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No, that's not phonetic
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Discussion Starter · #1 ·
Your distress at hearing speculations of 140mm axle to crown forks on 5-Spots (not sure anyone actually suggested that... but anyway) resurrected an oft repeated debate here: what drives a manufacturer to specify or recommend a certain A-C aside from handling concerns. In other words, forgetting the chopper/floppy-steering ramifications for a moment and simply focusing on stresses transmitted to the frame, which of the following considerations is the basis for an A-C recommendation:

(A) A longer A-C acts as a longer lever. The longer lever arm can do more mechanical work to the heat tube area than it was designed to withstand.

(B) A longer A-C encourages a rider to push the speed/technical riding envelope beyond what the rest of the bike was designed to withstand.

(C) A combination of both A and B.

(D) Something I have not thought of.

Argument A seems sort of silly on the face of it. If you were to increase the A-C from 500mm to 540mm, you have increased the leverage arm by 8%, which is nothing. BikeZilla puts 18% more stress on his heat tube than I do on mine while running exactly the same fork simply due to body weight (i.e. the mass acting on the lever, give or take a donut or two). Does that mean I can run a 580mm fork on my spot assuming BZ is within limits on a 518mm Z1, purely from an engineering point of view?

Argument B makes a bit more sense, and is the one invoked by SC for the Blur's 100mm A-C limit, I believe. The idea is that the fork's travel naturally limits what a rider is capable of doing with a bike. The lack of travel "holds the rider back" and thus the blur with the 100mm Float won't be launched off the gymnasium roof while the blur with the Super Monster would. It is not the leverage on head tube per se which is the problem, it is the impact on the parking lot tarmac suffered by the rest of the frame. The fork is not the problem, the fork-inspired use is the problem.

To confuse things, it is often stated that a stiffer fork (such as a dual crown) transmits more stress to the frame. This also seems like a misleading (if tangential) argument since the stiffness of the fork simply determines how much the fork deflects under a given load, NOT how much force the end of the lever experiences. A fork which is twice as stiff deflects half as much, but the amount of force felt at the head tube is the same in either case. In fact, a stiffer fork may experience less binding and thus absorb energy better than a noodly fork. There are acceleration effects here, I'm sure, but I am ignoring those just now.

It is hard to get a definitive answer on all of this because it seems that ambiguity breeds conformity. This question is totally hypothetical and I am not looking for an excuse for anyone to do anything untoward, it just seems we have a unique opportunity to pick the brain of a real live bike manufacturer and engineer who has probably looked at and thought about this from every angle. Your candor here so far as been a real breath of fresh air and your involvement with the board a credit to your company.

I should point out that I am NOT an engineer. Feel free to simply point out that I'm talking out my butt...
 

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Interesting question Tscheezy

My Banzai has an added plate at the head tube and the rep told me they tested it so that it could take the abuse of a double crown fork. So the front end of my bike can take a double crown, however, I'm not sure the rear linkage could take the abuse a rider with a double crown fork will want to use it for! So, I assume your B point is mostly the reason.

I wish ALL frame manufacturers would tell us what fork the frame is "approved" to take and ALL fork manufacturers would put in their spec sheets the AtoC measure... After that, if a person wants to deviate from it, they are on their own.

Anyway, I like this thread and hope other people contribute to the discussion...

Cheers.

tscheezy said:
Your distress at hearing speculations of 140mm axle to crown forks on 5-Spots (not sure anyone actually suggested that... but anyway) resurrected an oft repeated debate here: what drives a manufacturer to specify or recommend a certain A-C aside from handling concerns. In other words, forgetting the chopper/floppy-steering ramifications for a moment and simply focusing on stresses transmitted to the frame, which of the following considerations is the basis for an A-C recommendation:

(A) A longer A-C acts as a longer lever. The longer lever arm can do more mechanical work to the heat tube area than it was designed to withstand.

(B) A longer A-C encourages a rider to push the speed/technical riding envelope beyond what the rest of the bike was designed to withstand.

