Upside Down Forks

[jesster]

heres a question i was asked by an mxer i work with that i couldnt answer. Why are trials bikes all conventional front forks these days whereas mx bikes are upside down. And as i thought up a creative answer, i realized that trials and mx have gone the opposite way. There used to be upside down forks on trials bikes (betas) and now there arent, whereas mx bikes have all gone the other way. Can anyone give me a good answer to give this guy on tuesday (Yes, long weekend here in Canada!! Too bad all the riding areas around here are shut down due to fire hazards!!)

Thanks

[toomanybikes]

I don't know. I too have asked this question to dozons of people. The most reasonable answer i have gotton so far is, "a skinny trials bike dose not look right with the fat part of the forks at the top." I know very lame, but that is the best and I have asked a lot of peaple that should know.

I hypothisise the real reason is fork stiction. In the trials world it is common to intentionally hit obsticals with the front wheel; zap, splater, double blip. Not so in the moto-world or enduro world where you try like hell to avoid obsticals with the front. (I had a HUGE problem with this when I came to trials and still do to some extent). This intentional hitting aplifies the horizontal bending force on the front forks. The bending force is greatest just under the lower tripple clamp. Upside-down forks are more ridged in that area so the bending is moved down to where the upper sanction ends. In that area the fork bushing rides, bent shafts in bushings tend to bind. A little bit of friction can drastically effect the transfer of energy for manuvers like the zap or splater. Trials forks being thinner and lighter than moto forks this could be a real problem especially for hard hitting top riders. Mayby only a few of the top riders notice the difference, mayby more. I don't know. Mayby the 'looks' explination has more merrit.

[pogbird]

Jesster

For what it's worth, my guess is based on some of the sales pitch I recall for upside down forks. (By the way. My Dad claims that these are the right way up based on some ancient old bone shaker he recalls)

One of the reasons given for conversion to upside down forks was the reduction in the ratio of unsprung weight to sprung weight which is crucial in world of high performance suspension.

On a road bike it was not initially thought to make much difference as the bike was heavy and the already low unsprung weight ratio was also more affected by heavy tyres, wheels and brakes than the fork outer, its use on road bikes was initially thought to be very gimicky.

On a light bike like a motocrosser with light wheels and tyres the extra unsprung weight added by the fork outer was deemed to be worth the trouble/investment to remove.

While I appreciate that the average trials iron falls into the latter category of light weight etc. The suspension of a trials bike is not being asked to deliver wheel control with the same speed of suspension movement and conditions. Thus the unsprung weight ratio is not so crucial. Add to that the likelyhood of damage to important and expensive stanchions on a bike with upside downies which is ridden on and around rocks for most of it's life and the balance probably comes out in favour of conventional forks.

Pog

[da make]

I think that it has something to do with "suspended weight". A MXer is needed to smooth every stone on the way when riding 6th gear/full thortle, suspended weight: less = faster action.

Compared to trials machines which are designed to work at slow speeds there are no need to usd.

(edit: "unsprung weight" is what I'm talking about. I don

Edited August 6, 2004 by Da Make

[beta boy]

i have usd's on my bike and they are alright but are really crappy sometimes i find they have not much front to back play like the bikes with rsu forks so when i want to get up something big and my front hits it real herts my wrists but it does not on other bikes with rsu forks and plus the fork sliders are more prone to chiping cause they are low down and cause the oil is pushing down on the seals if you have a slight leak it comes out and goes all over the disk

[bigfoot]

I like the upsprung weight argument but I'm not sure how much difference sprung weight has on a trials bike's performance.

I think the fact that the tube is so much more likely to be damaged way down there that that desired it. Also as stated above the benting moment is far greater on USD forks meaning you need bigger thicker tubes and that adds weight to a trials bike. Trials bikes are subject to greater off axis loads from front on impacts than a MX or other off roader.

[chris]

Conventional forks tend to be stiffer (USD forks tend to flex slightly) when turning due to the brace used to hold the front mudguard on. MX don't use a mudguard and brace like trials bike's and therefore steering flex is not affected by changing to USD forks.

Hope this makes sense, all IMHO

Chris

[toomanybikes]

Nice try with the unsprung weight argument. What doesn't make sence is you say the MX can benifit less unsprung weight, which is true. However the steel sanction of the fork is heaver then its aluminum counterpart. This puts the heavier steel sanction of the USD fork on the bike needing the less unsprung weight, the MX. Thinking, but I do not see where it make sence.

Note their is not much difference in fork sanction weight anyway. Most of the forks unsprung wheight comes from the oil, the cartridge and the spring.

It takes some skill to ride a MX at the level to tell the difference in rigidty between conventional and USD forks. But the difference is there. The USD is much more ridged!

[beta boy]

yeah i feal that on my bike they are much more rigid than conventional forks

[t-shock 250]

No, conventional forks are stiffer, for the reasons mentioned in the previous posts.

[beta boy]

yes in stearing flex but they are more rigid fron to back well i find anyway

[t-shock 250]

Front to back?

Your bike may have a a serious problem if you can move your forks forwards and backwards!

[jtt]

tshock, I think toomanybikes was referring to MX applications, without the fork brace. In that application, there is no question the USD forks are stiffer. In fact, I'd be surprised if they were not stiffer even with the brace. All modern Roadracers also use USD forks for the added rigidity.

For what it's worth BB, forks do flex fore and aft to some degree (although so little in our cases to be insignificant). In Pro MX conditions, it is significant. So much so that frames are replaced regularly because they actually elongate the wheel base because of bending at the steering stem

I have used both conventional and USD forks for MX and always found the conventionals to be plusher. I think it may have to do with the sideloading on the fork bushings being greater on USD?? I'm only guessing, but they always seem plusher and that added plushness would make sense in a trials application.

Edited August 5, 2004 by JTT