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-   -   Geometry and folding bikes (http://www.bikeforums.net/folding-bikes/837907-geometry-folding-bikes.html)

pengyou 08-08-12 08:32 AM

Geometry and folding bikes
 
What about the geometry of a folding bike:

- makes the steering less "twitchy"?
- makes it more comfortable for touring/long rides with some equipment?
- makes it easier to maneuver in city traffic?

fietsbob 08-08-12 10:55 AM

Quote:

What about the geometry of a folding bike:

- makes the steering less "twitchy"?
- makes it more comfortable for touring/long rides with some equipment?
- makes it easier to maneuver in city traffic?
you going to go into folding bike manufacturing ? it is what it is .


I put weight on the front rack and the small wheel twitchiness is no problem

comfort is your personal saddle and handlebar choice..

in the case of Brompton,The front load is on the frame's front, not the wheel
so its easy to turn the wheel to avoid stuff..

Bui I am OK with 2 panniers on the racks of my bike friday travel bike..

they turn with the wheel.. It's all Good.

invisiblehand 08-08-12 11:05 AM

You've asked a complicated question. Google "bicycle geometry" and there are lots of sources of information ...

ttakata73 08-08-12 12:36 PM

I think less "twitchy" and easier to maneuver contradict each other.
If you want faster steering it has to be twitchier.
You can slow steering response down by adding weight to the front wheel or get wider handlebars but then it will be less
maneuverable.
Small wheels are twitchy because they have less rotational mass than a big 26" or 700c wheel.

I say you just need to get used to the twitchyness and enjoy the benefits of faster steering.
I ride a 16" tire Dahon and even chopped my handlebars down to 490mm width so it can split traffic lanes easier.
It took a few hundred feet to get used to its super fast steering but its rideable.



GlowBoy 08-09-12 11:11 AM

Depending on the bike and the rider, the more-responsive, "twitchier" steering (as pointed out above, they are mostly the same) could result from a combination of one or more of the following:
1. The reduced rotational momentum of a smaller wheel at speed, providing less resistance to changes in direction.
2. Applying conventional bike head tube angle and fork offset to a smaller wheel, resulting in less trail.
3. For some folders, a shortened wheelbase compared to bigger-wheeled bikes.
4. A larger rider on a one-size-fits-all bike that would be considered "too small" for that rider if it weren't a folder.

Not much you can do about #1 except go with a big-wheeled folder. A smaller wheel does handle differently than a bigger wheel, period. #2 and #3 can be eliminated through proper design. #4 could be eliminated if manufacturers offered more (and bigger) sizes. Not unusual for a road bike to be offered in 6 or 8 sizes, yet many folders are offered in only one or two sizes.

FWIW, I think I only experience #1 on my Swift. The bike is designed to have similar wheelbase and trail to larger-wheeled road bikes, and I'm fairly short so #4 isn't an issue.

chagzuki 08-09-12 12:26 PM

I don't particularly mind the fast handling of the variety of folders I've ridden but given a choice I'd opt for more stable handling, particularly on a Brompton.
It doesn't strike me as a problem to improve on No.2 and increase trail. A combination of reducing rake and slackening the head tube angle a touch would work, plus in compensating for the slacker head tube effective stem length/forward stem reach would be increased which would also stabilize the ride.

pengyou 08-22-12 05:23 PM

Thanks! I am looking for a new bike and find a lot more options now than I did five years ago when I bought my Dahon. Now I know what to look for.

1nterceptor 08-22-12 06:08 PM

I think it's best if you test drive a few different
bikes to see which one feels good to you.
By the way, I like your username my friend :D

feijai 08-22-12 07:58 PM

Quote:

Originally Posted by GlowBoy (Post 14585459)
Depending on the bike and the rider, the more-responsive, "twitchier" steering (as pointed out above, they are mostly the same) could result from a combination of one or more of the following:
1. The reduced rotational momentum of a smaller wheel at speed, providing less resistance to changes in direction.
2. Applying conventional bike head tube angle and fork offset to a smaller wheel, resulting in less trail.
3. For some folders, a shortened wheelbase compared to bigger-wheeled bikes.
4. A larger rider on a one-size-fits-all bike that would be considered "too small" for that rider if it weren't a folder.

C'mon, don't overdo it: of the explanations above, twitchiness in small bikes is nearly entirely attributable to trail.

pengyou, trail is the degree to which the front wheel acts like a caster and so resists steering. You can measure trail pretty easily: draw a line straight down the steering axis and mark where the line intersects with the ground. The measure the distance between that spot and the spot where the wheel touches the ground. If the distance is large, the wheel will resist steering and want to go straight: it has a high degree of trail. If the distance is small, the wheel will feel "twitchy" -- you can easily steer it. It has a low degree of trail.

Trail may be useful or not. High-trail bikes are more stable at higher speeds, so racing bikes tend to have a lot of trail. But low-trail bikes are more maneuverable (so mountain bikes often have lower trail) and are much easier to carry loads on the front wheel (so porteur and other work bikes often have very low trail).

On a large-wheeled bike it's easy to make the trail large. In fact the trail can be so large that it's difficult to steer the bike at all. As a result, bike manufacturers may add a fork rake (a forward bend in the fork) to push the wheel axis forward and thus lower the trail.

It's hard to get lot of trail on a small-wheeled bike simply because of the wheel size. To get a lot of trail you'd have to move the wheel rotational axis backwards (a negative rake) or change the steering angle quite a lot (like a chopper motorcycle). Both of these might be weird to steer, and possibly not easy to incorporate into a folding scheme.

A zero rake is easy to achieve though. Oddly, manufacturers still add a positive rake on their small-wheeled bikes for what seems to be no valid reason at all. Dahon's 16-inch bikes (including the Jifo), the Mezzo, and even the Bike Friday Tikit have a small positive fork offset, thus slightly *reducing* an already dismally twitchy trail. The worst offender by far is Brompton, which adds a large fork rake. Brompton seems determined to make their bikes the twitchiest in the industry (a trail of just 24mm).

fietsbob 08-23-12 10:22 AM

... But I find the handling of both my Brompton, and my bike friday pocket Llama, just fine.
because they are utility bikes and I always have a front bag full of stuff on them.


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