(C) A combination of both A and B.

(D) Something I have not thought of.

Argument A seems sort of silly on the face of it. If you were to increase the A-C from 500mm to 540mm, you have increased the leverage arm by 8%, which is nothing. BikeZilla puts 18% more stress on his heat tube than I do on mine while running exactly the same fork simply due to body weight (i.e. the mass acting on the lever, give or take a donut or two). Does that mean I can run a 580mm fork on my spot assuming BZ is within limits on a 518mm Z1, purely from an engineering point of view?

Argument B makes a bit more sense, and is the one invoked by SC for the Blur's 100mm A-C limit, I believe. The idea is that the fork's travel naturally limits what a rider is capable of doing with a bike. The lack of travel "holds the rider back" and thus the blur with the 100mm Float won't be launched off the gymnasium roof while the blur with the Super Monster would. It is not the leverage on head tube per se which is the problem, it is the impact on the parking lot tarmac suffered by the rest of the frame. The fork is not the problem, the fork-inspired use is the problem.

To confuse things, it is often stated that a stiffer fork (such as a dual crown) transmits more stress to the frame. This also seems like a misleading (if tangential) argument since the stiffness of the fork simply determines how much the fork deflects under a given load, NOT how much force the end of the lever experiences. A fork which is twice as stiff deflects half as much, but the amount of force felt at the head tube is the same in either case. In fact, a stiffer fork may experience less binding and thus absorb energy better than a noodly fork. There are acceleration effects here, I'm sure, but I am ignoring those just now.

It is hard to get a definitive answer on all of this because it seems that ambiguity breeds conformity. This question is totally hypothetical and I am not looking for an excuse for anyone to do anything untoward, it just seems we have a unique opportunity to pick the brain of a real live bike manufacturer and engineer who has probably looked at and thought about this from every angle. Your candor here so far as been a real breath of fresh air and your involvement with the board a credit to your company.

I should point out that I am NOT an engineer. Feel free to simply point out that I'm talking out my butt...
 

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Waiting to exhale.
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SHHEEEESH!!!!
I can't believe not a single freaking engineer replied to this question!!!!!!!!!!
WHAT THE DEAL-E-O ! ! !
aLL i HEAR in this thread are cricketts. :mad:
 

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not so super...
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Argument B makes a bit more sense, and is the one invoked by SC for the Blur's 100mm A-C limit, I believe. The idea is that the fork's travel naturally limits what a rider is capable of doing with a bike. The lack of travel "holds the rider back" and thus the blur with the 100mm Float won't be launched off the gymnasium roof while the blur with the Super Monster would. It is not the leverage on head tube per se which is the problem, it is the impact on the parking lot tarmac suffered by the rest of the frame. The fork is not the problem, the fork-inspired use is the problem.

Tscheezy - I had to laugh when I read this statement. There is a Kid I see riding a local trail that hits stuff on his Blur that I wouldn't even consider doing on a DHR. IMO...A truely experienced rider can almost always do more with less. I understand that SC was targeting the majority of the riders and not the "gifted" few.

Marzocchi uses this same reasoning for not using 20mm thru-axles on the AM series of forks.
 

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The Ancient One
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SSINGA said:
Argument B makes a bit more sense, and is the one invoked by SC for the Blur's 100mm A-C limit, I believe. The idea is that the fork's travel naturally limits what a rider is capable of doing with a bike. The lack of travel "holds the rider back" and thus the blur with the 100mm Float won't be launched off the gymnasium roof while the blur with the Super Monster would. It is not the leverage on head tube per se which is the problem, it is the impact on the parking lot tarmac suffered by the rest of the frame. The fork is not the problem, the fork-inspired use is the problem.

Tscheezy - I had to laugh when I read this statement. There is a Kid I see riding a local trail that hits stuff on his Blur that I wouldn't even consider doing on a DHR. IMO...A truely experienced rider can almost always do more with less. I understand that SC was targeting the majority of the riders and not the "gifted" few.

