A cars weight doesnt affect its top speed...right?

[GTSRboy2000]

Someone asked on yahoo answers (i need to stop going there), what determines a cars top speed, and the winning answer included weight. I aswered and said that no, weight does not affect it at all, and i got bashed by people who quoted me as 'having no idea what i was talking about"

Its not the MASS that affects it, its the VOLUME (size). Yea a heavier car will accelerate slower, but it will reach the same top speed.

Viperclub.org can you please confirm that WEIGHT doesnt affect top speed so i can sleep tonight

thanks

 

[Bobpantax]

It is my understanding that given two cars which have identical aerodynamic properties where one weighs more than the other, assuming the one that weighs more has enough additional power and an adequate suspension to offset the additional weight, they both should be able to reach the same speed. However, more power will definitely be needed. This is probably why you received the responses you received. Wind resistance is a big factor. It takes alot of power to overcome it, particularly at higher speeds. We have some very savvy enginers on this Forum. If I am wrong, they will jump in and correct me.

 

[GTSRboy2000]

Thanks for the feedback,

So your saying, that if you took 1,000 pounds of WEIGHT off your viper, its previous top speed of 172 mph would go up?

Im arguing that a huge weight reduction would not affect its top speed, becasue the given HP is still fighting the same wind agasint the same body as it was before. It would accelerate faster, but its TOP speed would be the same.

 

[GR8_ASP]

This is kind of a trick question in that in a perfect world weight does not affect the top speed, assuming an infinite length surface to attain it. It only affects the acceleration in the perfect scenario.

However, real word is a bit different. The things that absorb the applied power are aero drag, rolling resistance and driveline friction. I am excluding those things which affect acceleration (like inertial effects). Now drag is not directly impacted by weight. However, adding weight to a sprung system changes the ride height (position of the added weight matters here also). So aero drag may be reduced (lowering the front or total vehicle) or increased (lowering the back and raising the front) by the addition of weight. Another factor is road load or rolling resistance. Adding weight should cause an increase in tire rolling resistance and bearing friction (wheel bearings).

So the final answer is .... the effect is minor but is there. For any specific vehicle the answer is not automatic.

 

[ACR steve]

Cant be correct ...HP ratio vs top speed must include weight in addition to aerodynamics . There is a formula somewhere that shows the theoretical top speed of a car and I believe the weight was a factor.

 

[ACR steve]

Think of it this way: If your tire surface inflated to the same pounds were to have 1000lbs of load on it or 4000lbs of load on it would they both have the same rolling resistance? I don't think so but I am not an engineer

 

[GTSRboy2000]

So the only way weight can affect top speed is if it changes aerodymanics or friction right? Weight cannot directly affect top speed was what i was thinking.

So a 2008 Viper has a top speed of 200 mph lets say...if you dropped 2,000 pounds of weight off of it, how much faster would it go? The car would raise up off the ground, make its aerodynamics worse, so less weight make its top speed slower maybe?

 

[GR8_ASP]

So the only way weight can affect top speed is if it changes aerodymanics or friction right? Weight cannot directly affect top speed was what i was thinking.

So a 2008 Viper has a top speed of 200 mph lets say...if you dropped 2,000 pounds of weight off of it, how much faster would it go? The car would raise up off the ground, make its aerodynamics worse, so less weight make its top speed slower maybe?

But the effect on rolling resistance is real. Like I mentioned there are 2 resistance forces. Aerodynamic drag (based on drag coefficient and frontal area) and rolling resistance (I will ignore driveline efficiency here if we are treating this as wheel hp). There are many components to rolling resistance (tires, brake friction, driveline friction, etc). The tire rolling resistance and wheel bearing friction elements are directly impacted by the addition of weight. And the impact of the added friction is not negligible. However, if you are talking about an additional 50 lbs for a 4000 lb vehicle it would be very small. However a 1000 lb increase should be easily measureable.

 

[GTSRboy2000]

So yes, weight would affect a cars top speed by changing friction (rolling resistance) and ride height (aerodynamics). I guess i was thinking of this in a perfect scenario

thanks for all the info

 

[j-rho]

Cars at Bonneville add a ton of weight to increase stability on the salt flats. Sure, there will be increases in rolling resistance, but assuming you put the car together to operate properly at whatever weight you're running it (rake, ride height, tire size/inflation pressures, etc.) the increase in that resistance will be very small, practically negligible, in comparison to the key factors of aerodynamic drag and power.

If the car has just a little bit of aerodynamic lift (most street cars have lift by default), at a really high speed, the actual load on the tires will be quite a bit less than at rest, like a plane getting going down the runway before takeoff. Old Le Mans cars would have their front tires in the air at the end of the Mulsanne.

Additional weight will slow acceleration (at least in the speed ranges where the car is power-limited in acceleration) but as noted above, most top speed runs scenarios assume adequate distance to achieve the vehicle's max, so acceleration isn't really a factor.

 

[GR8_ASP]

All very true. But noting that drag increases as the square of the velocity, aero drag is much more important an element in Bonneville vehicles.

I do not believe current Vipers have exhibited significant lift, but in fact have positive downforce.

