raceing/physics

[Berserker]

Raceing & Physics
I was discussing with someone the effect of weight on motorcycle topends. They said it really didn't mater. Its mostly a function of drag. To me that doesn't seem right. I would think same bike, fatter rider is gonna be slower.
I know we got alot college students and race fans here, and I am to lazy to think. Any comments?

[PowerManDL]

That's gonna depend on a lot of things, really......not just the weight, but the power/weight ratio of the engine, drag, etc.

[Reinier]

the weight of the driver is going to affect the acceleration not the top speed. however a fat person will likely be less aerodynamic, so that will take the top speed down

[nejar462]

I would think the weight of the driver would affect the top speed as well as the acceleration, because of the kinetic energy and momentum equations, though I I wouldn't be surprised if I was wrong

[HahnB]

Originally posted by Reinier
the weight of the driver is going to affect the acceleration not the top speed. however a fat person will likely be less aerodynamic, so that will take the top speed down

So if you put a 800lb guy on a bike it would go just as fast as it would with a 140lb guy on it?

[Ritzol]

the mass is going to play a role. But, more so on the initial velocity. It will take longer for the heavier mass to gain speed, but it will result in more force.

[Berserker]

p=mv and k=1/2mv^2. Those are the formula for momentum and kenetic energy. I actually remember those. Don't think they what I am looking for though. v=dx/dt
v=v0+at a=F/m in these equation m is a factor. F=would be the motor I think. Maybe a little to simple.

[Berserker]

drag force D=bv b=m/terminal velocity
So drag is dependant on mass. I think this might be what I am looking for. This for a projectile, without its own force.

[Narcissus]

my bike's indicated (and neutered) top speed is 186mph. i have been up to an indicated 176mph with at least a grand to spare on the tachometer. i weighed just over 300 at the time.

at the upper tier of current racing replica bikes, you cannot determine absolute top end without doing a modification such as a Timing Retard Eliminator. that being said, i think weight plays a minute role in truncating indicated top end.

[Berserker]

Narc, I get your point but not sure your reasoning. What is TRE? Also I don't thinking having 1000 rpm left before redline is an indicator of how much power you have left. Sometimes you can rev it more and be at the mechanical limit and get nothing.

[Narcissus]

a TRE is basically a GPS override system. It sets the bikes factory gear position sensor to 5th gear. This increases low rpm throttle response by making the ECU think that the bike is in 5th gear all of the time. The ECU then provides a “better” ignition map to provide better performance. Another thing this does is bypass the top speed governor.

and yes, the 1000rpm left before redline is an indication that i could have continued to increase my speed. when i got to 176, i was accelerating easily, and only ceased because i backed off the throttle (and frankly, i was quite scared). i have no doubt that i could have continued to open her up and reach the governed top speed.

[Berserker]

That you were still accelerating is an indicator that you more to go.
I am still not following you on the TRE. So what if you bypassed the bikes govenor or made it think its in 5th gear. What does that have to do with drag, gravity, ect.. That is a mechanical item only. Sounbds like you just improved performance. Sorry if I am missing something.

[Berserker]

I been thinking about this all morning. I can't see how you can take the same motorcycle but a heavier driver on and still have the same top speed. The engine can only exert so some much force, which is indepandent of the driver. Add more weigh and same force, to me equal slower.
If I push as hard as can on empty cart and cart filled with bricks the empty cart is gonna go faster. They both have the some aerodynamics if there is a lid on the cart.

[Ritzol]

Originally posted by Berserker

If I push as hard as can on empty cart and cart filled with bricks the empty cart is gonna go faster.

Because you are the force behind it..... both carts have the potential to reach the same final velocity. Initially the cart w/o bricks will go faster, but eventually the one with bricks will reach the same speed because there is a greater force working on it.

[Big & Tall]

f=ma
v=vo+a(dt)

On level ground, aerodynamics aside, weight shouldn't matter. There will be a nominal effect on friction forces (i.e. between road and tire) but it shouldn't have that much effect on top end, only acceleration. That said, acceleration is what makes the difference in racing, as top end is rarely reached for long/at all on most courses.

As far as what Narc said, remaining revs may or may not be indicative of remaining available speed. Remember that most horsepower curves start to drop off before redline (I've attached one for a piped gixxer 1000). I'm not sure, but I think as a result of this, most bikes have drag-limited top speeds, as open-wheel motorcycles have higher drag coefficients than most mini-vans (yes, even the aerodynamic looking bikes)

[Berserker]

Originally posted by Ritzol


Because you are the force behind it..... both carts have the potential to reach the same final velocity. Initially the cart w/o bricks will go faster, but eventually the one with bricks will reach the same speed because there is a greater force working on it.

NO.
My legs can only exert so much force, that is a constant. What your saying is you can run as fast with a backpack full of rocks as you can with an empty one.

[Berserker]

If you can sprint at a top speed of 10mph and lets say get there in 30 seconds. If I weigh you done and give you all day will you ever reach 10 mph?

[Berserker]

Originally posted by Big & Tall
f=ma
v=vo+a(dt)

On level ground, aerodynamics aside, weight shouldn't matter. There will be a nominal effect on friction forces (i.e. between road and tire) but it shouldn't have that much effect on top end, only acceleration. That said, acceleration is what makes the difference in racing, as top end is rarely reached for long/at all on most courses.

)

Even ignoring friction weight is gonna have an effect.
v=v0+a*t a=f/m, so v=v0+f*t/m So from that I see velocity is effected by mass.
Maybe I am missing something here. I not sure why you put newton 2nd law in you post cause it seems to prove me right. Maybe I am gonna go duh in a minute. But I am missing something here.

[Ritzol]

Originally posted by Berserker
If you can sprint at a top speed of 10mph and lets say get there in 30 seconds. If I weigh you done and give you all day will you ever reach 10 mph?

What's the distance of the sprint?
and how much weight are you adding?

[Berserker]

I read this on a differant forum, if accurate answer my question

At really high speeds (> 100 MPH) the wind resistance is so huge, it swamps inertia which means weight becomes less important.

[Berserker]

Well I am starting to wonder about the cart. If the weight wasn't to much and you got the cart going , would its own inertia even things out eventually? I wish I had a speedo on my lawn tractor. Either that or steal a kids moped.

[Ritzol]

Dude...... stop thinking, you're totally confusing.

[Berserker]

I know I am gonna quit thinking now.

[Coleman]

Anyone know why bikes arn't very aerodynamic? They look like they are.

[GonePostal]

Ummm.... of course mass effects top speed. If it didn't we would be able to reach speed of light and beyond. Case closed.