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PostPosted: Fri Nov 11, 2016 5:47 am 
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I've hear and read quite a few times people say that downhill bikes can be too light. That below a certain weight they get bounced around more than heavy bikes.

I'm not sure I see the logic in that. Could someone enlighten me?

Thanks!


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Posted: Fri Nov 11, 2016 5:47 am 


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PostPosted: Fri Nov 11, 2016 6:48 am 
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I've heard conflicting things as well, for instance:

http://www.redbull.com/us/en/bike/stori ... ht-weenies

That link shows the lengths pros will go to in reducing weight, so certainly lighter is faster. But then again Sam Hill's mechanic says DH bikes can be too light (also in the link). I think what people are referring to is when you start to trade-off stiffness or reliability for weight, then you've gone too light. So as long as you can make something with the same performance that is lighter (with the only increase being cost) then I think it will make you faster.

As for comfort, I could see a heavier bike being smoother (think of a big Cadillac) but at the cost of acceleration, braking, maneuverability, and cornering.


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PostPosted: Sun Nov 20, 2016 4:55 pm 
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Maybe you are right and people are referring to trade-offs instead of total weight...

As for comfort wouldn't a different suspension setting compensate for lighter weight?


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PostPosted: Mon Nov 21, 2016 4:17 am 
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vitriol wrote:
As for comfort wouldn't a different suspension setting compensate for lighter weight?


Not fully. Consider a DH bike as having 2 main suspension components, the 'rider' and the 'bike'. As you increase the weight of the bike, it will increase the percentage of the bike/rider system, meaning its momentum will do more of the work in resisting being bounced around.


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PostPosted: Mon Nov 21, 2016 11:08 am 
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I'd say it comes down mostly to personal preference. You'll have one rider saying anything below 16kg is too light and the classical "that'll never hold up for me"-argument and then you have the other rider wanting to have it sub 15kg.

Funny thing is, and this goes for other types of bikes too, that many people who have strong opinions on light bikes and how it's a bad thing have rarely spent a lot of (or any) time on a truly light bike. Of course going from a regular heavy bike to a super lightweight one will feel a bit weird and the bike won't feel as stable at first etc, but what matters is if you get used to it and start riding better thanks to the bike being easier to move around or if you notice you're faster monstertrucking on a heavy bike.
Also, not everyone is aiming to race and be the fastest, some just rides for fun and a light bike can be just that.

Was a couple of years now since I rode downhill but I've owned three Trek Sessions (26") and the carbon one I had down to 12,65kg and the two others weighed 13,50kg and 14,64kg.
Personally I really liked having bikes that light, they where super fun to ride and you could move them around so easily.
I found that this bouncing around thing (if present) has mostly to do with rotational weight, probably since downhill tires are so heavy overall. If you ran heavy tires the bike still felt planted and if you went with lightweight tires (just below 1000g for example) the bike felt a bit more nervous.
So my favourite setup probably was a superlight bike which made it easy to move and get where you wanted it, but with "regular heavy" tires for some extra stability and momentum through for example fast rockgardens etc.

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PostPosted: Mon Nov 21, 2016 5:20 pm 
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jonasd wrote:
vitriol wrote:
As for comfort wouldn't a different suspension setting compensate for lighter weight?


Not fully. Consider a DH bike as having 2 main suspension components, the 'rider' and the 'bike'. As you increase the weight of the bike, it will increase the percentage of the bike/rider system, meaning its momentum will do more of the work in resisting being bounced around.

That would make some sense - except it doesn't, say the bike weighs 20Kg, the rider ready to go with armour etc 110Kg (downhillers tend to be beefier), the suspension is setup therefore to cope with about 125Kg (leaving 5Kg as unsprung, wheels/tryres/lower suspension - all round numbers for simplicity), how much do you think it reacts to the 15Kg sprung bike, then compare that to if the bike was say 15Kg - 10Kg unsprung, would it make much difference? No as with the inputs from just 15kg the suspension will be to all intents and purposes rigid.

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PostPosted: Mon Nov 21, 2016 7:52 pm 
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TheRookie wrote:
No as with the inputs from just 15kg the suspension will be to all intents and purposes rigid.