Marzocchi uses this same reasoning for not using 20mm thru-axles on the AM series of forks.
In one of Tony E's replies on this board a long time ago he implied the same thing you are saying. I can't remember the context enough to do a search for it.
 

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Lay off the Levers
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SSINGA said:
...Marzocchi uses this same reasoning for not using 20mm thru-axles on the AM series of forks.
And yet their decision not to make the Z1 travel adjustable defies all logic :p .
 

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No, that's not phonetic
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Discussion Starter · #7 ·
It basically has to be a "use-inspired" problem, because the "extra leverage due to a longer A-C" theory just does not stand up.

That is unless I'm missing something subtle but important here.
 

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Baked Alaskan
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Well I'm no engineer, I have a journalism degree which is about as far away as you can get this side of philosophy, but the longer A-C argument makes no sense to me either. And I'm talking about running a Z1 as opposed to the Fox, not an 888 on a Sid equipped bike. You take the "fat guys with heavy bikes" club Smokey started and every time they're on the bike there's more stress being put on it than the 150 pounders. That would be under identical riding conditions and so forth.

And SC's Blur can only run a 100-whatever fork to me is a little overkill. I have a friend that's been running a 125 Vanilla for a couple years and he loves it. I've also seen one with the Mav DUC on the trail, same thing, the dude loves it and no problems.

I would say your use inspired thing is all that makes sense too. One of the cats I ride with is only about 155# on a good day and he usually rides a K2 Razorback, 3x3" all air and not exactly what I would call plush. I can't keep up with him no matter what. Part is fitness, part is he's towing 50# less up hills and part is he is really smooth, he just floats over everything. Funny thing is he's an ex motocrosser and a DH monster, and he's always catching stupid amounts of air on his tiny little 3x3. But he's so smooth. Now I know he's riding WAY beyond the limits of an XC race bike, yet its 4-5 years old and still getting abused.

Crap, I just got a phone call and I don't remember what my point was now. I'll finish this if it hits me....
 

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carpe mañana
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Maybe it IS stricktly handling concerns? When I first got my Burner I called up Casey and asked him what he though about running a 120mm Marathon on the bike. Wheather there were any concerns due to frame integrity, etc. His reply was very short. He said that I should go for it, since I don' t know what the bike handles like with a 100mm fork and I won't miss the handling. So I am guessing that bikes like Blur, Spot, etc, which are advertised to offer a certain characteristing when ridden only will exhibit that characteristic when setup a certain way, the recommended way.

_MK
 

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No, that's not phonetic
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Discussion Starter · #10 ·
That does not explain why certain manufacturers will void the warranty if the bike is ridden with a fork over a given amount of travel. Note that they NEVER cite A-C as limiting factors, just travel.

DT also became noticeably concerned when someone asked about putting a 6" travel zoke or Fox 36 on. He may have been worried about handling, but I think his remarks may have mentioned "that poor frame" or something.
 

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carpe mañana
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tscheezy said:
That does not explain why certain manufacturers will void the warranty if the bike is ridden with a fork over a given amount of travel.
that's a good point.
_MK
 

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Engineer Weighing In

Danger, danger, engineer weighing in!

Late to the thread, but I think you guys have it figured out anyway.

It's not the AC dimension and it's related leverage issues, it's the use aspect.

The leverage issue might be arguable between 4 and 6 inch travel forks, but not very defensible between 5 and 6.

JMO
 

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Exactly!!!

AK Chris said:
Well I'm no engineer, I have a journalism degree which is about as far away as you can get this side of philosophy, but the longer A-C argument makes no sense to me either. And I'm talking about running a Z1 as opposed to the Fox, not an 888 on a Sid equipped bike. You take the "fat guys with heavy bikes" club Smokey started and every time they're on the bike there's more stress being put on it than the 150 pounders. That would be under identical riding conditions and so forth.

