Author Topic: Comfort and curved forks  (Read 10659 times)

kwkirby01

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Comfort and curved forks
« on: September 28, 2012, 11:12:21 pm »
In the new the latest Raven brochure Andy Blance says the new C2 fork is "exceptionally comfortable", even more so with 2.0" Dureme tyres.
This got me thinking, how much comfort is due to the tyre and how much the fork?
Does a curved fork really make such a difference? Would a straight fork with the same wide tyre at the same pressure be noticeably less comfortable to the average rider?
Kevin
Kevin K. Glasgow

Danneaux

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Re: Comfort and curved forks
« Reply #1 on: September 29, 2012, 01:32:58 am »
Hi Kevin! (thanks for your kind words elsewhere)

Ah, this is one of those evergreen topics that is a real favorite of mine, so thanks for bringing it up!

As a hobbyist framebuilder with an interest in the science behind bicycles, I investigated this myself some years ago and built nearly identical forks of each type and then instrumented them. As it happened, so did professinal framebuilder Dave Kirk ( http://www.kirkframeworks.com/ ), who reported his results over at Road Bike Review. His methodology and results mirror mine: http://forums.roadbikereview.com/276129-post20.html

Here's how the differences between the two kinds of forks lay out...

= Straight bladed forks and raked ones (with curved blades) can result in identical geometry (trail, effective head tube angle) in the same bike. How/why? Well, the straight blades are also raked...but the rake occurs at the crown (the sockets are set at an angle). So long as the dropouts end up in the same place, it matters not what happens in-between; the geometry of the bike will be the same.

= Because curved blades are longer, they do tend to exhibit more deflection over their greater length than stiffer, shorter straight blades under load when hitting bumps. Because of this, curved blades tend to be more comfortable.  The margin is pretty slim, however, and all variables have to be held constant if one is to have any hope of a direct comparison. Because straight blades are shorter, they do also tend to terminate at a different section of the available blade length, and this affects feel as well because of a difference in cross-section.

= It costs considerably less to make a fork with straight blades because one entire step is left out. Curved-blade forks often start as straight forks, then are raked as a unit. Other builders rake the blades individually (or buy them pre-raked for convenience, then alter head tube angle to get the desired geometry or bend the blades further as needed), then build, but I preferred to rake my own forks as a unit to get the geometry I wanted. Blades are almost always raked "cold", though I have known some builders who used heat in the process (no! Heat -- and more of it -- can adversely affect the integrity of the steel, so builders take pains to keep temperatures as low as possible to avoid overheating the steel).

= Apropos nothing in particular, but it is useful to remember that raked blades have been bent beyond their elastic deformation into the plastic zone (cold-set). Some people argue that straight blades don't undergo these stresses, and so are naturally "springier". I would argue that once a blade has been cold-worked and normalized, any changes in the quality of steel are moot, and are affected to a much greater extent by (over)application of heat thereafter (in the building process).

= One oft-overlooked advantage of curved blades for touring forks is the greater ease with which front racks can be fitted, and better weight distribution as a result. Try putting a Tubus Tara or Duo on a straight-blade fork, and it will project considerably further forward than when mounted on a curved-blade fork (see attached pic for an example). Why? The lower attachment point of the rack often ends up *ahead* of the axle instead of behind it, thanks to the straight-blade's path. As a result, more weight is carried forward on the front wheel and handling suffers to a degree.

To sum up, a curved-blade fork will be a bit more comfortable than one with the same geometry that has straight blades because the curved blades are a little longer. The difference won't be much, but it is most likely to be felt when hitting bumps while riding -- when people will notice it most. Straight blades are cheaper to produce, and curved blades make it easier to place front panniers where their weight is more properly carried.

Regardless of how the blades are raked (or not), a greater determinant of comfort is likely to be tube-wall thickness. Remember, the C2 fork is made from lighter, competition-gauge tubing that is very thin, and cannot be fitted with lowrider bosses for carrying luggage. That light-gauge tubing is also longer than a straight-bladed fork, thanks to the greater length needed to curve the blades. No wonder it feels so nice and compliant! Fit this fork with a wide-cushy tire, and the result is real comfort.

All the best,

Dan.
« Last Edit: September 29, 2012, 03:57:48 am by Danneaux »

andrew_s

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Re: Comfort and curved forks
« Reply #2 on: November 26, 2012, 01:15:49 am »
To sum up, a curved-blade fork will be a bit more comfortable than one with the same geometry that has straight blades because the curved blades are a little longer.
I reckon the difference between a raked-blade fork and a straight-blade fork can be a fair bit more than that due to the difference in length.
You didn't consider the direction of the load on the fork.

