Why are front forks all upside down??

Hi,
Can anyone tell me why all MTB forks (all the ones I've seen) have the
stanchions (chrome inner part) at the top and the sliders (cast outer
part) at the bottom?

Two reasons why this is backwards:
-The max stress occurs at the top of the fork so this would be the
logical place to put the larger dia, stiffer slider.
-The bottom half of the fork is unsprung weight, which needs to be
minimized to help the suspension work well, so this is the ideal place
for the lighter stanchion.

I know it's a bit easier to manufacture them with the stanchions at
the top but that should only concern low-end manufacturers.

In article m,
Jon_C wrote:

Hi,
Can anyone tell me why all MTB forks (all the ones I've seen) have the
stanchions (chrome inner part) at the top and the sliders (cast outer
part) at the bottom?

Two reasons why this is backwards:
-The max stress occurs at the top of the fork so this would be the
logical place to put the larger dia, stiffer slider.
-The bottom half of the fork is unsprung weight, which needs to be
minimized to help the suspension work well, so this is the ideal place
for the lighter stanchion.

I know it's a bit easier to manufacture them with the stanchions at
the top but that should only concern low-end manufacturers.

Motorcycle forks go both ways, depending on the design preferences, with
a strong trend in performance bikes towards "inverted" (chrome bit at
the bottom) forks.

On bicycles, the biggest issue is probably keeping dirt and scratches
off the vulnerable chrome part. Also, there's some non-trivial sealing
issues with having downward-exposed slider seals, I'm guessing.
Inverted-fork dirt bikes deal with this using really big plastic slider
guards. If you have to add a half pound of plastic slider guard to save
a quarter pound of fork weight....

Note that one way or another, you need a fancy bottom mount for stuff
like the axle and the brake caliper, so inverting the fork probably
saves less weight than you would hope.

That said, there's the Marzocchi Shiver if you're really eager to go
inverted. It's a downhill-specific (and very race-oriented; not really
for freeriders) fork:

http://www.mtbr.com/reviews/2004_fro...t_122903.shtml

Note the complaints in the reviews that as nice as it is, there are a
lot of DH forks out there that are lighter.

--
Ryan Cousineau http://www.wiredcola.com/
"I don't want kids who are thinking about going into mathematics
to think that they have to take drugs to succeed." -Paul Erdos

"Jon_C" wrote: Can anyone tell me why all MTB forks (all the ones I've
seen) have the
stanchions (chrome inner part) at the top and the sliders (cast outer
part) at the bottom? (clip)
^^^^^^^^^^^^^^^^
Several years ago a friend of mine installed a set that are right-side-up
according to your standards. )I have forgotten the brand.) They used
elastomer inserts of various "constants" to adjust the springiness--he
carried an assortment in his pocket, but seldom found a need to change them.

I agree with you that the tubes should be on top. As I recall, though, this
involves complications in mounting caliper brakes, since you can't attach
them to the lower stantion. (I'm really fuzzy on this part.)

On Sep 24, 1:52 pm, Jon_C wrote:
Hi,
Can anyone tell me why all MTB forks (all the ones I've seen) have the
stanchions (chrome inner part) at the top and the sliders (cast outer
part) at the bottom?

Two reasons why this is backwards:
-The max stress occurs at the top of the fork so this would be the
logical place to put the larger dia, stiffer slider.
-The bottom half of the fork is unsprung weight, which needs to be
minimized to help the suspension work well, so this is the ideal place
for the lighter stanchion.

I know it's a bit easier to manufacture them with the stanchions at
the top but that should only concern low-end manufacturers.

Sliders on the bottom allows the support arch between the legs to stay
closer to the mid span between the axle and the crown. For non thru-
axle forks this is essential for maintaining rigidity. Also, the
stanchion isn't necessarily lighter than the slider. The larger
slider tube can be made with a thinner wall than the stanchion.
Assembling the fork the other way around would also be very hard to do
in a way that's lighter than casting the lower legs, support arch,
dropouts, and disc tab all out of a single piece of magnesium.

On Sep 24, 2:56 pm, "Leo Lichtman"
wrote:
"Jon_C" wrote: Can anyone tell me why all MTB forks (all the ones I've

seen) have the stanchions (chrome inner part) at the top and the sliders (cast outer
part) at the bottom? (clip)

^^^^^^^^^^^^^^^^
Several years ago a friend of mine installed a set that are right-side-up
according to your standards. )I have forgotten the brand.) They used
elastomer inserts of various "constants" to adjust the springiness--he
carried an assortment in his pocket, but seldom found a need to change them.

I agree with you that the tubes should be on top. As I recall, though, this
involves complications in mounting caliper brakes, since you can't attach
them to the lower stantion. (I'm really fuzzy on this part.)

Yup brakes need to move with the wheel so mounting caliper brakes
would be a real problem. Disc brakes wouldn't be an issue as they
mount right at the bottom of the fork on the unsprung part. (It's a
real shame to use disc brakes tho cos they add unsprung weight)

I don;t see a big issue with caliper/axle mountings. It would only
take a small casting on the end of the fork so you'd be saving
unsprung some weight for sure. It might not be a massive saving but
any saving is good and saving unspring weight is way more important
than saving overall weight.

I agree about the problem of damage to the stanchion chrome. Dirt
could be kept off with flimsy bellows type covers but if you ride
amongst rocks then you;d need something more substantial. However,
guards can at least be mounted to the sprung section of the fork.