And SC's Blur can only run a 100-whatever fork to me is a little overkill. I have a friend that's been running a 125 Vanilla for a couple years and he loves it. I've also seen one with the Mav DUC on the trail, same thing, the dude loves it and no problems.

I would say your use inspired thing is all that makes sense too. One of the cats I ride with is only about 155# on a good day and he usually rides a K2 Razorback, 3x3" all air and not exactly what I would call plush. I can't keep up with him no matter what. Part is fitness, part is he's towing 50# less up hills and part is he is really smooth, he just floats over everything. Funny thing is he's an ex motocrosser and a DH monster, and he's always catching stupid amounts of air on his tiny little 3x3. But he's so smooth. Now I know he's riding WAY beyond the limits of an XC race bike, yet its 4-5 years old and still getting abused.

Crap, I just got a phone call and I don't remember what my point was now. I'll finish this if it hits me....
We have a friend who rides with us, DH, Free-ride, and XC, all on a Giant NRS-2 from a few years back. That poor bike has been abused and tortured, but the guy just keeps riding it, kicking all our behinds on the xc stuff, and doing darned well on the FR/DH stuff. Granted, he's a bit more, how should we say, "apt" to take a large header on steep jumps, largish drops, and dirt-jumps, than those of us on 7" travel, 67 degree head angle bikes, but he does them, and the bike isn't dead......yet. I shudder to think of what he will be like on a bigger travel bike, that we are trying to get him on for this summer.

The thing you can always count on with guys like that is a resistance to go with a heavier bike, or one with more travel because they are seriously afraid they won't get to the top first anymore. I saw him walk away from the most horrendous crash I've ever witnessed this summer at Mont Ste. Anne. I pulled over to take a look from the other side, of a wooden ramp (you probably have seen them in the videos we have), but this particular one is quite a bit steeper, and at the end of a longer ski run, thereby generating more speed. I had just enough time to get off my bike, turn around and start to walk back up the hill when he hit that ramp so fast, that little xc bike just pitched forward and disappeared right under him, leaving him, oh about 12-13' in the air, travelling at a good clip, no bike to speak of. He waved his arms and legs a bit, rolled in mid air, catching the brunt of the impact on his camel back, sliding down the trail a good 25', and just kind of lied there in a twisted up mass. I thought he was knocked out but within about 5 seconds, he kind of rolled out of the trail, got up, brushed himself off and rode for the rest of the day. Still, on the same bike......Tough SOB.
 

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not so super...
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esquire said:
We have a friend who rides with us, DH, Free-ride, and XC, all on a Giant NRS-2 from a few years back.
One of the guys I ride with had a trek 8500 complete w/ the SID and 18 spoke wheels. We went to a local ski area for asome lift assisted fun and he ate it big time. The SID was soon replaced with a Z1 and for a little while it had a Jr.T. I'm sure that is not what Trek had in mind for that frame.
 

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tscheezy said:
That does not explain why certain manufacturers will void the warranty if the bike is ridden with a fork over a given amount of travel. Note that they NEVER cite A-C as limiting factors, just travel.

DT also became noticeably concerned when someone asked about putting a 6" travel zoke or Fox 36 on. He may have been worried about handling, but I think his remarks may have mentioned "that poor frame" or something.
Perhaps it's a combination of the 2. If you have a longer travel fork, you're going to go faster. If you do something catastrophic with the longer fork (like run into an immovable object), the forces on the frame will be that much worse than the one the frame is designed for. If proper engineering practice is followed when designing a frame, the safety margin should be large enough to prevent a frame failure (e.g. putting gussets at the head tube-down tube joint), or the mfr sets a maximum limit based on analysis and testing. But I'm sure some manufacturers are better than others at doing "real" engineering.

Another question: If you have a longer A-C length, does that change the force vectors acting on the head tube in a direction that is outside the frame's design parameters? This would mean that two riders could put the same amount of force into the head tube, but the one with the longer A-C fork would be applying that force into a different direction.
 

