Bicycle geometry, fundamentals

Perhaps someone with the knowledge would chime in on this. I have done this little sketch but really don't know where to start designing a properly sized bike taking into consideration handling and comfort. I would like to draw up a comfortable city bike that handles something in between agile and relaxed cornering:

For this i have used 20" wheels. The BB hight is 270mm or BB drop is +16mm. I have used a fork offset of 0 and heat tube angle of 77 degrees. Top tube effective length is 550mm.
First is the Fork offset. Does it have to have an offset or it could be 0?
How to calculate BB position in relation to Headtube, vertical front wheel axis to get it right?
Thanks

Fork off set on its own is not important, the important part of steering geometry is the steering angle and the trail.
The angle is just that, too little will make sharp very responsive steering, " fast steering" to much angle and the steering will be heavier and more predictable, "Slower steering"is the term used.
The trail measurement is the distance from a line projected to the ground through the axis off the steering pivot, measure from where this line contacts the ground (in front of the tyre contact patch) back to the centre of the tyre contact patch. Lots of things can change the trail measurement, fork angle, off set, wheel size etc. A greater trail measurement gives more stable slower steering, so the balance between fast and stable steering can be tailored with adjustments of the two to achieve the desired behaviour for a particular vehicle and its uses.

So what if a fork is without rake or angle? I still can get desired trail changing head tube angle:

I mean is the off set critical or must it work with appropriate headtube angle?

I measured a few bikes with as little as 50mm so 60 would be quite stable.

On frame design in general, I am doing one myself at the moment (22 deg 50mm trail) and have arrived at a usefull way of finding the position for the BB. I found the wheel base divided by 1.65 puts the BB 1 from the front wheel spindle and .65 from the rear wheel spindle, well actually all the MTB's I measured where between 1.6 and 1.65. It was a good starting point for laying out my frame.

Please file away the phrase "trailer hitch" for future reference.

Your fork angle looks very steep, this will make the steering very light and "flighty" more angle will add some weight (effort needed to steer) but also more to the trail, some off set can be used to reduce the trail. too little and it will tank slap on bumps at speed, to much and it wont want to turn at speed. There isn't a right and wrong, right for one vehicle is wrong for another but you will find that most bikes fall into a "window".

Thanks. What would be advisable head tube angle then? I think i could work out the fork offset then, having the desired head tube angle and trail.
So your BB position is spaced at 65% from the rear off the wheelbase? Mine is a tad longer so your method wont work

Imagine a wheel base of 1.65m, it would measure one meter to the BB from the front spindle and .65m to the BB from the rear spindle. Its a ratio not a percentage. most of the bikes I measured where with 1.6-1.65. Or putting it another way the rear distance is 65% of the front distance. Of course it wont work out for a long tail.

I started with 20deg, looked a tad steep, yours looks steeper! but a longer wheel base will add to steering weight due to front rear weight bias and over all weight, consider how the frame reacts to steering. turning the steering 90 deg can have a lifting action on the frame, this can be influenced by the geometry, off set etc, more angle bigger wheels and more off set creates more lifting creating more stability or more reluctance to turn. What is right for yours wont be for mine.

These are bicycles not MotoGp bikes, near enough is probably good enough, there really is no need to over complicate things for the speeds we are likely to be traveling at. But being aware of what influences the steering will help with diagnosing it if it doesn't behave how you intended.

Has anyone tryed zero trail steering at about 40 to 50 degrees? I don't think it would work with a hub motor, but otherwise you get much better steeriing and small diameter turns.

I knew a man that built a bike like that long tail cargo bike, and he did not like the fact that it took so much longer to turn. I recommend a two wheeled trailer with a heavy duty tractor like my bike. But use a 3 inch motorcycle tire on a 16 inch rim for the drive wheel. And if you want to haul children a long nose European cycletruck is better than a long rear bike or a trailer.

I started using short trail on my recumbent bike because I could feel the front end moving up and down when climbing hills slowly with cargo. But the only way I could create short trail on my short bike was to make a steeper steering angle.




http://www.sugarbearchoppers.com/

Back in the day when Mountain Bike Action was new, they did an article where they made an adjustable head angle and fork angle test bike. They could always find a combo that felt like "normal" steering, but with ever steeper fork angle the bumps felt harsher and harsher.

That would be the reason for the springs on the motor cycle and One third of the weight on the front end of the bicycle.....but here is a video that is very interesting.

It's too bad that most people reject these concepts because they don't even try to understand the counter weight of the tiller effect [human arms are heavier than you think]. Or the way that Sugar Bear's springy front end works.

More info here:
https://endless-sphere.com/forums/viewtopic.php?f=1&t=100374&p=1469140#p1469140






Very interesting:

Voltron said:

Back in the day when Mountain Bike Action was new, they did an article where they made an adjustable head angle and fork angle test bike. They could always find a combo that felt like "normal" steering, but with ever steeper fork angle the bumps felt harsher and harsher.

Click to expand...

[youtube]qM6zhH-0-fs[/youtube]

Hugh-Jassman said:

Has anyone tryed zero trail steering at about 40 to 50 degrees? I don't think it would work with a hub motor, but otherwise you get much better steeriing and small diameter turns.[/img]

Click to expand...

Trikes work well with zero trail, but bikes don't, at all. Bike and trike steering dynamics are so different that they should never use the same forks. They only do so for reasons of manufacturing cost and lack of understanding.

I once built a chopper trike with approximately 50 degree head angle, 20 inch front wheel, and about 4 inches of fork offset for near zero trail. It steered reasonably OK for what it was, but I would not recommend such a slack head angle over a more traditional head angle of close to 70 degrees.

Two wheelers need steering trail to stay balanced. Trail is the lever by which the bike steers itself back to equilibrium. There's a much smaller range of head angle that works satisfactorily for bicycles, compared to tricycles.

The moral of the story is that what works for bikes doesn't work for trikes, and vice versa; and that normal head angles are normal because they have proven to work best.

agniusm,

If you want real answers to your question, I suggest you check out the calculator from Bill Patterson.

http://www.wisil.recumbents.com/wisil/trail.asp

Bill is/was an engineering professor who spent many years developing the science of bicycle steering, with his students.

http://www.calpoly.edu/~wpatters/

Also check with Mark Stonich, who has built many sweet handling bikes.

http://bikesmithdesign.com/Design/12steps.html

http://www.ihpva.org/projects/tstrike/building/handling.htm