Differences in diamond frames
 

Now that my job/money life is approaching the light at the end of the tunnel, I am back on at looking for a good new commuter bike. I'm after roadsters, and I've got a question about the frame geometries.

My favorite is like this:
http://farm3.static.flickr.com/2568/...e378cfbd99.jpg

Flat up top. That "flat top" gives it the frame shape that looks classic and appealing. My road bike is like that, so I know it's good.

But then there's this type:
http://bikeportland.org/wp-content/E...mClassicGr.JPG

Where the top tube is sloped down. It's not a mixte or a stepthrough, so what is it? And why? Is it a matter of standover height? If so, why not just a smaller frame? My hybrid is like that.

Frame geometry is...mystery for now.

Thanks,

M.

2nd bike is a 'crank forward' bike , note bottom bracket in front of seattube instead of inline .
ride around leaning backwards, meant for grandma and casual riders
the toptube slopes up to lift the handlebars to the sky, else you can't reach them with that backwards posture

It's a completely different riding position. You may or may not like it. Advantage is you can put both feet flat on the ground while sitting on the saddle. Some people like that. If I did a lot of riding through congested urban areas I think I might.

A crank forward bike would seem to be a great bike for commuting. The fact that your feet easily reach the ground would be great for commuting during stops at lights etc.

the disadvantage of crank forward bikes
is that the rider position
is not ideal
for putting power to the pedals

and the op should know
that most bikes made today have sloping top tubes
but are not crank forward style bikes

and yes
sloping top tube is mainly intended to improve standover clearance
and make it possible for a greater range of rider sizes
to ride a smaller range of bike sizes

also
to the op
please notice that the previous two posters
who think crank forward bikes would be very good for a commuter bike
are recumbent riders
which
while not bad in and of itself
indicates a certain attitude wrt upright bikes

in my opinion
upright bikes are great
recumbents are great
crank forward bikes are kind of a poor comprimise

On that second bike could someone been thinking,, unisex frame?? just a thought,,,,,

Utility bikes have long had a rather low angle seat tube.
black Dutch Opa, are pretty well sorted out by now, being unchanged since before WW1

[crank forward have taken it even further back]

the riding posture is upright , a rate of speed will be stately.

so dress in your best clothes since the ride is about looking good ,
and arriving in the kind of clothes that put you in the GQ dress for success mode ..

Just be aware the fine wool fabrics will wear thin where your saddle on the bike contacts,

So a Polyester blend is probably better ..


its all about numbers , to learn more, bring an angle finder and a tape measure , and gather Data.

Crank forward bikes work very well for some people. You can easily touch the ground while sitting on the seat. They are very easy to control, but are not speed demons.

REI carries them, for example. Go test ride one and see if it works for you.

I'm not a frame design expert by any means, but I think I've boiled it down to the rule of thumb basics.

Rider position is determined by the three points: bottom bracket, seat, hand position. The three points are rotated and adjusted for every riding style, (except for trick bikes where you're standing up a lot but I leave that out of the scope). The three points also determine the center of gravity.

Given the rider configuration, seat tube length and height of the top tube depends on leg length. Top tube length is torso and arms.

Where the wheels are relative to the rider center of gravity determines the handling characteristics, and the foot clearance. Head tube angle affects trail, which means responsiveness and stability. And clearance with the feet from the front wheel, generally less of an issue. Chain stay length positions the rear wheel, longer for more straight stability and heel clearance (with panniers), at the expense of "nimbleness".

Some other measurements are more specialized. Bottom brackets are raised (cross bikes) for more clearance (feet and rear derailleur) over curbs, tree logs etc, sometimes pedal clearance when turning. The sloping top tube is mainly for standover clearance - there may be some impact on strength and rigidity, but I don't really see it. Level top tubes are usually prefered for cyclocross since they're easier to carry that way.

I'll probably take some heat for this, but I don't think that the seat tube angle really matters much. It roughly positions the saddle, otherwise is cosmetic.

Although I categorize these for different aspects, they're all inter-related for a given style of bike. A cruiser for example has the upright rider position and therefore doesn't strain to position the wheels to facilitate handling at high speeds, and so on.

