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Old 02-14-07, 01:55 PM   #1
mlh122
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Frame stress question

I have a Judy TT fork and am considering upgrading. since it's 80mm now i've read to not exceed 120mm because it will stress the frame and possibly break. I'm wondering if sag affects that?
For instance what if I got a 140mm fork with 20mm sag, or a 160mm fork with 40mm sag? since the only time the fork would use the upper 20mm or 40mm of travel is when the wheel is going down into a dip or if it's in the air, both times it wouldn't have much force applied to it. and even when landing a jump the force it would under go to compress the first 20mm or 40mm is pretty low (less than my body weight) that shouldn't affect the frame much right?
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Old 02-15-07, 07:46 PM   #2
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Since nobody responded I'll give it a go.
First of all, going from an 80mm fork to anything over 120 is going to make your bike feel ridiculous. It's going to be sluggish in turns and feel like a freakin chopper. The head tube angle is definitely not designed for that lenght fork if it came with an 80mm.
Second, chances are you will void the warranty on the frame if you put on a fork that is advertised as having over 120mm of travel, so I would not recommend it.


So I would say just don't go over 120mm. Anything more than that and the bike will probably ride like crap anyway. As far as stresses go, when you land and your fork is still at 140mm the force is not going straight up the fork, but rather straight up from the ground, so yes it will put more force on the frame that is safe before it gets a chance to go down to the 120mm point. I think... but i'm no engineer, so who knows
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Old 02-15-07, 08:15 PM   #3
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i went from the 100 or so to 140 on my hardrock and it feels spot on. i have a pike, so if i dont really like how its handling i can just change the amount of travel. but, i would think going from 80 to 140mm of travel is a bit extreme. i agree with zecanon. put it up to 120 at max
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Old 02-15-07, 08:46 PM   #4
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Quote:
Originally Posted by mlh122
I have a Judy TT fork and am considering upgrading. since it's 80mm now i've read to not exceed 120mm because it will stress the frame and possibly break. I'm wondering if sag affects that?
For instance what if I got a 140mm fork with 20mm sag, or a 160mm fork with 40mm sag? since the only time the fork would use the upper 20mm or 40mm of travel is when the wheel is going down into a dip or if it's in the air, both times it wouldn't have much force applied to it. and even when landing a jump the force it would under go to compress the first 20mm or 40mm is pretty low (less than my body weight) that shouldn't affect the frame much right?
don't do it. the forks aren't made to run that much sag. in lay terms, you will have a sloppy, wet noodle front end. depending on how sophisticated the fork design is, there can be poor dampening characteristics associated with hanging in the lower part of travel. there could also be collapsed coil spring or compressed air effects adding to an already deteriorated ride quality.

just go with a size for your frame.
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Old 02-15-07, 10:07 PM   #5
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What frame do you have?

If it came with a Judy TT, it's probably not designed to increase it's travel much. I would not recommend upgrading to any more than 100 mm of travel, unless perhaps it's a DJ frame.
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Old 02-16-07, 08:16 AM   #6
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Its a Trek 4500.. thanks for the input that makes sense. i'll either get a 100 or 120mm or i might just get a lighter spring for my Judy. right now on it's lightest setting its very very firm. which is great for road but with the warm weather around the corner i want it to be able to suck up bumps better than last year.
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Old 02-16-07, 11:23 AM   #7
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If you have an 80mm fork now and you like the way the bike handles, why not replace it with an 80mm fork? Why the need to go up to 100 or even 120? Going from 80 to 100 or even 120 is not going to give your bike or you any more ability to do anything different than you are already doing. It will just make the bike steer slower and drop to the side faster on slow turns. If you don't hate your Judy TT off road, I would recommend getting the lighter spring and be done with it. Otherwise stick with an 80mm fork like the bike was designed for.
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Old 02-16-07, 12:34 PM   #8
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all good suggestions. there are few observations...

I observed that people change forks for two reasons:
a. improve the ride quality
b. change the style of riding (from XC to DJ or DJ to XC)

So, if your intention is any of the above two, you are going in the right direction.

Some comments:
- I see people focus too much on the number when it comes to fork travel. and, dont pay much attention to fork brand (desing, mechanism) and model (features). to me the brand and model matters as much or more than the amount of travel you have. so, going from a low quality (brand/model) 80mm fork to a high quality 80mm fork will significantly improve the riding quality. Say, going from Judy TT to a Fox 80mm would definitely improve how the fork feels to you. Specially, if your intention is to change the riding quality of the bike rather than going from XC to DJ.

- Now, going from 80mm to a 100 is not a big deal. or even 120......sure it changes a few thing, but if you are not a pro or very specific about how you want your bike to feel....you are not going to notice much...

- Keeping a 20-40 mm sag is going to negatively affect how the fork and the bike will feel to you.

- why are we all so hung up on loosing warrenty for a longer travel fork. how are they going to know......you can deal with it like a dont ask, dont tell policy. I am very skeptical about bike companies warrenty policy......and how things are determined......if they showed me the mathametical proofe that a 20mm travel difference brakes their frame.....I would try to share more information about how the frame has been broken...

- if you are determined about getting a larger travel fork.......why not go with a fork with variable travel....like 85mm-100mm-130mm....or something like that.....it will allow you to adjust the required travel based on your needs...and if you dont feel like 130mm for a particular ride.....do the 85....

