Straight Forks

[ANTONISH]

When I started cycling in the late 50's front forks had a pronouced curve. Over the years the curve has lessened until now when many road bikes seem to come with straight forks. Doubtless there is some advantage to a professional cyclist or straight forks wouldn't be used. I'm interested to know what advantage they give the average cyclist. Do they make the ride harsher?

[le vieux grognard]

Carbon forks are straight because carbon can't be bent. The curve at the bottom of steel forks is replaced by setting carbon forks at an angle in the fork crown. That gives the castor effect needed for steering. Whether straight forks are harsher than curved steel depends on the curve in the steel and its quality, so its a subjective assessment. Pronounced curve - rake - is mainly used to keep the bike going in a straight line when being ridden slowly. That's why the rake is more pronounced on old-fashioned roadsters. A side-effect is that the bike is then harder to steer, a corollary of its being better at going in a straight line. Carbon frames are generally for nippier riders and nippier riders ride faster and don't need that centering effect. A nippy bike is closer to a fighter jet than a bomber, inherently less stable and made that way deliberately so the bike can be steered around other people's back wheels and round tight corners. But experienced riders have no trouble with that.

Hope that helps.

[RickH]

Carbon forks are straight because carbon can't be bent.

The Carbon forks on my Kinesis are curved, as are many other carbon forks.

Then there's always the "wiggly" carbon fork on some Pinarellos


Carbon can be manufactured into virtually any shape you want.

Rick.

[Brucey]

When I started cycling in the late 50's front forks had a pronouced curve. Over the years the curve has lessened until now when many road bikes seem to come with straight forks. Doubtless there is some advantage to a professional cyclist or straight forks wouldn't be used.

they are slightly shorter so can be slightly lighter. Also note that most curved forks are traditionally cold-set to the correct curve; if you build in the very strongest steels the blades can't be so easily cold set. Sometimes they can't be cold set at all.

I'm interested to know what advantage they give the average cyclist. Do they make the ride harsher?

I used to think not, and indeed a straight fork in very light gauge high strength steel could in theory be more flexible than a curved fork in slightly heavier gauge steel. However if such a thing exists I have not seen it, and I think differently now. I think that fork tip deflections (from within the blade itself) are certainly small, but that everything adds up. [Many modern forks are hamstrung by the use of a 1-1/8" steerer; this is typically too stiff IMHO...]

BTW straight blades were very modern and trendy in the 1990s and early 2000s but not so much now. When I bought a new road bike in 2001, I specced it with curved carbon fork blades, not straight ones. Whether it made any real difference I can't say, but I very much preferred the look of it both then and now.

cheers

[reohn2]

When I started cycling in the late 50's front forks had a pronouced curve. Over the years the curve has lessened until now when many road bikes seem to come with straight forks. Doubtless there is some advantage to a professional cyclist or straight forks wouldn't be used. I'm interested to know what advantage they give the average cyclist. Do they make the ride harsher?

Steel forks can be made quite flexible and the 'hockey stick' pronounced type of bend I think you mean,is more flexible(in a vertical plane) than a fork with a more 'graceful' bend starting at say halfway down the fork blades.
However the biggest aid to comfort comes from the tyres,23mm tyre at 120psi will always be a lot harsher than than a similarly flexible 35mm tyre at 50psi on the same fork,whatever that fork is.
I don't think the straight bladed fork was ever manufactured for any other reason than economics.
Compare the two disc braked bikes on this page(about halfway down, both bike are mine):- viewtopic.php?f=8&t=82822
Both bikes are very comfortable indeed and regularly ridden on some very rough tracks,they also perform very well on tarmac.
I've descended on both bikes at 50mph and regularly at over 40mph, sometimes on less than ideal tarmac,without so much as a flicker from them,it's all in the tyres

[Norman H]

So called “straight” forks are not really the end point of the evolution of progressively less curved fork blades. The blades may be straight but the fork ends are still offset (raked) with regard to the steering axis. The amount of rake is decided by the frame designer and will depend on the steering angle and the desired trail.

Steering geometry is a bit of a dark art but, assuming the steering angle is less than 90 degrees. Increasing fork rake actually decreases trail and gives less self centering.

The reason that older roadster bikes have a lot of rake is because they have slack head tube angles and the reason that modern bikes have less rake is because of the fashion for steeper head tube angles.

[Brucey]

....The reason that older roadster bikes have a lot of rake is because they have slack head tube angles and the reason that modern bikes have less rake is because of the fashion for steeper head tube angles.

That is true to some extent in general terms but it is most certainly not true in every case; if you look through old catalogues you will find that the amount of trail that is considered 'appropriate' has varied wildly over the years with fashion and application. There have been very many frames made with comparatively steep head angles and large fork offsets.

In the 1950s the most common fork blade curves were a comparatively tight radius near the end of the blade; this is more noticeable to the eye; the same offset with a more gradual radius (as found on many later frame designs) looks like it might be a smaller offset, even when it isn't. The 1950's design (in a blade with a well tapered tip) is probably more compliant, which was (I imagine) the intention.

cheers

[531colin]

I'm with Norman on this.
In the huge majority of bikes that you will see on the road**, a short offset fork is matched with a steep head angle in bikes where "lively" steering is required, and in bikes where stability is required this is done using a slack head angle and long offset.
The dangerous bit is the widespread misconception that its the long offset that gives the stability, a misconception repeated in "le vieux gronard's" post above.

http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry
http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics

**exceptions are everywhere. I have flagged one up here http://forum.ctc.org.uk/viewtopic.php?f=5&t=84308&start=15....if the geometry table is to be believed, its a touring bike with livelier steering than I would want on any sort of bike.

