What key factors determine a vehicles top speed?

Thanx in advance for any input.

 

Transmission final drive.

 

And redline.

 

aerodynamics to a certain extent

 

Probably after you get into really really high speeds.

 

a lot of factors.
HP, redline, torque curve, drag coefficient, altitude, weather, surface, grade of surface (level, uphill, downhill, etc), weight, traction, final drive for ur transmission.

 

Nice, lovin' the info guys. Thanx.

I've heard a lot about the final drive ratio affecting your top speed, but I don't quite understand it. Can someone clarify, say, what a good FDR would be for a top speed car? I know, I know, sounds nOOb and vauge, but any input on what a good transsmission setup for top speed runs would be greatly appreciated.

 

Driver fearlessness also. I would have to say this list pretty much sums it up.

 

Nice, nice. Still wondering on my question reffering to the final drive ratio of a transmission.

 

higher number=shorter gear.

higher gear ratio=slower acceleration but higher top speed. im sure you've ridden a bike when you were a kid, ever notice how on the first gear (high gear ratio)although its easy to turn and you can get going faster after a while your legs cant move fast enough to accelerate further? same thing. controversly, every try starting out in a higher gear (lower gear ratio) and you have to bust your *** just to get it rolling?
those 2 sentances describe how gears work in a nutshell. for a higher gear ratio there comes a speed where your engine cant move any faster/harder to make it accelerate more. in a lower gear ratio it takes more power to get you moving but you can go alot faster. so it really depends on how much power your car is making and where (a kid on the football team can peddle a lot harder than the chess club president if you catch my drift).

final drive ratio is how many times the pinion that is on your tranny must turn to the turn the axle once. again, higher number=smaller gear.

 

oh keep in mind that your engine has to be able to pump out enough power to drive the gear, so if you set the gear ratio too low it might not be able to create the power needed to accelerate....infact it might even decelerate then stall out. if you want a good example of this put your car in first gear, get up to about 2-3k RPMs, then switch to 5th and watch it stall out.
i dont know of any equations based off tq/hp to calculate what the best 5th gear and best final gear would be, far as i know its trial and error/necessity.

anyway if your looking at examples, a dodge viper has a .05 gear ratio for 6th and a 1.54 for final has a final speed according to dodge of 192 MPH. keep in mind it is a v10 pushing 490 tq and 450 hp.
on the other hand an integra gsr (128 tq 170 hp)has a .787 5th gear and a 4.4 final gear and acording to acura has a max speed of 130 MPH.
not sure if its a steady line between best gear ratio and HP/tq, that would be an interesting thing to compare for people who have custom final drives and/or gears. would be kinda hard to do though because you'd need 2 of the same type of car with the same build and numbers and you'd have to compare them side by side literally to get an acurate comparison.

update: sorry forgot to put this in earlier (SOMEONE CORRECT ME IF I HAVE THIS BASS ACKWARDS btw) . a gear ratio is how many teeth one gear (final drive in this case) has vs another (wahtever gear your in). so a gear ratio of 3.0 means there are 3 teeth on your final drive for every tooth on your gear, in other words the gear has to spin 3 times to spin the final drive once. its the direct inverse of using a wrench to screw something down as far as force exerted goes. when screwing something down it takes less force the further out the wrench goes, in this case it takes more energy since the engine is pushing the gear from the center, not the outside.
anyway if your car has a final drive of 3.0 and your current gear is 3.0, then your gear has to spin 9 times (3x3) to get the axle to spin once. for the same final drive if your current gear is .5, then your gear has to spin 1.5 (3x.5) times to spin the axle once.
good way to test this out for yourself (if your a hands on person like me) is to grab a dumbell that you can adjust the weight on and leave one end empty and put a 5lb weight on the other end. hold it towards the center and try to push the weight up using just your wrist. then move your hand to the empty end and try again: much harder.

 

Thanx xander1100, exactly what I was looking for

 

Its kind of sad though that most aftermarket vendors (at least the ones that I have browsed), sell high ratio gears for most import vehicles.

Anyone know of a vendor, or a custom distributor, that sells low ratio gears for import vehicles?

 

Drag, horsepower curve, gearing, and air density

Yes, grade matters, but top speed is usually rated on a flat surface.

If you want to do it mathematically, make a plot of drag vs. vehicle speed and thrust vs. vehicle speed. Wherever those two curves intersect is your top speed.

 

Mkay, great. Another point I'm trying to get at is this: is there any way to increase a vehicles initial top speed?

 

More horsepower, less drag

 

yes. as long as your engine can back it up you can change your tire size. a larger tire will cover more distance per rotation than a smaller one. you can make the vehicle lighter. not sure if 130 is an integra redlining or not, if its not though then increasing the HP will get you more RPMs. you can buy aftermarket gears/tranny (expensive as hell). one thing ive heard of someone doing (this was for a drag car, track only) was taking out the reverse gear and putting a 6th gear in there.

 

Gearing and tire size could make a difference, but keep in mind that changing these will change the engine speed at top speed. If the horsepower at this speed is lower, then the top speed will be lower, and vice versa.

Also, when the top speed is reached, the acceleration is zero, so weight doesn't matter. It's just a balance of thrust and drag.

 

shoulda posted this earlier but spaced:
a good way to lower your frontal area is to lower your car, it also means less air pushing your car up from underneath so you get better traction.

weight does make a difference for top speed, since the heavier your vehicle the more force gravity is working against your vehicle (you're trying to go forward, not downward.)

 

That's only if you're not driving on a flat surface.

 

even if it is on a flat surface. gravity doesnt dissapear just because you're going fast or because your on a level surface. its pretty much another point of friction (well 4 points actually), a heavier car will put more weight on its tires than a lighter one, tires are in contact with the road. the harder you push down on an object when trying to move it the more friction it creates.
gravity means friction. friction means resistance. more resistance means slower top speed.
granted, even in acceleration weight does make little difference (i think 100lbs averages .1 seconds less in a quarter mile), but if vehicle A+B are exactly the same except car A took out all of its sound proofing, back seats, etc and weighs 100 pounds less, it will go slightly (1-2 mph) faster than car B.

anyay if you want a mathematical formula to find your cars top speed (more or less, this does take your gears out of the picture but it will give you a guestimate, as well as can help you find the proper sized gear to make your max top speed):
F = 1/2(Cd)(A)(p)(v^2) + Cr(W) put ()'s to seperate each individual part ot make it easier to read. you'll need to reorganize this so you're solving for v, i feel too lazy to do that right now.
F is force behind yoru vehicle. its balanced (meaning its the same number) as your peak HP expressed in the proper term (metric if your using metric, etc). easiest way to do that would probably be with a dyno graph.
CD is the drag coefficient. you can find that easily with google.
A is the frontal area of the car (the size of hte hole the car has to punch through the air)
p is the density of hte matter the object is moving through (air in this case)
v is velocity.
cr is the coefficient of hte roling resistance
W is weight

 

Yeah, I guess weight matters for rolling resistance, but it's a pretty small factor. You're not going to change it by more than a few pounds.

 

So, in essence its kinda like this. . . : If the vehicle can provide the right traction and aero, accompanied by substantial power from the motor, then the top speed of a vehicle can increase exponentially until the motor can no longer feed the neccessary components for the transmission?

 

Thanx for this equation.

 

It's just a balance between thrust and drag (from aero and rolling resistance). I wouldn't really call it traction, because you're not going to be spinning the tires.

The less drag your car creates (from a more aerodynamic shape) and the more horsepower you produce, the higher your top speed is.

Also, drag increases exponentially with speed (see the above equation), so you have to produce a lot more power to go a little faster.