RoadBikeReview.com's Forum Archives - Cyclo-Cross
OK, dumb question-(5 posts)
|OK, dumb question-||Why straight blades???|
Dec 29, 2001 9:09 PM
|Many (but not all) cross forks, and many (but not all) rigid mountain forks have straight blades. I'm happy with my straight-blade cross fork, although admittedly it's got lots more raod miles than otherwise. But I've never heard a technical explanation as to exactly what the straight-blade advantage is.
Thanks for your insight on this nagging question!!
|re: OK, dumb question-||mackgoo|
Dec 30, 2001 9:48 AM
|Some may say one design is stiffer than another and mathamaticaly may be able to prove it, but in the end personally I think the only difference is one is more apealing than another to the individual. I never cared for the asthetics of the sraight blade, then I got on a retro Colnago kick and now I think straight blades are pretty cool looking. That's about it. Geometry wich can be achieved by either style and component material wich can be used on either style play a bigger part in how the fork feels.|
Dec 30, 2001 9:54 AM
|"I have always wondered why bicycle forks tended to be curved. Of course some kind of deviation from the steering axis is needed to produce what some call 'offset' and others call 'rake'. But forks sometimes have been made with straight, canted blades which achieve that offset -- why not do this always? |
I have a couple of thoughts, and no real answer.
It is often held that 'compliance' or shock absorbency
of the fork is due to the bend. But you actually get quite
a bit of flex in a straight-blade design too..... I would
have to calculate (again) the difference between them, but
how important could it be, since a fork appears to offer
less 'give' than the tire or handlebars?
Another possibility is that fork crowns are simply easier to
make with parallel holes.
Another effect of curved forks appeals to me very much, because it's kind of subtle: preventing the rim from rubbing the brake blocks.
If you think about it, forks are not all that stiff laterally. If you hold one in your hands and squeeze the blades together, there's noticeable motion. When you stand up and lean your bike, or when you swerve rapidly without good upper-body co-ordination, you create significant sideforce at the ground. The fork actually deflects some, but it's unusual to get actual rubbing at the brake pad (although I've had small amounts, evident with an old serrated rim and a near-dragging brake block).
I once went to measure the amount of rim travel at the brakes, and was surprised: there was almost none! A few inches around the rim, however, there WAS substantial motion -- the wheel was evidently pivoting about a point very near the brake, perhaps tilting about an axis which passed through the brake region. If you think about it, motion of the rim near the blocks has several parts:
1. axle bending
2. rim bending (if you push the rim sideways at 6:00, it
moves in the same direction at 12:00, though much less)
3. the fork blades act like a parallelogram, and permit
the hub to translate laterally as both blades become S-shaped
4. twisting, since the ground point is off the steering axis, a sideforce pushes one blade forward and one back
5. compression -- a curved blade is less of a column, so onewill shorten slightly and one will lengthen.
My interpretation of what I saw was that there was quite a bit of #3, which was canceled AT THE BRAKE by the wheel tilting (due to #5) so as to bring that point of the rim back where it was supposed to be. I couldn't figure out anything else which could act to cancel the effects of #3.
That's where it stands -- a speculation that bent fork blades
re effective at preventing brake rub (although they thereby
double the displacement at the ground point due to fork
blade bending alone).
Is there any better explanation for the curvature?"
info copied from Jim Papadopoulos
Dec 30, 2001 10:21 AM
|Most cyclocross forks use curved tubes for the crown and do not need parallel holes, the cantilever posts replace these. |
Other topics include it is cheaper to stock one fork crown and shape the fork curve/ rake to the rake degree desired. As opposed to making individual crowns for individual rakes. I have read that it is easier to change the curve than to replace the fixed fork crown to alter rake degree.
Also trends change, straight blade forks seem more popular today. I know I perfer the look of straight blade forks.
I am glad to see quality and detail becoming a paramount ideal to american frame builders. Hats of to IF bikes. Also with my custom Graham Weigh cyclocross from the UK, I requested a straight blade fork, and recieved one at no extra cost. I would like to think perhaps if I may say, it takes a bit longer to construct a straight blade fork.
|Thanks for the insight!!||Pablo|
Dec 31, 2001 8:49 AM
|Wow, I feel better, this is much less of a "duh" question than I thought!
As far as aesthetics go, I kinda like the straight blade forks. But I also don't dislike curved (my old-school rigid mtb has curved). Of course, I think that rigid disk forks on mtb's are rather sexy too.