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Old 10-22-2012, 07:04 PM   #21
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“ Quote:
Originally Posted by ZXWUT? View Post
What does it matter what you're falling off of if the speed is the same?
Easy with that logic there pumpkin... This is the Internet, not a higher education discussion.

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Old 10-22-2012, 07:19 PM   #22
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I'd rock it. My brother does downhill mountain bike racing and some of his helmets have taken some hard hits.

If the choice is between no helmet and a mtb one, it is obvious, but if it was between a mtb and moto helmet, I'd probably choose the moto helmet.

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Old 10-22-2012, 07:25 PM   #23
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“ Quote:
Originally Posted by Grantizzle View Post
Easy with that logic there pumpkin... This is the Internet, not a higher education discussion.
My bad.

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Old 10-22-2012, 07:47 PM   #24
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“ Quote:
Originally Posted by Ashex View Post
Bicycle helmets aren't rated for crashes at the speed you would be going on a dirt bike, don't expect it to hold up if you hit something hard like a rock or tree.
How fast do you go when doing single track on your Maxim or your Versys?

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Old 10-23-2012, 01:55 AM   #25
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“ Quote:
Originally Posted by ZXWUT? View Post
What does it matter what you're falling off of if the speed is the same?
Let's look at it because I am curious. If Force= mass x acceleration, then we can see what the difference in force is on the helmets while riding at the same speed. As long as the downhill helmet is rated to handle the force you should be fine. The problem is the mass of a rider+bike is quite a bit different when comparing a motor bike and a pedal bike.

Step 1 of 3
Determine how much mass the crashed object contains. There are 2.2 pounds for every kilogram (kg) of mass, so:
Mass of motor bike = 250 pounds / 2.2 kg/pound = 113.6 kg.
Mass of pedal bike = 30 pounds / 2.2kg/pound=13.6kg

Add the rider (200lb) to both masses giving us:
Motor bike= 113.6 + 90.9= 204.5kg
Pedal bike = 13.6 + 90.9= 104.5kg

Step 2 of 3
Determine the acceleration, or deceleration, involved in the crash. Imagine that the bikes are both traveling 11.175 meters per second (m/s)--roughly 25 miles per hour--when they hit a tree coming to a complete stop in 0.05 seconds--5 hundredths of a second. To calculate the acceleration, simply divide the change in speed by the time it took to change. Acceleration of the car = (0 m/s - 11.175m/s) / 0.05 s = -223.5 m/s^2 Note: the negative sign on the acceleration indicates that it was deceleration that occurred, and is not important when calculating the net force involved.

Step 3 of 3
Use Newton's Second Law to calculate the net force involved in the crash. Force = mass x acceleration
Motor bike = 204.5kg x 223.5m/s^2 = 45,705.75 Newtons (N)
Pedal bike= 104.5kg x 223.5m/s^2 = 23,355.75 Newtons (N)

All of that math means that the two different style of bikes have a pretty big difference in force applied during crashes, due to their difference in mass. The motorbike's force is 1.95 times greater than the force of the pedal bike.

I think I'll keep my moto helmet, but to each his (or her) own....

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Old 10-23-2012, 05:45 AM   #26
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“ Quote:
Originally Posted by BobbyBlackFinger View Post
All of that math means that the two different style of bikes have a pretty big difference in force applied during crashes, due to their difference in mass...
But the rider's mass is constant.



KevinD

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Old 10-23-2012, 05:49 AM   #27
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“ Quote:
Originally Posted by ZXWUT? View Post
What does it matter what you're falling off of if the speed is the same?
If you get whack by what you are falling off, it could.
There are also other dynamic loads that can come into play when you have a motor to accerate you into objects.

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Old 10-23-2012, 05:53 AM   #28
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“ Quote:
Originally Posted by KevinD View Post
But the rider's mass is constant.



KevinD
, the bike's weight doesn't matter unless you are bolted to it.

I took a tree to the forehead once on a snowmobile, and the sled kept going. It could have weighed a million pounds. Now if you turn completed up side down and land exactly on top of your head while hanging onto the bike, then yes you'll have greater forces involved.

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Old 10-23-2012, 05:55 AM   #29
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“ Quote:
Originally Posted by tod701 View Post
If you get whack by what you are falling off, it could.
There are also other dynamic loads that can come into play when you have a motor to accerate you into objects.
like what?

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Old 10-23-2012, 07:40 AM   #30
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“ Quote:
Originally Posted by RC51 View Post

I took a tree to the forehead once on a snowmobile.

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Old 10-23-2012, 07:44 AM   #31
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“ Quote:
Originally Posted by Texasl View Post
That's about what my eyes did.

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Old 10-23-2012, 07:53 AM   #32
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“ Quote:
Originally Posted by KevinD View Post
But the rider's mass is constant.



KevinD
True, but unless you fly off of the bike completely and hit a tree by yourself you need to use combined mass of the rider and bike.
The drivers mass is constant in a car crash too... I know there are tons of variables and that my equation only applies to a few of the possibilities in which one could crash. I just wanted to look at it for fun.

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Last edited by BobbyBlackFinger; 10-23-2012 at 08:25 AM..
 
Old 10-23-2012, 08:23 AM   #33
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I really don't give a rat's ass what anyone wears when on a scoot street or dirt to tell you the truth, hell look at some of the gnarly bmx guys doing crazy stuff with no lids.

But myself, I like a helmet that's gonna be best for not scramblin' the eggs upstairs if I whack my head. Boots, and helmets, two spots I don't skimp in the dirt!!