 

[Bobpantax]

So the only way weight can affect top speed is if it changes aerodymanics or friction right? Weight cannot directly affect top speed was what i was thinking.

So a 2008 Viper has a top speed of 200 mph lets say...if you dropped 2,000 pounds of weight off of it, how much faster would it go? The car would raise up off the ground, make its aerodynamics worse, so less weight make its top speed slower maybe?[/quote]

Not necessarily if the car had a front splitter and an adjustable wing, etc. producing enough downforce. This past Saturday at Homestead there was this little yellow number running around that had a modified 675 HP Corvette engine in it. It was approximately the size of a Radical. I bet it weighed well under 1800 pounds. If you look at the photos posted by Brooks you will see it. I agree with GR8 ASP. I forgot about rolling friction.

 

[Paul Hawker]

Interesting question.

What would have the highest top speed, a 3,500 lb. Viper or a 50,000 lb. Viper. (Same engine)

Logic would suggest the lighter Viper would be faster

 

[pteam]

Interesting question.

What would have the highest top speed, a 3,500 lb. Viper or a 50,000 lb. Viper. (Same engine)

Logic would suggest the lighter Viper would be faster

I agree there is no way a 50,000 lb viper is going to make it to 190+ mph, the lighter viper is faster.

 

[j-rho]

Guys, let's not get crazy here. If you put 46,500 pounds into a Viper you'd crush it into scrap. Yay grandma on a bigwheel has a higher top speed. So what?

The distinction many have trouble making is how slower acceleration does not necessary mean a lower potential top speed. Sure, it might not reach as high a speed between stoplights, or even on a long stretch of local freeway late at night, but given adequate room, the heavier-but-otherwise-equal car will reach a roughly equivalent top speed, subject to increases in mechanical drag due to the increased weight. Which - generally is a very very small contributor to the overall drag total when talking about something with a top speed around what a Viper can do (190+).

Both cars will continue to accelerate to a point where the pounds of thrust produced by the engine are equally countered by the pounds of drag (aerodynamic and mechanical, with aero being by far the greater contributor). Note how weight is not a factor in the above, again, except as a contributor to mechanical drag, probably <1% of the overall drag total near a Viper's top speed.

 

[dave6666]

This is certainly interesting stuff, and on more than one front.

First off, the engineering discussion. Like I said, interesting.

Secondly though, is how irrelevant all of this engineering talk actually factors in to what was likely the original concept being discussed on Yahoo.

I would imagine that anyone that asks a question like that on the Internet is thinking in terms of whole digits, if not even larger increments. You know, like 200 vs 201 mph, or 200 vs 210 mph.

Back to the engineering being discussed here... 6th decimal place kind of stuff. Which most of us never deal with personally on our cars because it is too advanced for the average Joe to do. We add some power or get stickier tires and the like and improve our 1/4 mile times, but even that ain't 6 decimal place kind of stuff.

So anyway, I just think it's interesting that someone on the Internet asks a question along the lines of is dad's truck slower than mom's car because it weighs more, and we're discussing changes in 10ths on suspension settings at speed due to increase in mass of the vehicle.

Now, back to the show.

 

[Chuck 98 RT/10]

Guys, let's not get crazy here. If you put 46,500 pounds into a Viper you'd crush it into scrap.

You missed the point. Paul was exagerating weight to give a better illustration.

For example:
Will 100lbs matter? Not significantly.
Will 1000lbs matter? Maybe.
Will 50,000lbs matter? Probably.

The conclusion is weight probably matters, but it may not matter very much.

 

[j-rho]

You missed the point. Paul was exagerating weight to give a better illustration.

For example:
Will 100lbs matter? Not significantly.
Will 1000lbs matter? Maybe.
Will 50,000lbs matter? Probably.

The conclusion is weight probably matters, but it may not matter very much.

If you put a 50k pound trailer behind a Viper, and that trailer contributed no change to the overall aerodynamic or mechanical drag profiles of that Viper, it would have no affect on the vehicle's top speed. It will take 14-15 times as long to reach that speed, but it would still do it. Not probably, not maybe, it will.

Weight, by itself, does not affect top speed given adequate distance. It affects acceleration. Most people have a hard time grasping that a car with lower acceleration can have an equal top speed. Your conclusion of weight "probably" mattering is incorrect, except in the case of a distance-limited top speed run.

I didn't exactly miss the point, my response was meant to indicate that often times in these sorts of discussions people reach for examples from extreme ends of the spectrum (in this case weight range), and that usually, the behavior of a system at those extreme ends is not indicative of the system's behavior when dealing with values in a more reasonable range. We could talk about our new top speed if we lightened the Viper to zero pounds (might have to get Stephen Hawking in on the discussion then), but it would add little to nothing of our understanding of what's really going on.

 

[PAvenomRT/10]

If i remember my physics 101, E = 1/2 mv2 where E is energy, m is mass and v2 is velocity squared. So in the absence of wind drag it takes proportionately more energy, i.e. horsepower to accelerate a heavier object to a given speed. Factor in wind drag which is equal in the scenario of the 2 cars mentioned above, and the lighter car still attains a higher speed assuming equivalent horse power.
In the case of the speeds attained in tests on the Bonneville salt flats, aerodynamics plays such a large role in the final equation that weight takes a back seat to aerodyamics. Put another way for a top speed test, i would rather have a more aerodynamic car than a lighter one.
PAvenom RT/10

 

[j-rho]

Energy is the application of horsepower over a period of time.