Easy to check that - put the o-rings down, pick the bike up and drop it, see if they've moved. I bet they will have.

The sprung weight of the bike is then acting as a mass damper between the wheels and the rider. It will have some effect, reducing a bit of chatter and vibration if nothing else.


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PostPosted: Tue Nov 22, 2016 3:21 am 
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TheRookie wrote:
jonasd wrote:
vitriol wrote:
No as with the inputs from just 15kg the suspension will be to all intents and purposes rigid.


As wingguy mentioned, the difference will really be in the small fast bumps. Imagine that 15kg bike encountering many small very quick bumps (50+ km/hr). Since they hit so fast, even the small 15kg weight will do a lot to lessen their severity (i.e., take the sting out of them).


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PostPosted: Tue Nov 22, 2016 4:06 am 
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No. I've owned DH bikes from 42 down to 30.5lbs and all were built to not compromise performamce or durability. The lighter bikes were always faster and better feeling. The one exception being wheel size. A 26" DH bike can be built lighter than an equivalent 27.5" bike. My 30.5lb* V10.5 is 26", feels great as a park/play bike, and used to be competitively fast in pro DH races. Now it's noticably slower than the heavier 27.5" V10.6 and feels harsher despite the ability to run more travel.

So, get 27.5" wheels unless smoother trails and jumping will be the main focus and then make it as light as you cancwithout compromising function.

*That's with 2.7" DHFs, a big seat, MRP G3 chainguide, and bar end protectors It could be a lot lighter with different tires, a racier seat, MRP SGx chainguide, and plastic bar end plugs.


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PostPosted: Tue Nov 22, 2016 4:15 am 
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Let me address the difference between light and heavy DH bikes over bumps. Yes, a heavy bike with feel plusher with less feedback to the rider. It might even be faster through some sections. The same could be said for a soft suspension setup. However a soft bike or heavy bike won't be faster or better feeling on most of the course and that's why top racers want a light bike with firm suspension.


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PostPosted: Fri Nov 25, 2016 1:18 am 
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Not sure if its a coincidence, but someone asked this on GMBN and they addressed it:

https://youtu.be/rD_FHvFZebc?t=3m31s


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PostPosted: Mon Jan 16, 2017 2:48 pm 
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vitriol wrote:
I've hear and read quite a few times people say that downhill bikes can be too light. That below a certain weight they get bounced around more than heavy bikes.

I'm not sure I see the logic in that. Could someone enlighten me?

Thanks!


Find a small child and push them over. Now come up to me and try the same. Unless the child you chose was obscenely obese, you will find I am much hard to push over due to my weight.

Same principal goes with bikes.

For sure I find lighter bikes tend to get kicked off line by lumpy terrain. I cannot put a figure on the threshold as surely that depends on many things such as terrain, speed, riders weight, rider's confidence etc.


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PostPosted: Mon Feb 06, 2017 6:18 am 
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I've ridden both a carbon dh bike around 30lbs and my trusty cannondale that pushes 40lbs and the difference is pretty remarkable. With the heavy bike, I feel stuck to the ground on bumps where the light one may catch air and push me off a line. I found that the heavier bike, makes for the more stable and confident ride down in my my case.


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PostPosted: Thu Feb 16, 2017 9:29 am 
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vitriol wrote:
I've hear and read quite a few times people say that downhill bikes can be too light. That below a certain weight they get bounced around more than heavy bikes.

I'm not sure I see the logic in that. Could someone enlighten me?

Thanks!



When people say a donwhill bike is too light, it usually is a part being lightened up to a point that it is not solid and as stable. XC racers to get an edge will lighten up bikes to unstable points. I raced SuperD (Donwhill without as many jumps) on a XC bike with a Sid World Cup. 4 races in the lowers had cracked. It was most likely too light for DH :) While waiting for the new fork to come in, I used my Downhill bike for the 5th race. Felt much less like I was going to die, but my time was slower.


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PostPosted: Sat Feb 18, 2017 7:53 am 
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In that case it's equating not strong enough with describing it as "too light".....which would seem a silly description!

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Posted: Sat Feb 18, 2017 7:53 am 


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