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I have to chime in and agree with just about everything in this post. I believe that most of the concerns come from poor handling (having a 64 degree head angle on a 3" XC bike wouldn't make the racer boy happy and therefore upset that his bike was holding him back) , limiting the application of use (there's that strapping on false confidence just because your bike has a 12" travel Super Monster), and the fact that the initial warnings were issued years ago when dual crown forks were just being introduced and headtubes weren't reinforced like they are today. Let's face it, there will always be someone wanting the one bike to "do it all" and another person saying that "to do that would mean riding a bike that's too heavy". Trying to compromise with a lightweight frame and a massive fork will just make in unbalanced and perform poorly.

Now the engineering theory. I don't know that it will greattly affect the headtube, but here's my thoughts. This is a dramatic example to show the effects of the relatively small changes that will occur. With a steeper headtube angle, the force applied to the headtube from the fork hitting an obstacle is more vertical. Yes, I realize that it's not completely vertical because of the 71 degree headangles that I'll use as the arbitrary number for XC headtubes. However, the force will be applied more linearly straight up through the fork and against the headtube which is more perpindicular to the ground. If you were to take the same frame and insert a fork that slackened the headtube a lot (this argument is for putting a massive fork on but to make the visual more clear) to say 64 degrees, the headtube "lays down" more and becomes more horizontal. When the front of the bike hits an obstacle the headtube would not be quite as "in line" with the force, rather it would have to take the vertical force that was at the end of the lever (the axle of the wheels). The force would be "off line" with the force and eventually damage the headtube (longer fork legs mean more leverage). Here's another way to think of what I'm trying to say (may not be coming out right). If you take a straw and hold it straight up and down on a table, you can press down on it with some degree of force before it crumples. This is because the fork (straw) is "in line" with your hand (force). If you were to take the straw and angle it, the fork will crumple under almost no force because it is "out of line" with the force you are applying. With a steeper headtube, you can use a thinner walled headtube because the forces will be more "in line". Using a massive fork in that thin headtube can eventually result in damage.

Back to reality. The major leverage differences between a 5" and 6" travel fork are probably minimal (although they do exist). Other companies are probably just protecting themselves. If you look at Turner's bikes, anything that's got 5" of travel or more are gussetted at the headtube and ready for big forks and lots of abuse. Therefore, after all of this typing, it probably boils down to how the frame will handle. After all, Casey did say "go for it" when a previous writer iniquired about putting a longer fork on since he wouldn't know how the bike handled with less travel.

Bryan
 

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No, that's not phonetic
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Discussion Starter · #17 ·
Squishing the head tube is not the issue. Pulling the head tube off the rest of the frame is the problem. Your lower head angle scenario would actually reduce stress on the head tube from frontal impacts (running into objects while rolling down the trail) but increase stress from vertical impacts (landing drops). The more in-line the force is with the telescoping direction of the fork, the more the fork's compression helps absorb the energy and the less binding (bushings) occurs. A force perpendicular to the fork's telescopic direction will not be absorbed at all. All of that force gets fed into the frame.

Optimally, the fork would always be pointed directly toward the force being applied to it.
 

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I agree, that's why the gusset on the 6 pack is only on the bottom. It's because they're trying to protect the headtube from being ripped off from the downtube from a large drop.
 

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Lay off the Levers
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SSINGA said:
One of the guys I ride with had a trek 8500 complete w/ the SID and 18 spoke wheels. We went to a local ski area for asome lift assisted fun and he ate it big time. The SID was soon replaced with a Z1 and for a little while it had a Jr.T. I'm sure that is not what Trek had in mind for that frame.
He should have kept the SID. That way he could have a detachable fork like this guy...
Note the excellent aftermarket organic red paint on the rim.



...no I don't know what happend. Just keep this in mind when you're bolting that Monster-T on your Flux, or trying to build a 23lb 6 Pack.
 

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A/C Dimensions

Hey, do you guys think that maybe the reason that DT expressed some concern over too long a fork on a Spot is just because he has been forced to talk to TE too much in the recent past?!?!
 
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