[stewartpratt]

Carbon forks are straight because carbon can't be bent.

Carbon fibre can be moulded to pretty much any shape.

The curve at the bottom of steel forks is replaced by setting carbon forks at an angle in the fork crown. That gives the castor effect needed for steering.

The castoring effect is provided by the head angle. The fork offset, or rake, actually reduces it.

Whether straight forks are harsher than curved steel depends on the curve in the steel and its quality, so its a subjective assessment.

Quality of steel doesn't make any difference per se. It may influence the wall thickness, which has a minor effect. But predominantly it's down to the external diameter and any shaping. A curved fork allows more bending. It's not really subjective: a curved fork, all else being equal, has more flex.

Pronounced curve - rake - is mainly used to keep the bike going in a straight line when being ridden slowly. That's why the rake is more pronounced on old-fashioned roadsters. A side-effect is that the bike is then harder to steer, a corollary of its being better at going in a straight line.

Again, wrong way round. Not quite sure what you mean by "pronounced curve" because to my mind it's not the same as rake, which is the offset from the steering axis. Rake is normally higher on framesets that use slacker head angles, to counteract the relatively large castoring effect from the angles.

Once you've positioned the axle relative to the crown, that's the steering geometry sorted. You're then free to join those two points however you see fit: you can use fat or thin tubing, you can bend it, you can profile it and you can ovalise it, or more (see Specialized "Zertz" for example), and that's how you manipulate the ride quality.

[reohn2]

We've wandered some way off topic into steering geometry,the OP was asking if straight forks were harsher than curved.

[bigjim]

In 2012 I hired a Specialised bike in Mallorca with straight carbon forks. I found it very harsh and ended up wrapping an inner tube around the bars to relieve the vibrations.

In 2013 I hired a Pinarello with the wavy forks. I had no comfort problems.

[531colin]

So there you have it.
Compare 2 bikes a year apart, put it on t'interweb, and its FACT!!
.......sorry, Jim, in this case I don't find your argument "proved"

[531colin]

We've wandered some way off topic into steering geometry,the OP was asking if straight forks were harsher than curved.

Actually, it was the first reply that wandered into steering geometry.....with factual errors that needed correcting.

[bigjim]

So there you have it.
Compare 2 bikes a year apart, put it on t'interweb, and its FACT!!
.......sorry, Jim, in this case I don't find your argument "proved"

Whoooaah.... Colin back off there. I did not state any facts and I doubt you can. What argument?
I was stating my experience in case it was of any help to the OP.
If I'm prepared to go to the trouble of replying to somebody, don't you come out all guns blazing! My post did not deserve that.
That kind of reply will end up up with people afraid to relate their own personal experience that may be of some help to the OPs enquiry.
I'm suprised at you. You're usually the most helpful person on here.

[Brucey]

In 20012 I hired a Specialised bike in Mallorca with straight carbon forks. I found it very harsh and ended up wrapping an inner tube around the bars to relieve the vibrations.

In 2013 I hired a Pinarello with the wavy forks. I had no comfort problems.....

So there you have it.
Compare 2 bikes a year apart, put it on t'interweb....

Does bigjim travel by tardis?

's funny, you would have thought with another 17999 year's development and all, that the 20012 model would be a bit better sorted out than that....

But seriously it is an interesting observation though.

However, I think that you can look at a steel fork, know roughly how heavy it is and make an educated guess at how stiff it is likely to be. A quick twang and it will become clearer.

But a CF fork can be (at the same weight even) almost any stiffness; you just can't tell from looking at it how it is laid up. The shape may or may not be important. It is somewhat unlikely that a CF fork actually compresses along its length appreciably (after all you can still draw a straight line along its length that lies within the blade profile); it is much more likely that it twangs back and forth and absorbs bumps that way, and the shape may be something of a decoy. By contrast the seat stays are built very light and may indeed flex differently; they don't have to see such large bending loads in normal use.

Comfort in CF is an area where some of the manufacturers have been working hard to improve matters; here they tread a fine line because some people buying racing bikes won't buy one if they think it is in any way 'compromised' (which shows you how mad they are- by contrast the lightest steel-framed road bikes are nearly always also the most comfortable...).

It wouldn't surprise me to find that the two bikes in question were indeed very different in their ride characteristics. Back to back you might be able to say for sure that one bike was a lot different to another, but in fairness (and I'm not 'having a go' here) a year apart, with different wheels and tyres in, maybe not with such certainty. A duff headset adjustment (or similar) in one bike could make it seem terrible too.

Years ago when steel was the material of choice quite a few riders and framebuilders got to understand roughly what kind of tube gauges might give a particular feel or stiffness in a frame or fork. These days the situation with CF is potentially waaaaay more complicated than that; I doubt anyone (outside of half a dozen people in each design office) understands how the layup 'works' and how that might alter the feel of the frame. If I were asked to test bike frames these days I'd have to give a subjective impression (with all the pitfalls that entails) and past that I'd want to put each frame in a jig and measure its stiffness, or instrument a frame that is being ridden. I doubt you would be able to make sense of it all otherwise.

cheers