Suspension Forks

chrisga

Hi,

Not sure if this is the right place to post but was just pondering this over and thought it would be interesting to hear other's opinions on this:

Why do the majority of mtb suspension forks have the thick stanchions at the bottom and the thin sliders on the top attached to the steerer crown. This seems to be the opposite of the motorbike world. I know that some mtb forks are "upside down" but aren't they high end downhill forks or am I wrong.

I can see a few advantages of them being upside down. On normal forks the seals around the top of the stanchions are at about the muddiest place on the fork with gunk being flung off from the tyre, and running down the brace in to the seals. The gunk is then just able to sit there wearing through seals, and stanchions etc etc. You also have lower unsprung mass with upside down forks, generally meaning that the fork performance is improved. The forks may also be stiffer but that is a matter for another debate I guess.
The only downside I could see is a slight weight gain so is this the possible problem?

toasty

I'm guessing because of the distance from the head tube, the bottom of the fork needs to be as rigid and flex free as possible. Motorbikes aren't counting the grams quite as much.

passout

Mud?

dave_hill

Weight.

USD forks have all the weight at the top - not so much of an issue with a motorbike, but with a push bike it's best to keep the weight (and thus the centre of gravity) down low.

There are/have been flirtations with USD forks on pushbikes in the past (e.g. Mountain Cycles Suspenders, Manitou Dorado and Marzocchi Shiver) and Maverick still make them.

In the motorcycle world, it seems to be accepted that there isn't any discernable advantage to either type - if you remember in the late 90's, Kawasaki's sports bikes (i.e. the old ZXR750 and 400) had USD forks - but a couple of years later, they went back to right-way-up. Triumph have never used USD forks as far as I know, and Honda were very late in adopting them.

supersonic

Flex too - need massive sliders and stanchions as you cannot fit a brace. Also cannot fit rim brakes.

Many cite unsprung mass as a reason to have USD forks, but if you have ever stripped a fork and weighed magnesium lowers...

Anonymous

The unsprung mass argument doesn;t hold true for MTB forks anyway, does it? I thought all the complex bits were in the leg uppers these days, meaning that the unsprung mass is as low as it could ever get.

supersonic

Yeah, it's a bad argument for the USD fork fans! mag lowers are so damn light anyway...

Daz555

Yes, it's all about unsprung weight and you won't see a single high-end motorcycle fork that is the right way up these days. If the MTB fork makers think that nothing is to be gained from USD fork in terms of unsprung weight (and/or total weight), then we won't see them - - even if intuitively it seems the obvious design.

dave_hill

yeehaamcgee wrote:

The unsprung mass argument doesn;t hold true for MTB forks anyway, does it?

Do you ever agree with anyone about anything?

chrisga

dave_hill wrote:

Weight.

In the motorcycle world, it seems to be accepted that there isn't any discernable advantage to either type - if you remember in the late 90's, Kawasaki's sports bikes (i.e. the old ZXR750 and 400) had USD forks - but a couple of years later, they went back to right-way-up. Triumph have never used USD forks as far as I know, and Honda were very late in adopting them.

Sorry to disagree Dave but most modern motorbikes, sportsbikes and motocross bikes all seem to have upside down forks, including triumphs as here:

.

The ZXR400 didnt go back to right way up forks, the other half has one and it def has upside downs on it, as does my old honda:



So for an mtb it must be a weight thing i guess, but whoever it was that made the point about the brace and not being able to connect the sliding parts made a good point that I hadnt thought of. Could a bolt through axle not be made stiff enough to control the legs?
I dont see mud as being an issue, I think it is far more an issue with the right way up forks we have now.
Rim brakes is another good point, hadnt thought of that, but if these were top end forks there arent many top end mtb's running vees any more are there?

Dont get me wrong im not complaining about the current crop of sus forks which have made significant steps forward in the last 10 years and these are all valid arguments, I was only wondering why.....

Chaka Ping

Also, the stanchions would be far more likely to get damaged if at the bottom of the fork.

They'd be so exposed that they probably have to be covered in some way, which is another drawback.

Anonymous

But what's the point? on "normal" mtb forks, all the weighty, complex bits are in the fork UPPERS.
Turning them into USD forks would actually INCREASE the unsprung mass.

DaveHill wrote:

yeehaamcgee wrote:

The unsprung mass argument doesn;t hold true for MTB forks anyway, does it?

Do you ever agree with anyone about anything?

I don't blindly follow trends, and I'd rather have something proven to me than just take it at someone's word.

chrisga

There is no "point" to this thread really, I was merely wondering why, and some people have come up with some ideas that I hadnt considered. So job done really.

Anonymous

Fair enough, that's the best sort of thread I reckon. Get some good debate going.