the turn/curve with room to spare. So what's the difference? By the numbers it's the trail figure.

I have a traffic circle at the bottom of a hill on my regular loop which is just fantastic for comparing bike handling. It really tests a fairly tight radius at medium speed, a good facsimile of what are usually the diciest moments on a big mountain descent.

[MarshMellow]

'power-on-oversteer' from the earlier post was meant as ''...-understeer' leading to oversteer once corrected.

Still makes zero sense since the rider is always the ballast and, there's no negative camber setting entering into the opposing radius, like a Car at Bathurst's Mount Panorama.

Hope OP is okay, not wedged into a Spruce Tree somewheres...

[Hellgate]

Let's not even introduce the notion of counter steering.

[RyanH]

I'd like to redirect this conversation to geometry and frame design rather than external factors. Certain bikes handle differently with the exact same contact points and rider and wheels.

Take for example when I had two Litespeeds a medium T1sl and a small T3. The medium T1sl was a calmer descender. The frames are nearly identical geo but one is different than the other.

[MarshMellow]

I'd like to redirect this conversation to geometry and frame design rather than external factors. Certain bikes handle differently with the exact same contact points and rider and wheels.

Take for example when I had two Litespeeds a medium T1sl and a small T3. The medium T1sl was a calmer descender. The frames are nearly identical geo but one is different than the other.

Yet one is a Small and one is a Medium...

Here's a pretty picture which applies to anything generating forward acceleration, is referred to as Instant Center and a factor of torque vectors during moment of propulsion, during moment of inertia and, applied geometry and its stacked tolerances.
Also happens to be a Bicycle. Any vehicle generates an IC however. Be it with full suspension, fully rigid, flexible stays or Titanium Woven Carbon Fibre seat stay bridges.

There's alot more going on than just jacking the handlebar on two bikes that 'have the same geometry' yet are completely different sizes.

http://www.dw-link.com/physics.html

[Mr.Gib]

Struggling to understand the relevance of acceleration/propulsion. Is anyone having issues when they accelerate? I thought this was about being able to corner fast.

[tjvirden]

I'd like to redirect this conversation to geometry and frame design rather than external factors. Certain bikes handle differently with the exact same contact points and rider and wheels.

Take for example when I had two Litespeeds a medium T1sl and a small T3. The medium T1sl was a calmer descender. The frames are nearly identical geo but one is different than the other.

Yet one is a Small and one is a Medium...

Here's a pretty picture which applies to anything generating forward acceleration, is referred to as Instant Center and a factor of torque vectors during moment of propulsion, during moment of inertia and, applied geometry and its stacked tolerances.
Also happens to be a Bicycle. Any vehicle generates an IC however. Be it with full suspension, fully rigid, flexible stays or Titanium Woven Carbon Fibre seat stay bridges.

hqdefault.jpg

There's alot more going on than just jacking the handlebar on two bikes that 'have the same geometry' yet are completely different sizes.

http://www.dw-link.com/physics.html

Dave Weagle's suspension designs are good; your waffle is simple nonsense.

[Karvalo]

Also happens to be a Bicycle. Any vehicle generates an IC however. Be it with full suspension, fully rigid, flexible stays or Titanium Woven Carbon Fibre seat stay bridges.

hqdefault.jpg

There's alot more going on than just jacking the handlebar on two bikes that 'have the same geometry' yet are completely different sizes.

http://www.dw-link.com/physics.html

What the what now?

IC is the point the rear wheel is rotating around as it moves through its suspension travel at a given moment in time. For a single pivot bike like an Orange IC is always just the main pivot, for a linkage bike the IC moves around in various different ways as the bike gets deeper into its travel.

What does any of that have to do with road bike cornering characteristics?

[Alexbn921]

I'd like to redirect this conversation to geometry and frame design rather than external factors. Certain bikes handle differently with the exact same contact points and rider and wheels.

Take for example when I had two Litespeeds a medium T1sl and a small T3. The medium T1sl was a calmer descender. The frames are nearly identical geo but one is different than the other.

Corning speed is only loosely tied to geometry. It has far more impact on the feel of a bike, but little to no impact on performance.
Even if you are one of the best in the world on the bleeding edge of what a bike can physically do, slight changes to the HTA or CSL will have little effect on a road bikes ultimate speed. As long as the ride can adequately weight both wheels and physically turn the bars fast enough to make a turn, it’s the tires that are the limiting factor. It’s only when you get on to loose surfaces and crazy steep descents that geo limits corning performance.

Rider fatigue plays a roll and there is a tradeoff between too much input (nervous) vs too little(lazy).

When you lean over your bike in a turn the frame/wheels become a side loaded spring and the dynamics of this spring have a huge effect on how the rider interacts with the tire. Having the right system stiffness absorbs bumps in a predictable manner and transmits information to the riders without overloading the tire or muting feel.

[RyanH]

Corning speed is only loosely tied to geometry. It has far more impact on the feel of a bike, but little to no impact on performance.

I agree with this which is why I described the one bike as a calmer descender. There are two aspects of a good descending bike I think: lack of movement in a straight line at speed (going into a turn) and ease of holding and making small changes to the line (in the turn). Both of these are psychological I think, or just reduce the amount of skill needed to extract the performance from the bike.

When I had the T1sl, T3 and Crux together, I did a descent I was familiar with over and over on the 3 bikes. I was able to get similar times on each bike because I knew what one bike was able to do so I more or less just sent it on the others. But, it took the better (or easier) handling bike to set the bar and then I was able to improve the times on the other bikes.

So, a great descending bike gives the user the confidence to hit the turn at higher speeds and get closer to the theoretical max cornering speed of the bike.