Forks - Inverted VS Conventional

[roadwarrior]

I hope this question isn't too elementry, but here goes. What are the advantages, and disadvantages bettween conventional, and inverted forks? Why do moto cross bikes (YZ,CR,RM) thend to come with inverted forks, and enduro bikes (DRZ,XR,KDX) with conventional. I've never owned a bike with inverted forks, so I'd apprietiate your opinions.

 

[dirt bike dave]

Inverted forks tend to be more rigid, with less flex. A plus for MX and SX. On the trail, some fork flex may help the bike feel more forgiving. However, fork internals and the proper valving for the application are going to be the most important factor in performance.

 

[Jeremy Wilkey]

RW,
To add to DDB, inverted forks are not only more ridigid but allso designed to be landed front wheel first. IF the rear tuches down before the forks then the resulting stress cause severe fork harshness. In MX this is normally not an issue as all good riders land nose first. In offroad the oposite is true and landing without harshness is a major plus!
Regards,
Jer

 

[Dirteater]

New twist

Lets add a new trist to all of this. What about Conventional forks witha fork brace VS inverted. I would imagine the fork brace would make the conventionals more rigid. Anyone else?:think

 

[longtime]

Can somebody please weigh in on DirtEater's question? I'm interested, too.

 

[Jeremy Wilkey]

DE,
I really like the fork brace equiped forks. They are way more ridgid and give a more precise feel. I would strongly recomend them, as they add the feel with out the slap down issues..
Regards,
Jer

 

[longtime]

Jeremy -- the best. :)

 

[GlennP]

I jumped off a rock wall Sunday and landed flat, almost losing some upper fillings. In these situations, I miss the conventional fork plushness. Braced conventionals seem like a good setup.

Glenn
'00 GasGas XC250

 

[dirt bike dave]

Jer - thanks for your input. This thread has evolved in a way that raised some questions in my mind, such as:

For woods and trail, is there a diamater at which conventional forks are sufficiently rigid that they would not benefit from a brace? Are 46 - 50 mm forks stiff enough, while 41-43 mm flex too much?

Does the quality of the fork tube material make a difference, or are all the forks pretty similar in tube quality?

How much does bike weight effect fork flex? Can bikes with more rigid frames tolerate more fork flex and still handle well?

Thanks for any information.

 

[BrentBlain]

now that you mention that i do notice it is alot harder on my hands if i land rear wheel first but i didn't know my forks were designed that way. They feel great if i land on the front wheel but bruise my hands if i land on the rear wheel. I just thought i had the forks setup wrong.

Kali: I noticed a big difference when i switched from my XR (coventional forks) to my CR (inverted forks) on the trail. Espically on the whoops, i land with my front wheel first in the whoops and even if i miss it completely i don't feel it. I ride so much faster with my cr suspenion with out killing me. So i would think the YZ forks would be an upgrade if you are an aggressive rider.

I always thought that the inverted forks were better because of all the R&D put into them from the racing bikes. Seems like they don't change the forks much on the XR's and such.

-Brent

 

[mxneagle]

They aren't designed that way on purpose. The issue is overlap (or lack there of) of the fork bushings. On inverted forks there is only about 2" between the bushings with the forks extended. When you slap the front wheel down it wants to push the front wheel out and this binds the bushings.

 

[DanS]

My '99 KTM 300 has 50mm conventional WP's and I notice the same harshness landing rear first as the son's '01 CR250. Would this be do to less flex or just the WP fork. Also, would a brace be worth it on these forks?

 

[James Dean]

mxneagle, The conventional forks are the ones with less overlap aren't they??

Has anyone weighed the fork unsprung weight on USD forks and compared it to a conventional fork??

I had both a '89KX (45mm conventional) and '91KX (43mm USD) and felt the USD was slightly stiffer despite the smaller diameter. The '89 conventional ALWAYS seemed to be the better fork for my offroad riding, more compliant, tracked well, and bottoming resistance.

 

[Jeremy Wilkey]

The big difference with USD forks is not the bushing overlap but how the tube flexes.. If you notice modern USD forks are widest at the lower triple clmap and have a taper from top clamp and from bottom clamp. Also the chrome tube is machined at it's top.

The USD fork bends and distorts the tube at the lower clamp. To use an old anlogy pushing a pecil through a bent straw is hard..
The tubes have been getting better and better as they figure out the best combination of stree over the arc of the taper. There are many varibles but the technolgy continues to improve.

Ohlins has a really cool fork setup. They run very long upper tube with two sets of inner bushes... The fork has the standard bushing at the seal and then one about 4 inches into the tube.. This is a major help to slap down landings.. I'm not sure if it is patented or just everyone else is too cheap to do it.. BTW.. that is the only cool thing about Ohlins forks.. Other than that you might as well have a set of early 90 KYB's...

DBD...

Tube quality and size are all issues... I look at it like this.. A fork with less flex is better as it aides in control. You want good flex, you don't want bad flex. This is a function of rider ablity. (in terms of what is best) The diffrent types of flexs and strains happen with diffrent types of loads. As an industry we are still making the discoverires about what we want to bend and how. As a rule the Eruo metals seem to be of higher quality. (They can withstand higher coating temps when TI coatings are apllied..)

Regards,
Jer

 

[James Dean]

I suppose we should add that the lower triple clamp bolts have a reduced torque specified because of the stress point where the internal bushing rides. The manufacturers are aware of the problem. It's hard to tell how many bike owners will torque them properly.

Jer, Is WP the only fork maker that uses the scalloped rebound stack in their USD forks?

I have made a prototye rebound check valve (in the passive valve) that had a similar scalloping. The plate had to be indexed with a hex shape on the inner hole to hold it's position. The flow to the basevalve shims gets a more direct path and becomes more responsive without increasing port size. I have since sold the bike and the current owner certainly doesn't appreciate the design concept within his fork. Some day I may offer to help change his fork oil to see that it is still intact.

James

 

[HiG4s]

About the harshness of USD forks. I was under the impressions from reading the Euro forum that the new EXCs USD forks were quite plush? I know my old KTM 89 EXC with USDs were. Some one with a new KTM enduro with USDs want to elaborate?

 

[Jeremy Wilkey]

JD,
Yes WP is currently the only one.. (That I know of).. I've done lots of testing with the return port and found it to make no differance. I feel beacuse we are no where near flow threshold the path of the fluid seems to be a non issue.

Not to name drop but Ross and I had a long talk about this and KYB came out with the piston design on 28mm forks to prevent such an issue.. Well it turns out it made no differance and infact the factory bikes use a piston design that is more tradtional..

Whats worse is GQ came up with the whole design flaw deal..When I seriously believe the major issue was poormachining..

Starnge but true.. Not instantly logical either..

Jer

 

[johnf3]

Not to split hairs, but '89 KX's came with 46 mm forks, I believe. The only year for these, at least on my 500 they are 46mm.