Not related to the second bike really, but see the little article here, which sort of explains the frame design of modern road bikes.
What they don't say is that making a frame slightly smaller and lighter makes it slightly cheaper.
http://www.giant-bicycles.com/en-us/...ompactroad/57/

Both of those bikes are pedal forward styles. And both are "Dutch" styles with fenders and chain guards, etc. Drop a vertical line through the seat clamp and the crank center is well forward on both. the angles of the seat tube are similar. Pedal forward bikes feel very different from more conventional styles, and are very popular with those who may be limited in movement by age or other infirmity. they are also comfortable to those who just like to have their feet flat on the ground when stopped. I actually have one, and while it is very relaxed to ride, it just feels very slow. It makes me feel very old. I never ride it.

Another difference:

Taller headtube means less stem quill to yield the same bar height.


http://farm3.static.flickr.com/2568/...e378cfbd99.jpg

http://bikeportland.org/wp-content/E...mClassicGr.JPG

I hadn't noticed the positioning of the cranks. The fiets style bike / roadster / upright bike / city bike is basically the style I'm aiming for now. I have a road bike with the more aggressive lean-forward riding approach, and I have a hybrid with a posture between the upright and racing one. My SO had purchased a cruiser a while back (which is currently a restoration project due to a rear-end incident) which is upright. I'd not paid much attention until I started messing with this old 70's "All Pro", which is upright. I realized I really liked it, and it renewed my interest in getting myself a roadster style one. It was the top tube design that I was most curious about, as I was interested in knowing if there's anything more to it than style. Looks like it does have some practical application.

The SO really likes the cruiser style bike, where she can put her feet on the ground from the saddle. I'm not too sure how this is a factor of BB position and not saddle height, though - is it just because you can run your leg downwards and behind the cranks easier?

I'm looking to get myself a good roadster style bike sometime in the next few months since I'll actually have the money to do it now. It'll be replacing my big box Schwinn as my general-purpose mobility machine - grocery getting, commuting, etc. I'll keep my road bike for going fast, though I don't think my fitness / general physique is really going to make a difference between them. I'm going to need to test that out with a cyclocomputer sometime.

I figure since I know what goes into a good bike now, why not go spifferific?

Got my eye on a few, haven't made any decisions yet. The Windsor Kingsington IGH series seems like it would be a good platform for all the extras that makes a "Dutch bike" without having to go whole hog and buying a Paisley all at once, but we'll see. Because I am so short, I'm thinking I might look into 26" wheel bikes. This will be my long term "investment" bike.

M.

BB forward allows you to achieve full leg extension with a lower saddle height since the distance from BB to saddle is increased without increasing saddle height above ground.

You can easily flat-foot the ground at stops with a crank-forward bike whereas even on a 68° Seattubed bike you could only get toes and maybe ball of your foot down while seated.

That Raleigh could be even slacker than that, and has a fair amount of BB drop, so may be flat-footable, I dunno.

you are mistaken as to the meaning of
crank forward

crank forward designs have a the bottom bracket ahead of where the seat tube meets the chainstays and down tube
not just forward of the seat clamp
very very few bikes do not have the bottom bracket forward of the seat clamp
i actually cant think of any off the top of my head

the black bike has what is called a slack or shallow seat angle
but this does not make it a crank forward bike

also
dutch bikes
are generally traditional geometry
and are not crank forward style

Black one is perhaps 30~40 years old, ..I note the cottered cranks..

the green electra is currently sold stuff.

currently sold dutch bike

http://www.gazellebikes.com/~/media/...?mh=400&mw=600
from royal dutch
also has cottered cranks i think

All you have to do is go there buy one ride around on it and have it shipped back with you on the plane.
[don't forget the VAT export tax rebate paperwork, get it stamped at the departure gate.]

I dont see a cotter on that crank. maybe a steel arm but for square taper fitting.

RANS crank forward bikes locate the BB at the intersection of the chainstays, seat tube and down tube. Very far ahead of the seat.
http://www.rans.com/bicycles/crank-forward.html

Trek Pure J curves the seat tube so there is less leverage on the end of the seatpost..
the post nearer to a normal angle.. but still well laid back..

http://www.trekbikes.com/us/en/bikes...ion/pure/pure/

My next-door neighbor is an avid mountain-biker, 30-ish, who rides a high-end CF-framed full-suspension mtn bike.
For tooling around town, he also has a steel crank-forward bike, and loves it.
Go figure.