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I am into downhill (DH) but I don't live 1) in a country with a lot of DH parks with lifting services etc 2) near such parks. Still I like riding rough stuff (in forests or in town) so that I would finally opt for an enduro bike.

Concretely: I initially planned to buy a DH bike, the Commencal Supreme DH 29 signature 2019 and finally I will end up buying an enduro bike, the Commencal meta AM 29 Signature 2019.

But, I would like to replace the fork of the enduro bike with the one from the DH bike.

Is this a good idea, as it would affect the geometry of the bike?

How would this specifically affect the bike? Would it affect pedaling a lot so that at the end I wouldn't be able to use it for enduro (flat + uphill pedaling) as efficiently? What would be other effects?

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    One clear difference, a dual crown DH fork can't turn more than ~80 degrees left and right. A normal single crown fork can turn up to 180 degrees limited by cables. This affects very low speed manoevering. – Criggie♦ Nov 26 '18 at 19:11
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Let us assume that the wheel size stays the same at 29". Let us also assume that a DH fork can be installed on that frame with no clearance or mounting issues. What remains are questions of frame geometry and safety.

How would this specifically affect the bike?

I would say that the main parameter that changes is fork's axle to crown distance. DH forks have more travel than enduro: let's say 200 mm against 170 mm that the bike you link to has. So the top of the bike gets raised up for 30 mm. Each 10 mm of change corresponds roughly to 0,5° decrease of head tube angle. So that makes it 1,5° or more. It means the whole front end would raise up; if you stand on a perfectly horizontal surface, with a new fork it would feel as if you ride 1,5° uphill. Needless to say such a change won't help you climbing up hills, but exactly the opposite.

1,5° is a lot of a change for head tube angle, which becomes 64°. Most likely you will find it harder to make turns or to control your bike in turns.

Bottom bracket would be raised as well, possibly making the bike unstable in controlling, see this answer. Less pedal strikes, however.

What would be other effects?

It would weigh more, and it would not be covered by vendor's guarantee any longer because using a longer travel fork certainly voids it. Remember that an enduro bike frame and remaining components, such as handlebar and wheels, were designed, tested and certified for certain types of drop heights. Downhill riding assumes harsher loads; you would not want your handlebar to snap.

Is this a good idea?

No it is not.

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  • Well that conforts my subconscient opinion :) One question : I'm obviously not into doing Rampage or Hardline, nor Fort William, but is there a real visible difference on what such a DH bike could take that the enduro one couldn't ? Visible for someone like me doing urban stairs, jumps etc, not being a pro etc' – Olorin Nov 26 '18 at 19:02
  • The statement "..with a new fork it would feel as if you ride 1,5° uphill. Needless to say such a change won't help you climbing up hills, but exactly the opposite" is highly questionable. Gravity laws are not affected by steering angles! In my view, as long as you ride straight ahead, the effect of raising the fork crown is roughly the same as raising the handlebar. In curves the change in steering angle will be noticed, for bettter or worse. I replaced a rigid fork in my travel bike by a suspension fork with 3 cm higher crown, and got quickly used to it. A DH-bike is surely more critical. – mathieu van rijswick Nov 26 '18 at 19:14
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    I would add cost - for the same money I would almost always prefer a better bike than an bike+upgraded fork. – mattnz Nov 26 '18 at 19:33
  • @mathieuvanrijswick I certainly agree with you. My intention was to convey that with such a setup one is 1,5° closer to tumbling on one's back, so to speak. Of course, then one can fix that with adjusting a saddle for the same 1,5° degree. But the rest of the bike will still be "rotated" back. As for replacing a rigid fork with a suspended one — it is kind of expected that the suspension new fork has longer axle-to-crown to accommodate for the initial sag and some of travel that shortens it. – Grigory Rechistov Nov 26 '18 at 19:40
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    Its possible to get headsets that correct head angle up to 1.5degrees. – mattnz Nov 26 '18 at 21:04
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The main problem that stands out to me is - as strong as Commencal's bikes are - when you slacken the head angle and add more weight to the front end, especially simultaneously, you are putting a ton of stress on your head tube which is likely designed around a specific travel fork. As you could imagine there's already a ton of stress on the head tube as it is and despite their bikes being build like tanks that's a lot more stress so maybe if you ride delicately?

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    There's an excellent implicit point here, but to state it clearly "doing this fork swap would null and void any and all warranty, even if you reverted later." – Criggie♦ Apr 1 at 7:26
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You can get a Rockshox Boxxer 29" DH fork in 180 mm length, that would do the trick.

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    Hi, welcome to bicycles. This doesn't really answer the question, since there's no information about how this would change the geometry, handling, efficiency, etc. – DavidW Nov 29 '19 at 15:29
  • The question is specifically asking for consequences of this fork swap. What would be different when comparing the before with the after? Successful fitting is only part of the problem. – Criggie♦ Nov 30 '19 at 5:06

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