Can someone explain Fork flex: what, how, why, when, where...other than the obvious that the fork indeed flexes? TIA
What is Fork Flex?
LordDRIFT
1 - 18 of 18 Posts
Joined
·
9,514 Posts
That's pretty much it, the fork flexes.
On the trail the place this is most obvious to me when ramming the fork into a rock garden at speed or high speed turns. The fork will actually deflect from the direction you're traveling, and bike will track differently than where the bars are pointed.
There are a lot of ways to fix this including making the stanchions a bigger diameter, using a through axle, making the arch stiffer, or making the crown stiffer.
On the trail the place this is most obvious to me when ramming the fork into a rock garden at speed or high speed turns. The fork will actually deflect from the direction you're traveling, and bike will track differently than where the bars are pointed.
There are a lot of ways to fix this including making the stanchions a bigger diameter, using a through axle, making the arch stiffer, or making the crown stiffer.
Joined
·
9,514 Posts
Are you using disc brakes? That's most likely your fork flexing.LordDRIFT said:
)
Joined
·
50,076 Posts
3 axes;
Lateral. This is the tendancy of the wheel to "push over" when turning. Wheel/spoke flex is a bigger contributer these days. This is probably the least important and in modern forks is not an issue. I saw some old inverted forks from back in the day (cannondale Moto) that were horrible for this, but it's been a while.
Tortion. This is the tendancy to "twist". This can be pretty bad on inverted forks, and terrible on single-crown inverted forks. 20mm axle interfaces help (assuming solid lowers). Some older forks had 20mm axles like Stratos forks, but due to dated/insufficient chassi engineering they still twisted. A dual crown fork (even a QR one from back in the day) adds a LOT of stiffness to combat tortion. Dual brake-arches such as on the Pace and magura forks help a lot. Bigger stanchions and a big thick crown obviously help as well. When the fork twists it limits your ability to make the bike go where you want it and it binds some and doesn't absorb the impact very well.
Fore-aft. This is probably the least talked about, yet it is probably the 2nd most important. Small stanchions coupled with a small steerer tube/crown can lead to a lot of fore-aft flex. You may not notice it much, but there are quite a few pictures that show forks flexing during impacts. This creates binding and keeps the fork from absorbing the hit as it should. Having bigger stanchions only helps so much, and a good example of this can be seen with most long-travel single crown forks. Turn a 1.125 steerer marzocchi 66 upside down and you'll notice the steerer tube is so thick you can't even really fit a pencil in the space. They have to make it ultra-thick to give the stiffness and strength needed to support the fork. Everyone on the 29er board is crying that the latest 29er forks "only" use 32mm stanchions, and while bigger stanchions would be nice the fact that they're using a 1.5 steerer now makes a bigger difference in the fore-aft flex and is probably the primary reason they are using the 1.5 tapered system now. My point is that bigger stanchions only works up to a certain point and you have to do something else to maintain the fore-aft stiffness. With dual crown forks tehre's the entire 2nd crown. With most shorter-travel forks it's not a huge issue, but when the axle-to-crown length gets excessive, as with a single crown freeride fork or 29er, you simply have to do something to make it "stiff enough" in this direction. Another good example is the white brother's 150mm 29er fork. That thing SHOULD have a 1.5 tapered steerer, but instead it has to have a crown CNCed from a MASSIVE chunk of aluminum. It's not light and it's not an efficient use of materail, but it's what they have to do barring more expensive manufacturing processes.
Lateral. This is the tendancy of the wheel to "push over" when turning. Wheel/spoke flex is a bigger contributer these days. This is probably the least important and in modern forks is not an issue. I saw some old inverted forks from back in the day (cannondale Moto) that were horrible for this, but it's been a while.
Tortion. This is the tendancy to "twist". This can be pretty bad on inverted forks, and terrible on single-crown inverted forks. 20mm axle interfaces help (assuming solid lowers). Some older forks had 20mm axles like Stratos forks, but due to dated/insufficient chassi engineering they still twisted. A dual crown fork (even a QR one from back in the day) adds a LOT of stiffness to combat tortion. Dual brake-arches such as on the Pace and magura forks help a lot. Bigger stanchions and a big thick crown obviously help as well. When the fork twists it limits your ability to make the bike go where you want it and it binds some and doesn't absorb the impact very well.
