Greater Fork Rake/Trail = Stability

[Lavs]

Hi All,

I'm a newbee, with only about 10 hours practice so far, and don't want to start more than a discussion.

I have posted that I feel the Silvio 2.0 tracks well, so the fork rake and head-tube angle are well designed. This was a note/observation from my initial slow speed parking lot practice.

Now that I'm on the roads and starting to come up to "cruzing" speeds (20+mph), my Silvio seems quick to change course, to me. This is likely because on each of my custom DF-TT bikes we have specified a slacker steering-tube angle and aero-forks with most "trail" available for stable tracking. Truthfully, those are the only bikes I have ridden for the past 12 years, so I'm used to slow/dull handling but stability at speed.

My goals and riding style are to keep the wheels going in as straight of a line as possible (limiting the small oscillations) most of the time, with limited 90 degree cornering (every few miles) and I don't ride side-by-side with others or in groups, and almost never in town.

Yes practice, practice, practice,... and again I only have in about 10 hours on my Silvio.

I'm just starting a conversation. Maybe future Silvios (2.x) and Vendettas (2.x) will investigate this, if there is actually any validity to it.

Insights?

Thanks, Lavs

 

[billyk]

bicycle geometry is complicated

Hi Lavs -

I've also thought about this, but bicycle geometry is complicated.

First, increasing the rake will _decrease_ the trail (more trail = more stable). See the nice diagram on the Wikipedia page "bicycle geometry".

You can search through the forum pages for discussion of these questions. I think the bottom line is that the designers have given this a lot of thought and experiment.

It helped me to learn to ride the bike no-hands - that really taught me how it handles. Since your feet can control the steering this is actually much easier than on an upright bike, and the riding position doesn't change the way it does on an upright so you can still apply power. I can go for miles without hands, perfectly comfortably, including fairly sharp turns. Give this a try and see if you don't feel more "one with the bike".

BK

 

[John Tolhurst]

Bicycle frame geometry is a

Bicycle frame geometry is a very interesting field, as bicycles do not have any handling characteristic at all, until a human climbs aboard. Humans are also very interesting. alt="indecision"

 

[Lavs]

Human Factor

I'm the first to admit that this human has balance issues, in general. I like Billyk's advice about developing the ability to ride no-hands (eventually), once there, wow.

I must commend you on designing, building, and manufacturing the line of Cruzbikes. It must take an enormous amount of effort (and patience) to go through the entire process, and then to receive and provide feed back online. There have got to be so many variables that can be adjusted in different directions, all interacting, then yes the human variables at the end.

Please don't get me wrong, I feel you have done a wonderful job. My Silvio's manufacturing quality is very good.

This human just needs to keep practicing.

Thank you!!! Lavs

 

[marshall2389]

It is hopeful to think that

It is hopeful to think that there exists an accurate dynamic model of the biker-rider system. However, it doesn't exist. The system is simply too complicated. There are decent models of two-wheeled inline vehicles alone without a rider. Adding suspension gets more complicated. There are no analytical models that incorporate tire compliance and suspension. Analytical models are useful because they are based on general equations that can be optimized. In other words, one could use mathematical tools to solve for the optimal head tube angle given necessary parameter values (such as frame and wheel inertia, tire and suspension compliance, geometry, etc.). Even if this analytical model existed, adding a rider makes the system vastly more complicated. People don't act like steel. We are not a passive system. We are active and control our own bodies variably. An analytical model could make some assumptions and get decent results, but the equations to be optimized would be so huge that only a computer with efficient and well written code could solve the problem.

To summarize, prescribing the right head tube angle for a Cruzbike (or any bike) is not a hard science. It is an educated guess tested with lots of trial and error.

 

[John Tolhurst]

The most useful research that

The most useful research that I uncovered was Tony Foale demonstration that very tall head tube angles were stable at high speed on a motorcycle. But designers pursue low head tube angles to improve the fork's resistance to bending, to shorten the frame, to move the handlebar pivot closer to the rider, to lower the resulting vertical height of the headtube and so on.

 

[marshall2389]

Most research like that is

Most research like that is either numerical or empirical. Neither of those approaches result in a formula for optimum stability, performance, etc. There are analytical models of motorcycle dynamics. Motorcycles benefit from the fact that the human rider is proportionally less massive than the bike and the rider doesn't move as much. This means it is easier to develop an analytical motorcycle model that yields acceptably accurate results. However, conclusions based on motorcycle models should only be applied to bicycles with caution.

 

[John Tolhurst]

Worth looking at that

Worth looking at that reference if handing is of interest. I didn't say it was a formula to be applied, so we are on the same page. I recall Honda motorcycle engineers preparing bikes for world champion Wayne Gardner commenting that their understanding of car handling was excellent, but motorcycles were far more complex!