How many that wear bicycle helmets for trail riding also wear pressure suits or body armor?

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Old 10-23-2012, 08:51 AM   #34
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“ Quote:
Originally Posted by BobbyBlackFinger View Post
True, but unless you fly off of the bike completely and hit a tree by yourself you need to use combined mass of the rider and bike.
The drivers mass is constant in a car crash too... I know there are tons of variables and that my equation only applies to a few of the possibilities in which one could crash. I just wanted to look at it for fun.
Except in a car crash, the driver keeps moving and the vehicle is stopped. So when the driver hits the seatbelts and airbags, it is the driver's mass and driver's acceleration from 60mph to a stop that creates the force during impact.

The car is merely a vehicle (bad pun) to get them to a certain velocity before the impact. When the branch hits your head on a bike, your body (arms, legs, neck, back etc.) acts as a spring which spreads the acceleration over a longer period of time, creating less force since you and the motorcycle are not one rigid item. Of course if you have a deathgrip on the bars, and never let go, a hit on a motorcycle is going to be harder than bicycle. But who does that in reality when faced with a tree branch that will take them to a dead stop?

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Old 10-23-2012, 09:54 AM   #35
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You are right. The seatbelt stops the driver's mass only in a car crash. There is a certain scenario where you could hit a tree on your dirt bike with your noggin acting as the point if the pin so to speak. That's the type of crash I am referring to. While this may be a worst case scenario, that's what I want to be protected from. Just in case.

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Old 10-23-2012, 10:00 AM   #36
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Joined: Sep 2007
From: Kent, WA

I Ride: 05 KTM 625 SMC
“ Quote:
Originally Posted by BobbyBlackFinger View Post
Let's look at it because I am curious. If Force= mass x acceleration, then we can see what the difference in force is on the helmets while riding at the same speed. As long as the downhill helmet is rated to handle the force you should be fine. The problem is the mass of a rider+bike is quite a bit different when comparing a motor bike and a pedal bike.

Step 1 of 3
Determine how much mass the crashed object contains. There are 2.2 pounds for every kilogram (kg) of mass, so:
Mass of motor bike = 250 pounds / 2.2 kg/pound = 113.6 kg.
Mass of pedal bike = 30 pounds / 2.2kg/pound=13.6kg

Add the rider (200lb) to both masses giving us:
Motor bike= 113.6 + 90.9= 204.5kg
Pedal bike = 13.6 + 90.9= 104.5kg

Step 2 of 3
Determine the acceleration, or deceleration, involved in the crash. Imagine that the bikes are both traveling 11.175 meters per second (m/s)--roughly 25 miles per hour--when they hit a tree coming to a complete stop in 0.05 seconds--5 hundredths of a second. To calculate the acceleration, simply divide the change in speed by the time it took to change. Acceleration of the car = (0 m/s - 11.175m/s) / 0.05 s = -223.5 m/s^2 Note: the negative sign on the acceleration indicates that it was deceleration that occurred, and is not important when calculating the net force involved.

Step 3 of 3
Use Newton's Second Law to calculate the net force involved in the crash. Force = mass x acceleration
Motor bike = 204.5kg x 223.5m/s^2 = 45,705.75 Newtons (N)
Pedal bike= 104.5kg x 223.5m/s^2 = 23,355.75 Newtons (N)

All of that math means that the two different style of bikes have a pretty big difference in force applied during crashes, due to their difference in mass. The motorbike's force is 1.95 times greater than the force of the pedal bike.

I think I'll keep my moto helmet, but to each his (or her) own....
This all looks nice except it's all completely irrelevant because you hit the tree with your mass and the bikes bounces off whatever it hits with its mass. The helmet goes on the rider's head, not the bike's. You get that, right?

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Old 10-23-2012, 01:06 PM   #37
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“ Quote:
Originally Posted by BobbyBlackFinger View Post
True, but unless you fly off of the bike completely and hit a tree by yourself you need to use combined mass of the rider and bike.
Only if the bike is moving directly behind your head in the force vector. It could happen, but not often, unless you do a lot of stoppies.

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Old 10-23-2012, 01:08 PM   #38
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“ Quote:
Originally Posted by uber1 View Post
I have that same helmet for riding my DH bike..(mostly at Duthie Hill)...
I have taken big hits to the front and back of that thing and I'm not more stupid than before the crashes...
http://www.youtube.com/watch?feature...L-15JEJAE#t=9s
My cousin is currently in Overlake with a double pelvic fracture and a double spinal fracture from mountain biking at Duthie!?! But his skull is fine.

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Old 10-23-2012, 04:02 PM   #39
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Joined: May 2009
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I Ride: 2007 450EXC, 2002 EC300
“ Quote:
Originally Posted by ZXWUT? View Post
This all looks nice except it's all completely irrelevant because you hit the tree with your mass and the bikes bounces off whatever it hits with its mass. The helmet goes on the rider's head, not the bike's. You get that, right?
lets try a different angle then.
The difference in force proportionate to acceleration.
Do we agree that motorbikes with 25-50 horsepower can accelerate at a higher rate then a pedal bike with 1 human power (lets say the ground is flat)? If so, the higher rate of acceleration would create more force on the riders mass.

like I said earlier you would probably be OK most of the time with the downhill helmet, but the one time you goose your throttle into a tree on accident, you might wish you had a Snell or DOT helmet designed for impacts in motor vehicle crashes.

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Old 10-23-2012, 04:34 PM   #40
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Still irrelevant. Downhill bikers go much faster than you're going to on a dirt bike in the woods. Wear whatever you want though

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