Fore-aft. This is probably the least talked about, yet it is probably the 2nd most important. Small stanchions coupled with a small steerer tube/crown can lead to a lot of fore-aft flex. You may not notice it much, but there are quite a few pictures that show forks flexing during impacts. This creates binding and keeps the fork from absorbing the hit as it should. Having bigger stanchions only helps so much, and a good example of this can be seen with most long-travel single crown forks. Turn a 1.125 steerer marzocchi 66 upside down and you'll notice the steerer tube is so thick you can't even really fit a pencil in the space. They have to make it ultra-thick to give the stiffness and strength needed to support the fork. Everyone on the 29er board is crying that the latest 29er forks "only" use 32mm stanchions, and while bigger stanchions would be nice the fact that they're using a 1.5 steerer now makes a bigger difference in the fore-aft flex and is probably the primary reason they are using the 1.5 tapered system now. My point is that bigger stanchions only works up to a certain point and you have to do something else to maintain the fore-aft stiffness. With dual crown forks tehre's the entire 2nd crown. With most shorter-travel forks it's not a huge issue, but when the axle-to-crown length gets excessive, as with a single crown freeride fork or 29er, you simply have to do something to make it "stiff enough" in this direction. Another good example is the white brother's 150mm 29er fork. That thing SHOULD have a 1.5 tapered steerer, but instead it has to have a crown CNCed from a MASSIVE chunk of aluminum. It's not light and it's not an efficient use of materail, but it's what they have to do barring more expensive manufacturing processes.
Joined
·
721 Posts
jayem - i have fore-aft flex in my 29er Reba fork from 2007. going over rocky stuff at speed causes a lot of vibration in the head tube area of the bike (sultan) and i can see the fork being pushed backward by part of the energy of impact rather than all the energy being transferred to stanchions action (my fork is not set up stiff). so maybe part of your argument about 29er forks is right. interestingly, the A to C of my 100mm Reba (505 mm) is similar or even shorter than most 140 mm forks for 5.5" travel 26" bikes with the standard 1.125 steerer tube (515mm). do they suffer from the same fore-aft problems? thanx!
Jayem said:
Is fork flex ever exacerbated by rider weight, the reason i ask is im currently sitting at 215lbs, will more than likely be back down to ~200lbs by the time i get round to getting my new bike.
At the mo my option for forks is either the 150 32 Float with 15QR or the 36 Float 20QR (modded down from 160mm to 150mm)
Its very much most likely going to be on a 5 Spot (2010)
My ridding is currently XC and trying to keep up with my mate on the downhills (im on a HT at the mo) im hoping this bike will bring my game on abit
.
At the mo my option for forks is either the 150 32 Float with 15QR or the 36 Float 20QR (modded down from 160mm to 150mm)
Its very much most likely going to be on a 5 Spot (2010)
My ridding is currently XC and trying to keep up with my mate on the downhills (im on a HT at the mo) im hoping this bike will bring my game on abit
.
Joined
·
2,380 Posts
I'm pretty light (~75kg) and I still noticed it. I would imagine the riders weight affects this a lot, but the terrain affects it even more (I also noticed it a lot in rock gardens or very rooty trails).
However, I do believe there is an acceptable amount of flex, and an unacceptable amount of flex... I went from the Magura Wotan to a Pike 454 and noticed it was clearly flexier. The flex however did not bother me very much, I felt I could still keep my line very well. I went from the Pike to a Revelation 426 with QR and there the flex was far worse - and affected my riding a lot too. I then went to a DT EXC 150 with 20mm through axle and it felt great again.
I have a Fox 32 with 15QR coming on my next frame, and hope it will be sufficient. I've seen a lot of talk about 15 vs. 20mm axles on this site, but unfortunately the "scientific" studies linked to have IMO been lacking... Let's hope it's not a noodle, as that would be quite a hassle for me.
