1. The most important thing about buying a new bike is to make sure it fits. The only way you'll know if the bike is right for you is to size up the bike and make sure that the bike's geometry matches your body's geometry. Ask questions and do some research.
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2. If possible, try to find a shop that will let you demo the bike on real dirt. Five minutes in a parking lot won't cut it. You wouldn't buy a car without a real world test drive, and a bike should be no different.
3. Don't belive the hype. Just because your favorite rider or best friend rides a certain bike, that doesn't mean that's the best one for you. Have an open mind and be realistic about your needs and ability.
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  1. #1
    Singletrackmac
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    20mm longer fork weakens frame, voids warranty?

    I read in the August issue of MBA in the 'ask MBA' section that increasing a fork from 3.9inches to 4.7 inches would "shorten the life of the frame" and "void the frames warranty".

    I could see how going form 100mm to 150mm of travel could weaken a frame, but I do not see how increasing just 20mm could have any effect on frame strength.

    The article says the frame is designed to handle 3.9 inches of travel, but the bike does not ride with 3.9", it should ride with 2.6 to 3.6" of travel with the sag set up properly. I always thought that frames were designed to handle a range of fork travel, not an exact number, since many riders ride at different sag%. I figured they have at least about a 1" to 1.25" range for fork travel and that it probably exceedes the travel of the stock fork by a little just to be safe.

    I am curious about this because I just replaced my old 100mm fork with a 120mm fork. I really do not see how this could weaken the frame if it was designed to handle up to at least 100mm of travel. If I ride with +20% sag, then that means my fork travel when riding wil be under 96mm, so how could that weaken my frame?

    As far as the voiding of my warranty, I would only send back the frame, not the components, so how would the manufacturer know how much travel my fork had? Even if I needed to include the fork for some crazy reason, I would just put the original fork back on.

    Is MBA talking out their ass on this one or is my frame now weaker and and my warranty voided?
    Get out of the gutter and onto the mountain top.

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  2. #2
    rebmem rbtm
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    Specialized only recommends a 10% increase in travel.

    100mm can be increased to 110mm but not 120mm
    120mm = voided warranty

    80mm can be increased to 90mm but not 100mm
    100mm = voided warranty
    * They'll give you an extra 2mm on a bike with a 80mm fork, you can increase to 90mm instead of 88mm

    How many fork manufacturers make a 90mm or 110mm forks ?

  3. #3
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    MBA are talking out their arse on the "weakening the frame" front but the manufacturer may fail to honour the warranty if they see a longer travel fork on there.

    Going from a 100mm fork to a 120mm fork only increases the A-C height by from say 470mm to 490mm. ie. an increase in leverage at full extension of 4%. This is a negligible increase in frame stress compared to other factors like how heavy you are and how aggressively you ride.

  4. #4
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    It may be a nominal amount but it comes down to testing, if they haven't tested it then you wouldn't expect them to offer a warranty (even if the theory says it's 'probably' O.K) and if they have tested it then there may have been an issue. For that reason they're totally within their rights to say "run whatever fork you like but if you crack the frame it's your fault".

  5. #5
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    20mm longer fork weakens frame, voids warranty?

    It really depends on the frame. Some are intended to handle a range of travel but some are not. Especially if you are looking at older bikes when 80mm was considered long travel.

    It also seems like frames sold as complete bikes are oftentimes sold with one travel option in mind, and because of testing as mentioned before, mfr warranties may not cover longer travel.

  6. #6
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    Re: 20mm longer fork weakens frame, voids warranty?

    I'm not necessarily agreeing with MBA, but when mfcrs give forth travel specs for the frame, they are definitely taking sag into account already, so, you can't say 'well my new fork sags down to the max travel spec'd for the frame'.

    Each mm of additional travel raises the front, slackens the head angle, and increases the leverage that the fork puts on the frame. When mfctrs spec a max travel, its not like there is some cutoff or breaking point where one more mm of travel and the frame will just break. When they set the max limit, its just a cost vs revenue equation: knowing that each mm increases the chance of failure by some small percent, increasing their warranty cost, but smaller travel limits simultaneously decrease the desirability of the bike, decreasing sales.

    Obviously the actual peak stress that you put on the frame will be a combination of your weight, riding style, whether you do jumps & drops, etc.

    So, in short, if you're close or just over the travel limit, but you're light, and/or you ride light and don't do a lot of jumps and drops, don't worry about it. If you're worried about voiding the warranty, keep the old fork and put it back on before you take it in for warranty claim.

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  7. #7
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    Quote Originally Posted by singletrackmack View Post
    Is MBA talking out their ass on this one or is my frame now weaker and and my warranty voided?

