Excuse my ignorance here, but how come mountain bikes don't have USD forks ?

Seems pretty much all decent motorbikes come with them now, so why not mtb's

they do, WAKEY WAKEY!!

Unless you have a 20 mm axle they will be flexy or heavy ( to avoid flex) as they cannot have a fork brace / bridge.

Mainly fashion on motorcycles anyway – there are marginal benefits for racing – slightly less unsprung weight and increased rigidity in the upper tubes and longer overlap possible.

Mainly fashion on motorcycles anyway

You do talk some shite sometimes.

Stiffer for a given weight, although I understand dearer to produce.

singlespeedstu – Member
> Mainly fashion on motorcycles anyway
You do talk some shite sometimes.

Aww – that's not very fair. It's most of the time.

Give me a reason why they have them on many fairly ordinary bikes thats not fashion led. Most of the motorcycle world is fashion led these days.

Druidh in "funny twice in one week" shocker.

Jeremy.
Ride an MXer flat out round a GP track and then tell me that having a stiffer more direct fork is a fashion thing with only marginal benifits.

Try living in the real world with real experiences instead of just reading about it.

surely simple engineering (bending moments) says a USD fork is a better design. I'm sure us meer mortals would never tell the difference on a motorbike(I've owned both and sure as hell can't) but it has to be stronger for a given weight. Oh, and motorbikes don't use a brace/arch even on traditional forks.

mark, yes they do, its often aftermarket though to stiffen things up.

USD forks just arent as stiff as their conventional counterparts. Yes you can fit the thicker/stiffer parts at the frame end, but you loose a brace.

Very much a fassion thing on motorbikes, the USD forks on a touring bike have about as much in common with the ones on rossi's bike as custard.

n.b. dont confuse stiff enough with stiffer

USD = stiff enough

conventional forks = stiffer

A motorbike has a minimum weight limit for competition, so you may as well beef the forks up to get a marginal increace in performance rather than go for a conventional design with subtly different performance, but in a stiffer chassis.

Mountainbikes just strive to be as light as possible, so the lightest desing wins.

Take the dorrado, bloomin expensive and fancy materials, but the boxxer is infinately stiffer for a similar weight from plain old aluminium tubes and magnesium lowers.

motorbikes don't use a brace/arch even on traditional forks.

Of my most recent 6 m/cycles – 3 hondas, 2 suzukis, a bmw all with fork braces. Only the BMW was an aftermarket one.

That must explain why all modern production MXers come with an inverted fork then.Makes perfect sense to have a fork thats not as stiff as it could be when plowing into a flat out rutted whooped out corner. 🙄

😉

That must explain why all modern production MXers come with an inverted fork then.Makes perfect sense to have a fork thats not as stiff as it could be when plowing into a flat out rutted whooped out corner.

So you're saying that USD forks are stiffer than conventional ones?

My ZX-6R didn't have a brace, neither did my VRF. What bikes were yours?

SST – for race use they might have some advantages – reduced unsprung weight and better resistance to stiction and bending moments ( but worse for twisting)- especially at long travel. There will still be a fashion element tho in that everyone expects USD forks as thats what Rossi and Carmichael have.

For the majority of motorcycles tho its purely for fashion as I said. Hyosung 650 with USD?

Marky – have a look under the mudguard on RWU forked bikes. Every bike I have ever seen with a RWU fork has a solid connection between the top of the fork legs – either combined with the mudguard mountings or under the mudguard

Why is the Lefty USD, surely if this logic follows cannondale could have made it lighter the other way up?

Surely the limiting thing with MTB design is the 20mm axle (without making it out of steel which screws the weight).

under the mudguard on RWU forked bikes. Every bike I have ever seen with a RWU fork has a solid connection between the top of the fork legs

Not a VFR (NC-30)or ZX-6R (J1). I have removed the forks from both bikes personnally. Like USD forks they relied on the axle and pinch bolts.

So the mudguard did not bolt solidly onto the fork each side with a metal mounting bracket under the guard?

I'm pretty sure I've only owned one motorbike with USD forks, but it certainly had the most capable front suspension of them all:

Mudguard was plastic, no metal. Surely commom sense says no flimsy thin guage sheet metal bracket hidden under a mudguard is going to add any strength to forks relative to a hulking great 30 or 40mm dia steel axle.

USD forks are mainly used to reduce unsprung mass which makes the suspension respond faster.
Every bike I've had with RWU forks has had some sort of brace near the mudguard mounts.

i rememmber reading the US dirtbike mag shootout between desert bikes a few years back..ktm 640 vs honda XR650..they tested both stock and tuned bikes (engine and suspension)and the XR had the better suspension (conventional) but dirtbikes do have mostly usd forks…i remmember a few years ago when racing where they changed back and forth…
i do know personally coventional fork seals are easier to change…
i still think mtb fork prices are a massive rip off… 😮

snaps – americans can't make motorbikes, they make things out of cast iron and mild steel and rubber belts.