I think this question (Fox 32 vs. Fox 36) also is very much dependant on what kind of trails you ride. For me I noticed an even bigger difference then the flex on my climbing ability with the lighter forks. I have a lot of technical climbs where I ride that I can't get up without ditching the bike. Swapping to a lighter fork made a huge difference in that regard. I'm saying lightness because for example the DT fork was taller (AC height) then the Pike, but at over a pound lighter made a huge difference in climbing. However if I was riding on different kind of terrain the scale might tip towards stiffness bringing a bigger advantage.
However, I do believe there is an acceptable amount of flex, and an unacceptable amount of flex... I went from the Magura Wotan to a Pike 454 and noticed it was clearly flexier. The flex however did not bother me very much, I felt I could still keep my line very well. I went from the Pike to a Revelation 426 with QR and there the flex was far worse - and affected my riding a lot too. I then went to a DT EXC 150 with 20mm through axle and it felt great again.
I have a Fox 32 with 15QR coming on my next frame, and hope it will be sufficient. I've seen a lot of talk about 15 vs. 20mm axles on this site, but unfortunately the "scientific" studies linked to have IMO been lacking... Let's hope it's not a noodle, as that would be quite a hassle for me.
I think this question (Fox 32 vs. Fox 36) also is very much dependant on what kind of trails you ride. For me I noticed an even bigger difference then the flex on my climbing ability with the lighter forks. I have a lot of technical climbs where I ride that I can't get up without ditching the bike. Swapping to a lighter fork made a huge difference in that regard. I'm saying lightness because for example the DT fork was taller (AC height) then the Pike, but at over a pound lighter made a huge difference in climbing. However if I was riding on different kind of terrain the scale might tip towards stiffness bringing a bigger advantage.
Joined
·
50,076 Posts
That's not suprising, I have the same fork and the long axle-to-crown coupled with factors like fairly small crown, small/thin stanchions and 1.125 steerer make it possible. I don't know if you've looked at the steerer on something like a marzocchi 66 but it's super-thick.starre said:
The Reba is an XC fork, so it's going to do this more than something that's a little more "all-mountain" specific. On the other hand you'll probably find some AM forks that do this to the same extent. A bigger rider is obviously going to push the equipment harder. The best fork for someone of this nature may be something like a marzocchi 55 (for a 26" wheeled bike) or 1.5 tapered 44 (for a 29er). Even though the 44 has smaller stanchions the 1.5 steerer shoud help a lot with fore-aft flex.
The newer version of the Reba is supposed to be a bit stiffer, but in reality it's not going to be a huge change. The white-brothers forks have absolutely massive crowns and may not flex as much, but I gotta say that if I had a long-travel 29er I would NOT get a white brother's fork due to the extreme axle-to-crown distance. They make a 130 and 150mm fork, but at that axle to crown you can only beef up the crown so much and then you gotta do something else, like use a 1.5 steerer, bigger stanchions, etc.
Joined
·
1,611 Posts
dropadrop - please re-post when you get time on the 15qr.
I have a Fox F100RLC that is too flexy for my tastes. I am undecided on how to address this:
1. get new 15qr lowers
2. sell it and get a Manitou Minute w/ 20TA
(reba w/ maxle is not an option since I run a 650b front wheel)
I have a Fox F100RLC that is too flexy for my tastes. I am undecided on how to address this:
1. get new 15qr lowers
2. sell it and get a Manitou Minute w/ 20TA
(reba w/ maxle is not an option since I run a 650b front wheel)
Joined
·
721 Posts
thanx for your point of view as i am kind of mixed up on what's the best replacement fork for my 2007 Reba and it would be silly to buy a new fork (and new wheel with hub if needed) that doesn't address this problem of fore/aft flex. i checked the Fox site to compare 26 to 29er specific forks. comparing air sprung forks with 15mm QR for 1 1/8 steerer tube, the 150 mm Fox 32 Float comes in at 3.88 lbs and the 100 mm Fox F29 comes in at 3.96 lbs. both have more or less the same A/C and so i don't really think they can be substantially built much different. in going with your line of thinking, both of these 'longer' lighter weight forks should have a tendency toward fore/aft flex regardless of wheel size, no? again, many thanx!
Jayem said:
1 - 18 of 18 Posts
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