Any load applied will be purely radial between the initial bump contact and the hub.
With a straight-blade fork, the load direction will be more or less straight along the axis of the fork blade, so you will be compressing it rather than bending it.
With a raked blade on the other hand, the same applied load will be at a fair angle to the end of the blade (up to about 40 deg with an old fashioned rake), so there is a reasonable bending moment.

The difference will depend on the shape of the fork rake - forks with an even curve from top to bottom ("banana" forks, which were widely castigated for their lack of comfort when they first started to appear widely) will be much closer to a straight blade than those with a sharp terminal bend.

Danneaux

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Re: Comfort and curved forks
« Reply #3 on: November 26, 2012, 05:46:42 am »
Hi Andrew!

I think you raise a very good point, and I thank you. Yes, the curvature of the fork does affect the transmittal of force-loads up the fork -- either directly (or nearly so) or tangentially as you observed.

There's another factor as well, not often discussed, and that is fork blade section width and diameter as well as taper. Some early mountain bike forks were essentially cylinders of a constant section width. Sturdy? Yes, but they were pretty unyielding. In comparison, a nice taper-gauge blade doesn't always maintain a constant wall thickness, either, with considerable thickening at the dropout ends (at least in the forks I've built and raked; the difference is noticeable when the blade ends are slotted to accept the dropout) -- all the while reducing in outside diameter as the blade tapers toward the dropout. Framebuilder Dave Molton made an interesting comment about section width and fork comfort here ( http://davesbikeblog.blogspot.com/2007/12/in-search-of-perfect-fork-blade.html )...
Quote
If I were building a touring frame, and wanted a flexible fork for a more comfortable ride, I would cut from the top end and leave the blade thin at the bottom end. The framebuilder creates the perfect fork blade, by selecting the best place to bend the blade, and by choosing how much to cut from either end.
This is an additional factor beyond the shape of the fork rake and -- yes -- even the length.

Fork flexibility/comfort is the result of many factors, and we can't forget the crown design as well. Some early forged fully-sloping crowns effectively shortened the fork blades in two ways -- first, their slope reduced the blade length. Second, if the crown had long internal tangs, the hollow blades were shortened even further. Unfortunately, this design (often seen on older Cinellis) could precipitatefatigue failures -- not from braking, as one might suppose, but from vertical road shock acting on the stress riser at the end of the insert. See: http://yarchive.net/bike/fork_failures.html

Really, there are probably too many variables to come to a definitive conclusion as to which fork design is more flexible and compliant/comfortable -- even framebuilders disagree on the resulting ride quality of curved vs straight forks even when variables are held as constant as possible. One thing they do agree on is the geometry (rake and resultant trail and wheelbase) can be identical. Another thing they (sometimes reluctantly) agree on: Straight-blade forks are less costly to produce because there is less labor and tooling involved. Me? I think effective blade length still plays a role in ride comfort, but certainly not the only one; as I noted earlier, "The margin is pretty slim, however, and all variables have to be held constant if one is to have any hope of a direct comparison".

Best,

Dan.

Relayer

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Re: Comfort and curved forks
« Reply #4 on: December 12, 2013, 10:26:52 am »
I reckon the difference between a raked-blade fork and a straight-blade fork can be a fair bit more than that due to the difference in length.
You didn't consider the direction of the load on the fork.

Any load applied will be purely radial between the initial bump contact and the hub.
With a straight-blade fork, the load direction will be more or less straight along the axis of the fork blade, so you will be compressing it rather than bending it.
With a raked blade on the other hand, the same applied load will be at a fair angle to the end of the blade (up to about 40 deg with an old fashioned rake), so there is a reasonable bending moment.

A straight blade fork WILL bend; because of the rake the fork is not vertical and therefore not compressing.

I have recently bought a bike with straight blade forks and when I was researching this topic I looked into carbon straight blade forks and found that many riders reported that they could actually see their straight carbon forks bending.

I also recall in one of the Thorn brochures that Andy Blance was critical of straight bladed forks, yet SJSC supply a straight bladed fork option with the Mercury which is described as "comfortable" here

http://www.sjscycles.co.uk/ora-titanium-cyclocross-700c-disc-fork-1-1-8-inch-steerer-prod24706/

In the end I opted for a bike with straight steel forks and for me they are comfortable running with 2.25" Marathon Extremes. I accept that there would be a marginal gain in comfort terms with a curved fork but I was more than happy to compromise that to get the superior braking performance of disc brakes.