Yep, I rode a Buell once – horrible vibration, thought it was going to fall to bits, confirmed my vision of American bikes – give me a Jap bike any day!

surely RWU vs USD is mainly a cost thing, with RWU being cheaper to manufacture as there is no requirement to machine the outside of the lowers, where as on USD fork you need to. You also need additional parts to carry the caliper/axle on the end of the lower leg on USD and these need to be presicion machined and pressed onto lower legs. More parts, more steps in machining, more assembly = more cost.

Anyway….that's me for the evening….

Buell is about the only exception to the cast iron and mild steel comment (when they use rotax engines at any rate)

No it was a 97 White lightning with a Harley engine, bloody horrible! It vibrated so bad at tick over that the mirrors were useless & I thought it was going to knock itself into gear, the week after I rode it the exhaust fell off & the drive belt wore through the rear brake line covering the owner in fluid 🙄
I think you're probably right with the cost thing as the lowers of RWU forks are cast & fitted to budget bikes.

I had a pair of Simons USD forks fitted on one of my mx bikes in the early 80's- 83-84 I think. The guy who invented them worked for Fox, and had designed the Fox Air Shock.
The selling points then were: larger diameter hence stiffer tubes, better slider overlap, again for stiffness, and the elimination of the need for a leading axle fork- as fork travel started hitting the 12" mark the extension below the axle could actually ground in the right conditions.

TJ et al, MX bikes have no fork brace- the mudguard mounts onto the bottom triple clamp.

The best performing fork I ever ridden/owned was my Avalanche Dhf8, forget Fox40's they're crap, that Avy was awesome. USD design-only issue was when you crashed and 'dug' the bars in you needed to loosen the crowns and straighten the forks back up. Never any flex with its 20mm when riding.

http://www.avalanchedownhillracing.com/dhf8forka.html

RWD vs USD on motorbikes IS mostly fashion. Compare a GSXR600 SRAD with a GSXR750 SRAD, one has USD forks, one RWU. The RWUs (as long as they're the later model with the full adjustability) are ever so slightly better than the USDs. But nobody ever rode an SRAD 600 and said "Oh, these forks are too noodly"- in fact mostly if you rode the 2 back to back the only front end difference you'd notice was the crap brakes on the 750 (also fashion led)

Everyone tells you that it's for unsprung weight, but which of the 2 had less unsprung weight? Clue, it's not the 750 😉

It's no different than radial calipers and master cylinders. Road bikes end up with what's best for race bikes, not what's best for road bikes. Any quality RWU or USD fork is stiff enough.

Anyway, getting a wee bit OT there. And yes, my motorbike does have USD forks, off of a GSXR750K4, and it was a purely fashion led decision 😉

I love my Mavericks, especially as they are so out of favour, and hence picked up a brand new set including the 24mm axle hub for under £200 last year..

The inverted design has a great benefit of gravity helping to keep the crap away from the seals instead of on it, they seem plenty stiff for and aft wise where it counts but if you hold the wheel between your knees and twist the bars yes they flex… In use you dont notice any, and any for and aft flex there is comes from the crown much more than the legs themself's, this would be the same whatever design of single crown fork.

I was racing when they became the norm in MX (we still called it Scrambling then), and they were touted at the time as stiffer, having the larger tube at the top where the leverage is greatest this makes sense, but the biggest benefit was that as travel had increased so much during the suspesnion revolution, RWU forks had such a huge overhang past the leading axle to acheive the enormous travel & this could and did catch on the inside of deep ruts. USD forks dont require the overhang.

Both my pairs of maverick sc's are soooo flexy that i can't ride my bike in a straight line. NOT 😈

thisisnotaspoon – Member

A motorbike has a minimum weight limit for competition.

Not in my days of motocross and enduro riding they didn't, and AFAIK they don't now either.

Anyway, tinsy has hit the nail on the head – the big advantage of USD forks was that you could achieve long travel with sufficient slider/stanchion overlap without having the end of the sliders practically dragging on the ground.

RWD vs USD on motorbikes IS mostly fashion

That may be so. But, even you admit it's MOSTLY fashion, so there's no doubt there is some performance advantage, even if it's only a bit. And that's how technology progresses – Bit by bit, slowly. If it didn't, we'd all still be riding round on noodly things that needed an extra brace like those in the pic above. Can you get braces for modern bikes? Do they need them? I think not! 🙂

And what's wrong with fashion anyway? My Monster would have looked bloody stupid with RWU forks 🙂

http://farm4.static.flickr.com/3421/3742709337_3b454be555_b.jpg

Mainly fashion on motorcycles anyway

TJ, as usual you're talking pure bull.

For the majority of motorcycles tho its purely for fashion as I said. Hyosung 650 with USD?

Fashion and Hyosung together in the same sentence? Why not?

I remember back in the day (probably about 2000) Marzocchi announced a 1000g USD XC fork, the RAC, which IIRC (just to get another TLA in there) was 'Reversed Advanced (or Action) Composite.

Unfortunately they could never make it work properly, and the production version of the RAC was a short travel fork which ended up weighing quite a bit!

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