- finally, buy the best fork (brand/model) you can afford....after the frame....fork is the most important component that can determine your riding experience....

good luck..sorry for a long post...
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Old 02-16-07, 01:00 PM   #9
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Quote:
Originally Posted by IAMTB
Why the need to go up to 100 or even 120?
Cuz they're more betterer!
hehe just kidding. It's because my range for what i want to spend is around $200 to $400 and I'm not really after that much travel, the 80mm on my Judy has been fine all except a few occasions like a badly landed jump or something. but i did a short internet search and found theres a lot of 100 to 160mm forks in my range, and not very many 80mm forks, then as you go up in price there are a few really nice 80mm forks around $525. That could have just been the inventory on the few sites i looked at though, if i do more searching there could be a smorgasboard of $250 80mm forks i haven't seen yet.

but that's what raised the question about the frame and stress. My Judy fork does ok for what I do. But I rode my wife's Fuel with a Reba and it was quite a bit better. I don't remember the travel on her Reba but it didn't look like a whole lot more than mine. It didn't bob hardly at all, but soaked up big and small bumps just great.

I'm considering getting a lighter spring. Sram's chart is wierd though, it says the stock spring is for people around 160lbs, and i wan't a lighter one than that, so i guess the one for people less than 140lbs? which is strange because I am around 200lbs, so according to the chart i should actually have a firmer spring, but then i think the fork wouldn't move at all!
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Old 02-17-07, 09:52 AM   #10
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If it came with a judy TT... it doesn't mean much.

If warranty says up to 120mm, then 120mm is the max. Although , the 4500 is not a FR bike and increasing the travel will make it feel taller and all, and for an XC bike, that's not always good. 140mm on a hardrock is okay, beucase its frame is probably more aggressive than the 4500s. I wouldn't go over 100mm anyways, I appreciate the low down steering.
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Old 02-17-07, 11:44 PM   #11
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The Reba you rode is 100mm. I would say go with a 100mm or 120mm fork if you want something that will suck up bumps better without drastically changing the handling characteristics of your bike. When it comes to forks, you get what you pay for (most of the time) so buy the best fork your budget will allow.
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Old 02-18-07, 09:46 AM   #12
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hmm, so if I were to have a self adjusting fork like RS U-Turn or Fox Talas, and If i were to change the fork travel to the longest, it means I will stress up my frame? and I will also have a sloppy fork? Does changing your travel longer make your fork softer? I wonder that
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Old 02-18-07, 01:20 PM   #13
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yep today i found out they don't make the springs for the Judy's anymore. So if I can't find a store with one in stock I might upgrade a Judy or something similar
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Old 02-18-07, 02:36 PM   #14
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Quote:
Originally Posted by wheelhot
hmm, so if I were to have a self adjusting fork like RS U-Turn or Fox Talas, and If i were to change the fork travel to the longest... I will also have a sloppy fork? Does changing your travel longer make your fork softer? I wonder that
you wonder some excellent things. i hope that you are actually retaining some of the info from this site so that you can help others in the future.

sloppy? well, it should have more straight-line stability, be less twitchy/nervous/squirrelly (ie more straight-line stability), and adopt slower "corning" characteristics.

Quote:
Originally Posted by wheelhot
Does changing your travel longer make your fork softer?
good intuition wheelhot. it certainly can have this effect. however, in my opinion, this should not be a primary reason to lengthen travel. if it is, then i believe you are going about your suspension tuning the wrong way or you have been misinformed.

edit:

stop hanging out in Foo. you will rot your brain.
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Old 02-18-07, 04:00 PM   #15
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Quote:
Originally Posted by mlh122
Cuz they're more betterer!


but that's what raised the question about the frame and stress. My Judy fork does ok for what I do. But I rode my wife's Fuel with a Reba and it was quite a bit better. I don't remember the travel on her Reba but it didn't look like a whole lot more than mine. It didn't bob hardly at all, but soaked up big and small bumps just great.

I'm considering getting a lighter spring. Sram's chart is wierd though, it says the stock spring is for people around 160lbs, and i wan't a lighter one than that, so i guess the one for people less than 140lbs? which is strange because I am around 200lbs, so according to the chart i should actually have a firmer spring, but then i think the fork wouldn't move at all!
The Judy forks are not the best around but I have a friend that that uses one, weighs 220lbs and his Judy bobs a lot aswell. I loaned him a 135 Marzochi Freeride with the stiffest springs fitted- oil changed to 20 weight and overfilled with oil to stiffen it up more. He gets full 135mm travel and it bobs a lot. Checked his bike again and he is a big lad with a 23" frame- the biggest he could get and a long stem fitted. It is about 130mm. I changed the stem to a 100mm and no bobbing. It was not the fork causing the problem but the leverage of his weight on a long stem.

Check the wifes bike and try fitting her stem to your bike if it is shorter. Then try fitting the reba to your bike to see if it cures the problem. Then give up and go and buy a new quality fork, but get in touch with the fork manufacturer to see what they recommend.


Edit----Reason I looked at the stem was I test rode the bike- I weigh 150lbs and I had the fork bobbing. Incidentally the Freeride Fork I loaned was set up for a 400lb team on a Tandem and it worked fine.
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Old 02-19-07, 12:01 PM   #16
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yep today i found out they don't make the springs for the Judy's anymore. So if I can't find a store with one in stock I might upgrade a Judy or something similar
I meant upgrade to a Reba...

Today i found the preload knob on my Judy TT was jammed up with dirt, when i cleaned it out i got to turn it several turns softer. its noticeably softer now but still pretty firm. i'm gonna ride it for a bit and see how it goes.

also i noticed some tiny (pinpoint sized) rust spots on the stanchions... I'm gonna try to polish those out and hope they go away.

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