I dont know much about seal design but i'd imagine gravity to be a
negligible force compared to the pressure during compression so I
doubt seals would be an issue.

One other benefit is overall weight saving. By putting the naturally
larger part of the fork where the stress is, you end up needing less
material to withstand a given load.

http://www.maverickbike.com/main/do/Fork_Technology

why all MTB forks (all the ones I've seen)

Now you've seen one

--
DTW .../\.../\.../\...

On Sep 24, 12:56 pm, "Leo Lichtman"
wrote:
"Jon_C" wrote: Can anyone tell me why all MTB forks (all the ones I've

seen) have the stanchions (chrome inner part) at the top and the sliders (cast outer
part) at the bottom? (clip)

^^^^^^^^^^^^^^^^
Several years ago a friend of mine installed a set that are right-side-up
according to your standards. )I have forgotten the brand.) They used
elastomer inserts of various "constants" to adjust the springiness--he
carried an assortment in his pocket, but seldom found a need to change them.

I agree with you that the tubes should be on top. As I recall, though, this
involves complications in mounting caliper brakes, since you can't attach
them to the lower stantion. (I'm really fuzzy on this part.)


There are various forks like the OP describes, and I think I know
which fork Leo's friend had: the Halson Inversion. I've never seen it
myself, but I do recall seeing magazine ads for it in the early- and
mid-1990's, years before disc brakes were generally available for
MTBs. I don't know if Halson is still in business, but I on Ventana's
website I found this snippet describing the way that this fork solved
the brakes-must-move-with-wheel problem:

"An inverted suspension fork is a great idea on paper. The strongest
part of a fork needs to be at the top, next to the crown. It makes
good engineering sense to use the large-diameter alloy part on the
top, and put the smaller-diameter, moving part on the bottom. Not only
is this a better use of materials, supporting the bearings and sealing
its moving parts is made easier with an inverted arrangement. Known as
"upside-down forks," the idea has been borrowed from motorcycles (even
though professional motorcycle racers are returning to right-side-up
forks).

The fly in the ointment of upside-down forks is the brakes. They must
be close to the rim, and that means ten inches above the front axle.
Since the upper part closest to the rim of upside-down forks doesn't
move, it's hard to get the brakes to follow the rim. This problem has
sidetracked most inverted fork concepts into the waste can. Halson
Inversion forks solved this problem by slotting the upper alloy
section of its fork. The cantilever bosses move up and down inside the
vertical slots without having to depend on a disc brake. You get
superb bearing overlap, increased rigidity and less unsprung weight
(the part of the suspension that follows the ground on upside-down
forks)."

The link below has the full article from the April 1995 issue of
Mountain Bike Action:

http://www.ventanausa.com/mba0495.html

Stephen Greenwood

I searched around and found these too:
the Groove by White Brothers Cycling
http://www.whitebrotherscycling.com/...groove_200.jpg

Still, it seems like right-way-up forks are incredibly rare in the MTB
world which is odd when u consider that just about every sports
motorbike uses them. Road, moto-X and even trials bikes..

wrote:
On Sep 24, 1:52 pm, Jon_C wrote:
Hi,
Can anyone tell me why all MTB forks (all the ones I've seen) have the
stanchions (chrome inner part) at the top and the sliders (cast outer
part) at the bottom?

Two reasons why this is backwards:
-The max stress occurs at the top of the fork so this would be the
logical place to put the larger dia, stiffer slider.
-The bottom half of the fork is unsprung weight, which needs to be
minimized to help the suspension work well, so this is the ideal place
for the lighter stanchion.

I know it's a bit easier to manufacture them with the stanchions at
the top but that should only concern low-end manufacturers.

Sliders on the bottom allows the support arch between the legs to stay
closer to the mid span between the axle and the crown. For non thru-
axle forks this is essential for maintaining rigidity. Also, the
stanchion isn't necessarily lighter than the slider. The larger
slider tube can be made with a thinner wall than the stanchion.
Assembling the fork the other way around would also be very hard to do
in a way that's lighter than casting the lower legs, support arch,
dropouts, and disc tab all out of a single piece of magnesium.

hit the nail on the head - it's all about rigidity.

On Mon, 24 Sep 2007 21:47:56 -0000, Jon_C
wrote:

I searched around and found these too:
the Groove by White Brothers Cycling
http://www.whitebrotherscycling.com/...groove_200.jpg

Still, it seems like right-way-up forks are incredibly rare in the MTB
world which is odd when u consider that just about every sports
motorbike uses them. Road, moto-X and even trials bikes..

You might do us all a favour and stick with the accepted convention;
sliders inside the stanchions is UPSIDE DOWN.

For a bicycle, USD forks only really make sense if you solve the brake
and side to side connection problems, which means you must have a disc
brake and a clamped hub axle, commonly a 20mm.

For XC bikes with QR 9mm axles and optional rim brakes, USD forks
won't fly. USD designs also pretty much have to have cartridge
dampers, whereas an open bath design can be lighter.

I have Shiver SC (100mm) on my play bike, but it's nearly 2lb heavier
than a good 100mm XC fork, so only an option if you want plenty of
stiffness and don't care too much about weight.

Fashions change in motorcycles too; after USD forks became all but
universal, Showa/Honda switched back to RWU for, for example, the
Fireblade, because they were able to get a better balance of
stiffness, unsprung weight and steering inertia that way.

Kinky Cowboy*

*Batteries not included
May contain traces of nuts
Your milage may vary