A.I.I.Raciing
Dezert 8MYMNEY
There some very smart guys in here and I was wondering something, if a 35" tire weigh's 83 lbs, would it be about the same power lose as running a 37" tire that weighs 68 lbs?
Unsprung Weight vs. Rotating Mass
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KaiserM715
Kaiser Söze
There are some calcs you could do to determine the difference in rotating inertia, but give the relatively large weight difference (~20%) vs the relatively small diameter difference (~6%), it is pretty easy to approximate. Go with the lighter 37". It will be less power to turn than the heavier 35".
Last edited:
Cleave
FRF Addict
I think that the total acceleration loss would be similar, but how the truck reacts to each set would be completely different, with larger tires that weigh less you're losing acceleration due to gearing but you gain cornering ability and the ability to soak up the rough stuff, whereas the smaller tires that weigh more would probably lose a little less in the acceleration department but a lot more in actual offroad running ability
MagicMtnDan
FRF Addict
Reducing unsprung weight is one of the most critical factors affecting a vehicle's road (and off road) holding ability. Unsprung weight is that portion of a vehicle that is not supported by the suspension (i.e. wheels, tires and brakes) and therefore most susceptible to road shock and cornering forces. By reducing unsprung weight, lighter wheels and tires and brakes can provide more precise steering input and improved "turning in" characteristics.
Rotating mass must be accelerated/decelerated every time the speed changes, and it's harder to accelerate the rotating mass. Since most rotating mass (aside from the engine and the tranny) is unsprung weight (half-shafts, wheels, brake rotors), a reduction in unsprung weight generally entails a reduction in rotating mass, which helps acceleration. It is rotating mass that is roughly 3 times harder to accelerate than normal weight. Plain old unsprung weight is not any harder to accelerate. Basically, rotating mass must be accelerated twice every time the car speeds up, it must be accelerated linearly (in the direction of the vehicle's travel) and rotationally (in the direction of its spin).
As for a reduction in unsprung weight independant from rotating mass, this generally improves the suspension's ability to work properly. Unsprung weight contributes to inertia in the suspension, which in turn affects the geometry assumed by the suspension.
Unsprung mass has the same effect as sprung mass when it comes to acceleration as long as it is not a rotating part in the driveline.
If it is a rotating part (Such as a tire/rim combo),It will take more hp to accelerate a heavy rotating mass to a specific velocity than a lighter rotating mass. The same holds true for deceleration.
In racing we try to minimize unsprung weight because it hurts handling. When the weight isn't supported by the springs, the shocks and tires control that weight. On bumpier tracks, a large amount of unsprung weight can become difficult to control, and reduce tire contact to the racing surface, which will reduce grip. Also, since the tires are controlling the unsprung weight it can, in certain situations, cause heat build up, affecting tire performance. Also, really light race cars (mine is 2500 pounds, so it's considered light), are very sensitive to unsprung weight, so we attempt to minimize it whenever possible.
Turn a bike upside down, take the back tire/tube off & put the bare rim back on.
Turn the pedals.
Now put the tire & tube back on, & do the same thing.
See how much harder it was to turn the pedals?
Also the unsprung weight (tires, suspension) is supposed to react fast to the bumps in the road (or off road). The lighter these parts are the less force is required to move them and the faster they can react to the bumps resulting in a safer and smoother ride. Additionally you need to consider the rotational energy. The heavier they are, the harder it is to change the axis of rotation (angle, up/down).
Above from various sources on the interwebs
---------- Post added at 03:48 PM ---------- Previous post was at 03:46 PM ----------
And this is from an article on the subject (and probably much more relevant):
Unsprung Weight and Inertia
Unsprung weight and polor moment of inertia. Sounds like physics mumbo-jumbo. However, this is key to understanding the pros and cons of plus sizing your wheels and tires.
Unsprung Weight
Unsprung weight is the term used to describe weight that is not damped on the vehicle. The wheels, tires, brakes, and some suspension components are unsprung, whereas everything else is sprung. This buzz word is often thrown around with the implication that reducing a car's unsprung weight will make the car faster.
This is untrue. (Rotational inertia is what impacts speed.)
Decreasing a car's unsprung weight will increase its sprung-to-unsprung weight ratio, and that directly leads to improved ride quality. When your tire hits a bump in the road it sends a shock upwards into the chassis that must be absorbed. If it is a 3G shock, then the the chassis must absorb three times its unsprung weight. This jolt will cause transfer into the chassis, which causes the unpleasant feeling caused by hitting a speed bump. Cars with stiffer springs and harder bushings transfer this jolt more directly, which is why they seem harsher.