    Can't speak to the specific warranty you have, but your frame is fine, and Mountain Bike Action is pretty commonly referred to as Mountain Bike Fiction.

  8. #8
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    I don't read MBA but can speak from experience, had a frame that was designed for a 100mm fork, installed a 120mm fork and in withing months the head tube cracked at the lower end (close to the headset cup). The manufacturer never asked how much travel the fork had and got a replacement frame, maybe just lucky or they new about something that I didn't knew.

  9. #9
    Singletrackmac
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    Quote Originally Posted by ddprocter View Post
    I'm not necessarily agreeing with MBA, but when mfcrs give forth travel specs for the frame, they are definitely taking sag into account already, so, you can't say 'well my new fork sags down to the max travel spec'd for the frame'.
    I thought about the mfcrs taking sag into account, but how much sag? I know ideally you should have 20% to 30% sag, but I also know there are more than a few who like to ride with very little sag. I would think the mfcrs would need to consider all possible sag set ups as well as lockout when determining how much travel the frame can handle. If they didn't, and the max frame travel was based off of travel with sag, then they would be taking a bigger risk with possibly having to honor more warranties for riders who set up their bikes with little to no sag.

    If this is true, then I feel like I can say my new fork sags below the max travel spec for the frame so increasing just 20mm shouldn't weaken the frame as long as I run enough sag. I like to ride with a lot of sag, +30% so my fork travel with sag is about 84mm, well below the 100mm stock fork.

    I am pretty sure this is not an issue for my bike frame since there were 6 versions of this model all with the same 5" rear travel, but with fork travels ranging between 100mm, 120mm, 125mm and 130mm depending on the version. Plus, it was built in the good old USA. But even on bikes with one specific fork travel set for the frame and built in china or wherever most bikes are built these days, I would think that as long as the sag is set up below the recommended shock travel length, the frame shouldn't be weakened. Although, I did read the post regarding the first hand experience with longer travel and cracking the frame, but that can also happen without adding travel to the fork, so.....
    Last edited by singletrackmack; 07-30-2013 at 01:16 PM.
    Get out of the gutter and onto the mountain top.

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  10. #10
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    Quote Originally Posted by singletrackmack View Post
    I thought about the mfcrs taking sag into account, but how much sag? I know ideally you should have 20% to 30% sag, but I also know there are more than a few who like to ride with very little sag. I would think the mfcrs would need to consider all possible sag set ups as well as lockout when determining how much travel the frame can handle. If they didn't, and the max frame travel was based off of travel with sag, then they would be taking a bigger risk with possibly having to honor more warranties for riders who set up their bikes with little to no sag.

    If this is true, then I feel like I can say my new fork sags below the max travel spec for the frame so increasing just 20mm shouldn't weaken the frame as long as I run enough sag. I like to ride with a lot of sag, +30% so my fork travel with sag is about 84mm, well below the 100mm stock fork.

    I am pretty sure this is not an issue for my bike frame since there were 6 versions of this model all with the same 5" rear travel, but with fork travels ranging between 100mm, 120mm, 125mm and 130mm depending on the version. Plus, it was built in the good old USA. But even on bikes with one specific fork travel set for the frame and built in china or wherever most bikes are built these days, I would think that as long as the sag is set up below the recommended shock travel length, the frame shouldn't be weakened. Although, I did read the post regarding the first hand experience with longer travel and cracking the frame, but that can also happen without adding travel to the fork, so.....
    So why ask if you already knew the answer you wanted to hear?
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  11. #11
    Singletrackmac
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    Quote Originally Posted by TwoTone View Post
    So why ask if you already knew the answer you wanted to hear?
    Because I wanted to know if the answer I wanted to hear was right. MBA wrote something that didn't make any sense to me so I wanted to see what others who know about this stuff thought. I have to say the prospect of weakening my frame and at the same time voiding my warrenty worried me.

    I don't know how mfcrs determine the max fork travel for a frame and wanted to know if they took sag into account and if so, how much sag or what range of sag.

    I didnt mean to imply that I know what I am talking about, and that thats why I say 'I think' a lot.

    I also am not sure how diligent mfcrs are when checking to see what caused a frame to crack when trying to warranty and wanted to see what others thought about just putting the original fork back on if the frame were to crack due to a longer travel fork.
    Get out of the gutter and onto the mountain top.

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  12. #12
    Nouveau Retrogrouch SuperModerator
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    20mm longer fork weakens frame, voids warranty?

    Quote Originally Posted by singletrackmack View Post
    Because I wanted to know if the answer I wanted to hear was right. MBA wrote something that didn't make any sense to me so I wanted to see what others who know about this stuff thought. I have to say the prospect of weakening my frame and at the same time voiding my warrenty worried me.