PAVenom, using your formula for kinetic energy, the 50000lb Viper will have a lot more energy when it reaches its top speed. What you're missing is the engine is adding energy to the equation the entire time the car is moving. The reason it takes longer for the car to reach top speed, is that it must output more energy to accelerate a larger mass to speed.

 

[GTSRboy2000]

j-rho, at first i agreed with what your saying, but would the increased weight have an affect on the tires contact w/ the ground, causing more rolling resistance? If you added 1,000 pounds of dumbells into a 2008 Viper, would the increased rolling resistance mean the power was'nt able to get it to its previous top speed? This is the best argument that makes me think adding lots of weight might indirectly change the top speed

If your drivetrain were lighter you would have more wheel HP and thus a higher top speed...?

 

[j-rho]

Variables...there are many at play.

Part of the reason I suggest people keep from getting outside of reasonable ranges (in this case weight) is because it tends to make important, variables that wouldn't normally be a factor in reasonable ranges. At 50,000 pounds, a Viper would not move very well, because it would be crushed. This teaches us about tire maximum loadings and a frame's failure mode, but little about the affect of weight on a vehicle's top speed.

As mentioned in my first post in this subject, weight is likely to cause an increase in the mechanical drag (tires, bearings, etc.) a vehicle experiences while in motion. But there are other things that change too, in a positive or negative way, like ride height and rake.

It is difficult to quantify the magnitude, and in some cases the sign, of effect increased weight would have on each of these variables. Also, in a hypothetical situation, you have to ask to what extent the owner of a vehicle adding 1000lbs. of dumbbells to his car is otherwise configuring things for that weight - is the weight evenly distributed, or more on the nose, or all in the trunk? And what are the aerodynamic implications of such?

I think the basic lesson here is that additional weight decreases acceleration, but that a decrease in acceleration does not necessarily dictate a decrease in potential top speed. It most certainly increases both the time and distance required to achieve a given top speed. BUT - any change to the potential top speed is due only to second-order effects of the additional weight, such as increased mechanical drag, or aero drag perhaps.

Once we accept the above paragraph, we can debate all day about whether you could produce an improved overall coefficient of drag by lowering the car 2" with 1000lb. of extra weight, and how much you'd want f/r to optimize the lift/drag profile, and if the aero improvements would be worth the additional mechanical drag. Maybe the slammed car would have additional negative camber and decreased contact patch and thus rolling resistance? This sort of nonsense can go on for eternity and not get anywhere, so it's kind of pointless.

As to the drivetrain thing, I'm not sure I'm ready to try to explain why one that is simply "lighter" would not produce a higher top speed if we can't first swallow the "slower acceleration does not mandate lower top speed".

 

[GR8_ASP]

Well, after many posts and commentary you have arrived at what I said (maybe not as well) in post #4:

This is kind of a trick question in that in a perfect world weight does not affect the top speed, assuming an infinite length surface to attain it. It only affects the acceleration in the perfect scenario.

However, real word is a bit different. The things that absorb the applied power are aero drag, rolling resistance and driveline friction. I am excluding those things which affect acceleration (like inertial effects). Now drag is not directly impacted by weight. However, adding weight to a sprung system changes the ride height (position of the added weight matters here also). So aero drag may be reduced (lowering the front or total vehicle) or increased (lowering the back and raising the front) by the addition of weight. Another factor is road load or rolling resistance. Adding weight should cause an increase in tire rolling resistance and bearing friction (wheel bearings).

So the final answer is .... the effect is minor but is there. For any specific vehicle the answer is not automatic.

 

[Bobpantax]

I think post number two said it in a straight forward and simple way.

 

[dave6666]

I think post number two said it in a straight forward and simple way.

Bob... That was YOUR post!

YOU silly head...!

 

[j-rho]

Well, after many posts and commentary you have arrived at what I said (maybe not as well) in post #4:

You're totally right, I tried to acknowledge I was largely reiterating your post. I really **** at being succint. It just seemed like it hadn't sunk in for some folks.

Bob, your post #2 is a good example. The heavier car will not need more power to reach the same speed in this hypothetical situation, unless it is also a requirement that it reach the same top speed in the same amount of time/distance as the lighter car.

 

[GR8_ASP]

Ha, I was trying to give you credit for writing it better.

And Bob sorry but it was your inaccurate statement about needing more power that lead to my posting. I tried to demonstrate that without knowing the specifics you cannot know if increased weight would increase or decrease the top speed. But no queston the change in top speed for any vehicles who's top speed is high (say above 150 mph) will be impacted very slightly. Vehicles with a top speed of say 70 mph would have a significant impact due to weight, as rolling resistance is a significant factor at that speed. It's rolling resistance does not go down with higher speed, it is just that the drag force increases exponentially.

 

[Newport Viper]

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