Jim
« Last Edit: December 12, 2013, 10:41:09 am by Relayer »

il padrone

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Re: Comfort and curved forks
« Reply #5 on: December 12, 2013, 11:00:37 am »
Straight-blade forks are not "straight".

For the same amount of trail they will actually have more rake to the fork, just no bend in it. The rake of the fork (angle off the line of the steerer) will allow the fork to flex/bend fore and aft. Maybe not as comfy as a swept fork from the 50s, but it is stil going to bend.


mickeg

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Re: Comfort and curved forks
« Reply #6 on: December 12, 2013, 03:36:17 pm »
I took this photo in the Smithsonian this past spring.


Andre Jute

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Re: Comfort and curved forks
« Reply #7 on: December 12, 2013, 07:01:06 pm »
I took this photo in the Smithsonian this past spring.

Paint that bike yellow or pale green and you could sell it as the latest town bike fashion for responsible citizens.

Check how hight the handlebars are in relation to the saddle!

il padrone

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Re: Comfort and curved forks
« Reply #8 on: December 14, 2013, 12:09:04 am »
Almost identical bike from a private bicycle collection here in Melbourne. Those forks are actually sprung. The rear curved section is the spring, the front straight blades are pivoting. The bike alongside has front suspension as well - not much new in the bike shed  ;)

These are solid-tyred safety bikes from the period 1885-1890.

« Last Edit: December 14, 2013, 12:12:52 am by il padrone »

David Simpson

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Re: Comfort and curved forks
« Reply #9 on: December 14, 2013, 12:13:55 am »
The handlebar on that antique bike reminds me of the Jones H-Bar handlebar.  (http://www.jonesbikes.com/h-bar.html)

I wonder how many other "new" ideas we could get by studying these old bikes.

- Dave

il padrone

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Re: Comfort and curved forks
« Reply #10 on: December 14, 2013, 12:19:36 am »
Try putting a Tubus Tara or Duo on a straight-blade fork, and it will project considerably further forward than when mounted on a curved-blade fork (see attached pic for an example). Why? The lower attachment point of the rack often ends up *ahead* of the axle instead of behind it, thanks to the straight-blade's path. As a result, more weight is carried forward on the front wheel and handling suffers to a degree.

The rack mounting set-up in the photo you've posted here is suffering more because the rack has been mounted with the rear hole on the lower arm, and the forward hole on the fork, as much as the straight fork issue. If the owner had spaced the rack to clear the QR end and used the forward hole on the rack (and perhaps the rear fork mount as well) to mount it, then the rack would be 1-2cms further back.

« Last Edit: December 14, 2013, 12:29:35 am by il padrone »

Andre Jute

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Re: Comfort and curved forks
« Reply #11 on: December 14, 2013, 10:29:57 am »
I wonder how many other "new" ideas we could get by studying these old bikes.

In Peter's pic from the private collection above, the central bike appears to have very tunable front suspension. You just add or subtract a curved blade to each side. The twin blades per side fitted would probably suit a medium-weight rider, and one blade a very light one, with three blades for a very heavy rider.

The bike behind, with the short helical spring per side, is an engineer's solution, only theoretically as tunable; these short springs never manage fine adjustments well. The lever arm is supposed to offer more adjustability by changing it out for a different length, but that would then affect the handling of the bike by moving the axle. Not as impressive by far as the curved suspension version but undoubtedly cheaper to make.

Notice the coat guard on the back of the central bike. A commonplace fitting today on Dutch city bikes -- I have two bikes, including one from Trek, that came from the factory with very useful coat guards.
« Last Edit: December 14, 2013, 09:53:20 pm by Andre Jute »

moodymac

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Re: Comfort and curved forks
« Reply #12 on: December 14, 2013, 09:46:06 pm »

Andre,

Also note the matching pink in the tires and ladies hat feathers.  Ah, those were the days.

Tom

Andre Jute

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Re: Comfort and curved forks
« Reply #13 on: December 14, 2013, 09:55:09 pm »
Andre,

Also note the matching pink in the tires and ladies hat feathers.  Ah, those were the days.

Tom

Have you ever wondered whether those black and white checkered tyres they used to sell were faster than the plain black ones?

moodymac

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Re: Comfort and curved forks
« Reply #14 on: December 14, 2013, 10:09:31 pm »

"Have you ever wondered whether those black and white checkered tyres they used to sell were faster than the plain black ones?"

Definitely, if you had an Ace of Spades clipped to a seat stay.  Or riding in the land of Oz.  Bet the ride gets bumpy on a hot day though.

Tom