    I don't know how mfcrs determine the max fork travel for a frame and wanted to know if they took sag into account and if so, how much sag or what range of sag.

    I didnt mean to imply that I know what I am talking about, and that thats why I say 'I think' a lot.

    I also am not sure how diligent mfcrs are when checking to see what caused a frame to crack when trying to warranty and wanted to see what others thought about just putting the original fork back on if the frame were to crack due to a longer travel fork.
    MBA prints a lot of things that do not make sense or are flat out wrong.

    A longer than intended fork does not "weaken" the frame. The frame strength is the same no matter what you use.
    It does increase the amount of stress on the frame and can lead to failure.

    The European CEN testing, that is required for a bike frame to be sold, is done with a specific fork length (no sag IIRC).

    Removing the longer fork before seeking warranty replacement is a scumbag move.
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  13. #13
    > /dev/null 2&>1
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    Re: 20mm longer fork weakens frame, voids warranty?

    I agree you're probably not adding a ton of risk with a 120 mm fork, but there is definitely some additional stress here, no matter what your sag is:

    Quote Originally Posted by singletrackmack View Post
    I would think the mfcrs would need to consider all possible sag set ups as well as lockout ...

    Again its not really a binary outcome where 120mm fork weakens the frame, 119mm doesn't (or 29% sag does, 30% doesn't). Each additional mm in axle to crown distance puts more leverage on the head tube junction, which over time, tends to increase the risk frame failure, especially if you have an AL frame where fatigue strength is a factor (didn't see whether it was al or carbon).

    I like to ride with a lot of sag, +30% so my fork travel with sag is about 84mm, well below the 100mm stock fork.
    Using that logic, a 100mm fork with typical 25% sag will sit at roughly 75mm extension, right? So, you've still added probably 10mm of axle to crown when the fork is in a resting position with no dynamic forces. But, the stresses on the fork & frame obviously peak when the bike is doing rough terrain, from rocks and ruts up to small drops up to full on jumps. During these periods, the fork will be taking a lot of hits in the fully extended position or at least in the first 25% of travel. So, during the highest dynamic stresses, the fork also tends to be in various stages of extension, which, in turn puts more leverage on the head tube junction, witch over time increases risk of failure.

    I would think that as long as the sag is set up below the recommended shock travel length, the frame shouldn't be weakened
    . . . as much



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  14. #14
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    Umm, not to interrupt, but coming from another manuf. discipline, the builder might want to see the bike in it's entirety, not just the frame, to 'evaluate' the damage on the fork and verify a legitimate damage claim. I know I would.
    The original fork had better look like it was part of the accidental frame damage or I'm guessing your warranty is void... Definitely Not Ok'ing the practice of stiffing the builder, just saying...

  15. #15
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    20mm longer fork weakens frame, voids warranty?

    Quote Originally Posted by StumpyH View Post
    Umm, not to interrupt, but coming from another manuf. discipline, the builder might want to see the bike in it's entirety, not just the frame, to 'evaluate' the damage on the fork and verify a legitimate damage claim. I know I would.
    The original fork had better look like it was part of the accidental frame damage or I'm guessing your warranty is void... Definitely Not Ok'ing the practice of stiffing the builder, just saying...
    Accident/crash damage is not covered by warranties.
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  16. #16
    Singletrackmac
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    Quote Originally Posted by shiggy View Post
    Removing the longer fork before seeking warranty replacement is a scumbag move.
    So is charging more for a bicycle than a motocross bike. No love for manufactures because they are ripping all of us off. I have absolutely no problem being a scumbag towards the manufacture who is gouging as many people as possible. As far as I am concerned it is those who ride for fun vs. corporate greed and those who profit from their greed.

    Don't agree? Then please explain why a bicycle should cost more than a motocross bike.
    Get out of the gutter and onto the mountain top.

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  17. #17
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    Quote Originally Posted by singletrackmack View Post
    Don't agree? Then please explain why a bicycle should cost more than a motocross bike.
    It's known as 'Economy of scale'.

  18. #18
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    How dare those greedy bike companies make a profit off my recreational activity by selling me products that I freely choose to buy in an open market. I bet they rake in enough money to pay their employees a fair wage, contribute to the global economy, and sponsor bike-related causes like trail building and bike advocacy. Those bastards. Clearly that justifies me being dishonest in my warrantee claim to "stick it to the Man". If everybody did that, hopefully those greedy bike companies will lose profits, lay off workers, and stop donating to IMBA.
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  19. #19
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    Quote Originally Posted by singletrackmack View Post
    So is charging more for a bicycle than a motocross bike. No love for manufactures because they are ripping all of us off. I have absolutely no problem being a scumbag towards the manufacture who is gouging as many people as possible. As far as I am concerned it is those who ride for fun vs. corporate greed and those who profit from their greed.

