fish1

I really didn't think the slightly heavier winter rims would slow me down much to any degree.
I have the nice heavy black 17" steel rims with winter tires (my friends and co-workers call it the "Mad Max" look).
Anyways, I decided today to measure my top speed with WOT in 2nd gear to see if my top speed would be any less than with my stock summer rims and tires.
Turns out to be quite a difference.
My top speed in 2nd with stock rims is about 110-111 klm/hour (then fuel cut-off).
With my beautiful winter rims the best I could do today was about 101-102 klm/hour.
I measured the speed with my GPS.

Kinda curious if anyone else with steel winter rims have noticed similar results??, and, are they consistent with my results??

Kennetht638

iTrader: (2)

Your 17" winter tires are smaller in diameter. Top speed in any gear but 5th and 6th are limited by gearing and will be directly proportional to the diameter of the tire. This has nothing to do with the construction of your wheels and tires.

RIWWP

iTrader: (2)

The numbers you are posting are contradicting the question that you have.

Using "Slower" implies you accelerate slower. However you don't mention anything else about this.

However, I assume you are talking about at a given RPM, your speed is lower. This is directly a function of the outside circumference of the tire that you have vs your summer set. A Bigger outside circumference (rolling distance per revolution) will result in a slower turning axle for a given RPM, so at a given RPM, you will be traveling faster (same revolutions, longer distance).

Flip it around, if you have a smaller total outside circumference of the tire, the axle will be spinning faster for the same MPH/KPH, which will result in a slower speed for a given RPM.

I'd guess that your winter tire package is about 9% smaller total outer circumference than your summer package.

My speedo actually matches my GPS with winter set, but shows ~3% high with the summer set.


It's all gear ratios, and your rim/tire package is in fact the actual "final drive", with the final drive actually being the 2nd to last 'gear'.

fish1

Yes, my winter package is a little smaller diameter, resulting in my speedo being over by as much as 10 klm/hr than my actual speed.
But I am measuring my top speed with my GPS at fuel cut-off in both cases at WOT?
okay, now I'm confused..........must meditate on this one.

wouldn't a heavier rim/tire package reduce your top speed and acceleration??

RIWWP

iTrader: (2)

Yes, you are correct, a heavier wheel/tire package will in-fact reduce acceleration rate. It generally will NOT reduce your top speed. The extra weight resists change, meaning reduces acceleration and reduces braking rate, but you can still get there. Absolute flat out top speed you might lose a KPH...MAYBE, due to the slightly reduced acceleration rate being countered by aerodynamic drag slightly earlier. Not that you should be doing those speeds on winter tires.

However, your speed at fuel cut-off in RPM has nothing to do with acceleration. Nothing at all. That is simple Engine RPM changed to drive shaft RPM via the transmission, changed to Axle RPM via the rear diff, changed to distance per revolution on the outer circumference of the tire. It's all fixed gear ratios. Changing your tire diameter will change the difference between your speedo and GPS speed, since GPS uses actual and your speedo uses axle speed. Your speedo does NOT use tire speed, or it would never be off.

Stop thinking about the speed at fuel cut-off at WOT. Downshift to the fuel cutoff point and hold it. Your speed is the same. Doesn't matter how you got there, it's gear ratios.

fish1

Thank you, it makes a little more sense to me now.
I need to read things a couple of times for it to sink in.....
the gear ratios will always be fixed giving the same driveshaft rpm.
So here is the point I always get stuck on:
If I have the same HP and torque acting on a heavier object then wouldn't only acceleration but also top speed be reduced?
the same hp and torque just cannot get a heavier object to the same top speed?
Am I beating this thread to death.....lol

mrl84

http://en.wikipedia.org/wiki/Velocity

Velocity(speed) is independent of mass, the only equations there that have an 'm' in them deal with energy or momentum.

if you have two identical cars and then add 500lbs to one they will both have the same top speed it just going to take the heavier one a lot longer to get there.

if you have two identical cars and put smaller wheels (and tires) on one then the one with the smaller wheels will have a lower top speed.

if you add weight and smaller tires you will accelerate slower and have a lower top speed, not because you added weight but because you added weight and smaller diameter tires.