    Don't agree? Then please explain why a bicycle should cost more than a motocross bike.
    See this is a perfect example of the sheep mentality. Right now the cool buzz phrase is 'corporate greed' and is thrown around by people that don't have a clue.

    As pointed out above, 'Economy of scale' Here's a good start Economies of scale - Wikipedia, the free encyclopedia do some reading before making yourself look ignorant again.
    13 Lenz Lunchbox punkass

  20. #20
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    Anybody who has ever been involved in fatigue testing (lightweight aluminium components especially) will know that a small change in geometry and corresponding small increase in max load and its orientation at the pivot can substantially change the stress range seen in say the head tube from standard activities such as braking and drastically reduce the number of load cycles till the thing would have a reasonable failure probability - it is far from a linear relationship - (google S-N curves for 6061 Al - curve being the key word).
    I would imagine that 100mm to 120mm should be OK on most frames - but I wouldn't guarantee it without an FEA analysis and an accelerated fatigue test to prove it. I don't blame manufacturers for saying this has been designed and independently reviewed and destructively tested in accordance with EN14766 for configuration X - if you want to use it in configuration Y or Z for which it hasn't been designed and specifically tested then - no warranty and at your own risk.
    Not only is there economy of scale to consider with cycles - design, materials and construction has to account for the very limited power output of the human, this involves additional time and processes in manufacturing - you can't just forget 1Kg here and there to simplify it; far more important to a cyclist than motoX rider - super light motorbikes or cars also tend to be hugely expensive.

  21. #21
    Singletrackmac
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    Quote Originally Posted by slapheadmofo View Post
    It's known as 'Economy of scale'.
    Yes, thanks for bringing that up. I was only comparing the cost of motocross components to mtb components to show how much we are all being ripped off. The theory of Economy of Scale also demonstrates just how much corporate greed there is.

    Economy of scale means the more you produce the cheaper the cost. If economy of scale was taking place like it should be, then mountain bikes would be getting less expensive, not more. But these greed will not let that happen.

    Back in the early to mid 2000's a top of the line mtb went for around 3k to 4k, much less than a mid to top of the line motocross bike back then. Mtb technology has not changed much since then and there are a lot more mountain bikes being produced today than back then. Also, back then there were plenty of bikes you could buy that were manufactured in the good old USA and made by professionals. Now they are just about all made in China or Indonesia or wherever by children which further reduces costs.

    So economy of scale is just something else that shows how much price gouging is going on and therefore I have absolutely no shame in sticking it to a manufacturer. They are ripping us off, so I will do the same to them.

    Like I said, it's those who ride for fun and recreation vs corporate greed and those who profit from their greed.
    Get out of the gutter and onto the mountain top.

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  22. #22
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    $3 to $4K will still buy a pretty damn good MTB that 99% of riders will be fine with and what most 1/2 serious amateurs ride. I don't go in for this ridiculously over priced carbon bling bling wheelsets that cost the GDP of a small country - but if people are willing to pay $10+ for a bike it's entirely up to them - not the fault of the manufacturers for providing stuff that people want to buy - I don't feel ripped off in the slightest

  23. #23
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    Quote Originally Posted by singletrackmack View Post
    Don't agree? Then please explain why a bicycle should cost more than a motocross bike.
    You can buy a Devinci Wilson Carbon SL for $7500 MSRP. Pretty close to being the same bike that just won the DH world cup. Doesn't look like you can buy a motocross race replica, but when the Ducati Desmosedici RR came out it was $72,500. Sure, a bottom of the barrel motocross bike will cost less than a top of the line DH bike, but that's an apples to oranges comparison.

  24. #24
    Singletrackmac
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    I think I need to cancel my subscription to MBA. Just about every bike they review runs from 5k up to 10k and it stresses me out. I still cannot fathom how a mtb could possibly cost more than a motocross bike and I know I am not the only one who can't wrap their head around this.

    It's like a canoe that cost more than a ski boat. Sure manufactures are always trying to improve every aspect of their canoe like weight, strength and efficiency, but to end up with a final cost that is more than a ski boat would be very hard to believe. But ****, if someone thinks it's worth the price, then party on I guess.
    Get out of the gutter and onto the mountain top.

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  25. #25
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    Apart from the engine, a motocross bike has all the same parts a mountain bike does. Sure, they might be bigger, but it's not the volume of material that makes this stuff expensive, it's all the machining, etc.
    Plus, most high end bike frames are hand welded and components CNC milled. I'm willing to bet that moto bike stuff mostly isn't. As others have said, economies of scale.

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