RIWWP

iTrader: (2)

It's ok Learning is always encouraged.


Lets back up first to what top speed is. Every vehicle in existance has a top speed. And every vehicle in existance has it's top speed defined 1 of 3 ways.
- Electronic
- Mechanical
- Aerodynamic

Electronic top speed is when an electronic component or computer detects that the vehicle has reached a pre-set max speed, and uses some method to prevent it from accelerating further. Pulling timing, cutting fuel, or disabling cylinders are the usual ways.

Mechanical top speed is when the engine simply can not spin faster. Granted, this is normally an electronic rev limiter, but it is still referred to as mechanical, as spinning faster would just destroy the engine without gaining speed.

Aerodynamic top speed is when the torque output in the appropriate gear is exactly equal to the effort needed to push the vehicle through the air. This is drag, and it generally goes up by the square of speed. The faster you go, the more drag you have. As long as you are producing enough torque to overcome the drag, (like 1st through 4th always will in our stock rev range), you will accelerate. If you aren't producing enough torque to overcome drag, you will decelerate (i.e, taking your foot off the gas). When these 2 forces balance out evenly, that is your aerodynamic top speed.


How does a heavier wheel/tire package come into play? Well, a heavier package absorbs more of the power, which means that at absolute flat out in 6th gear, the reduction is enough to impact your aerodynamic top speed, dropping it a bit. Depending on the weight difference, it probably won't be enough to register. (a 10% heavier package doesn't mean a 10% drop in max aerodynamic top speed)

However, this assumes that the tire circumference is identical to the lighter package (summer set in this case). If the circumference is different, then the aerodynamic top speed difference is significantly impacted, before weight can even play a role. If you put 13inch tires on your car, the tires have to spin way more (faster) to cover a set length of ground, which means you will have higher axle speed, higher driveshaft speed, higher engine speed (RPM) for the same actual ground speed (GPS). If you are in a gear other than 6th, then your top speed is mechanically limited by the rev limiter. You can't spin the engine faster than 9k, which means that you can't cover the same distance in the same amount of time as at 9k in your summer set.


Lets say that your summer set has an outer circumference of 1 meter. This means that the wheel (and thus axle) will rotate 1,000 times per kilometer. For the moment, we are going to ignore the rear diff and transmission. Assuming the engine was linked directly to the axle, this means that your engine will rotate 1,000 times per kilometer. 9,000 revolutions per minute means 9 kilometers per minute, or 540 KPH.

Now lets reduce your tire size, which reduces your outer circumference to 0.75 meters. This means that the tire, axle, and engine will rotate 1,333 times to cover 1 kilometer. 9,000 revolutions per minute means 6.75 kilometers per minute, or 405 KPH.

See? Just changing the tire circumference, we dropped the mechanically limited top speed at 9,000 RPM from 540 to 405. To get to 540, you would need to upshift to a different gear.


Obviously, we aren't going these speeds, the transmission gears and rear diff gear exchange speed for 'torque' (pushing power against the road here, not engine rated torque), otherwise this would be undriveable.

Your tire outer circumference is just another gear when it comes to determining your mechanically limited top speed.

Weight doesn't matter here. Just for your acceleration rate, and your aerodynamic top speed.




Heavier wheels/tires have more stored energy in them, so braking performance is reduced, although maintaining a given speed is easier.

fish1

Man!....where were you RIWWP when I needed help studying in highschool!!
(and thank you too mrl84).
Excellent explanations guys, had the "ah I get it now! moment".

Thank you.
me smarter now......haahaa.

RIWWP

iTrader: (2)

My Dad is an engineer / logic guy, my Mom was a creative writer. I've found I'm pretty good at explaining anything to anyone using language that they understand

I just realized I consistently used the term "diameter", when I mean "circumference". Editing now.

mrl84

i tried to explain it as simply as i could glad it helped.

"I just realized I consistently used the term "diameter", when I mean "circumference". Editing now."

that was driving me nuts when i read it but didnt want look like a dick by commenting on it.

i was going to add that i too use 17" wheels for the winter but i use a higher profile tire so the outside diameter of the wheel/tire package is very close to my summer 18" set, so the speedo differance is very minimal