If the car has 80lbs of unsprung weight per tire, than a 4G shock could send 320lbs of force upwards per tire. That's 640lbs per axle hitting you from below to be damped in just one or a few inches of suspension travel, which can be a lot. This is why some sports and most racing cars use forged suspension parts- it reduces unsprung weight, so the suspension does not need to counteract as large of a force. Dropping the unsprung weight by 25% (admittedly a difficult thing to do) would decrease the upward force the springs need to counteract by 160lbs per axle, which in turn can allow the use of slightly firmer springs (reducing body roll) without a degregation of the stock ride quality. This is also the most significant drawback of solid rear axles suspensions- they add the entire weight of the differential and driveshafts as unsprung weight- that's usually 80lbs or more!
The easiest way to decrease unsprung weight for the tuner is to use lighter wheels and tires. While a steel 16" wheel can weigh 22lbs or more, a cast or forged 16" wheel could be found 6lbs or 9lbs lighter, respectively. Lightweight tires, such as those from Continental or Toyo (or Hoosier for racing slicks) can further reduce unsprung weight. Saving just 8lbs of unsprung weight is an improvement of 10% or more on most cars, which can make a marked improvement in ride and responsiveness.
Rotational Inertia
While often considered to be synonamous with unspring weight, rotational inertia is a different term altogether. It is possible to have a heavy wheel that has little inertia, or a lightweight wheel with lots of inertia. A wheel and tire with a lot of inertia takes a greater armount of torque to slow or accelerate, making the car sluggish.
Have a pencil in front of you? Try this: hold the pencil upright by its eraser. Now spin the pencil in your fingers as if you were trying to make a dot on a sheet of paper. It takes almost zero effort to spin the pencil this way, right? Now hold it the pencil's center, between your finger and thumb. Rotate the pencil back and forth, as if you were shaking it to hear loose parts. Feel how it takes more effort to rotate it this way? That is because the mass you are rotating is further away from its center of rotation. Ever notice how an ice skater or karate man tucks their leg in to rotate faster? It's the same concept. If you don't feel the difference, try the same experiment with a larger object such as a broom handle or a baseball bat. Spinning a broom handle like a propeller will take more effort than turning it like a giant drill.
Likewise, the further a wheel and tire's mass is from the axle, the more torque will be required to accelerate it. If a car has 16" wheels and those wheels are replaced with 17" wheels of identical weight, the amount of inertia that wheel carries will probably have risen between 7 to 8 percent. Going up another inch would add another 7 to 8 percent, and so forth. It adds up quickly.
That's assuming the larger wheels weigh the same, which is often impossible. With a larger diameter wheel comes exponentially more surface area needed to create the outer edge of the rim, which is the worst possible place to add weight. Increasing wheel diameter AND increasing weight (even if only a modest amount) will produce rather noticable drawbacks- you can loose 1-2 car lengths (or more) in a 1/4 mile race. Going with wider wheels raises the amount of metal required too, although it does so only linearly.
However, the largest contributer on the entire car to rotational inertia is the tire. Tires are even further out from the axle than the wheel, and usually weigh more too. Some street tires weigh as much as 4 pounds less than their competitors through the use of lightweight materials. Hoosier racing tires, until recently, were made of fiberglass belts instead of steel for the purpose of weight savings (until new regulations prohibited this). That mere four pounds per tire extra will require about the same amount of force to spin as it would take to carry a date riding shotgun! A small difference in tire weight can make a large difference in rotational inertia.
This is a compelling reason to run smaller diameter tires, as larger diameter tires have more inertia per pound and are heavier due to the increased amount of rubber. The weight and inertia savings of going to a skinnier tires is comparitively smaller than decreasing diameter.
Of course, going to a skinnier, softer wheel and tire combination can sacrifice handling (detailed in another article), so it's up to the tuner to find the proper balance for their car.
Do the Math
Andy Welter has created a spreadsheet for calculating the gains or losses of various wheel and tire combinations, found here:
http://www.mazda6tech.com/files/rotational.xls
Kevin K has revised the original spreadsheet to more correct specifications. Remember that the weight calculation is PER wheel/tire.
http://www.mazda6tech.com/files/wheel_inertia.xls
Article is from: http://www.mazda6tech.com/index.php?...d=16&Itemid=50
Rotating mass must be accelerated/decelerated every time the speed changes, and it's harder to accelerate the rotating mass. Since most rotating mass (aside from the engine and the tranny) is unsprung weight (half-shafts, wheels, brake rotors), a reduction in unsprung weight generally entails a reduction in rotating mass, which helps acceleration. It is rotating mass that is roughly 3 times harder to accelerate than normal weight. Plain old unsprung weight is not any harder to accelerate. Basically, rotating mass must be accelerated twice every time the car speeds up, it must be accelerated linearly (in the direction of the vehicle's travel) and rotationally (in the direction of its spin).
As for a reduction in unsprung weight independant from rotating mass, this generally improves the suspension's ability to work properly. Unsprung weight contributes to inertia in the suspension, which in turn affects the geometry assumed by the suspension.
Unsprung mass has the same effect as sprung mass when it comes to acceleration as long as it is not a rotating part in the driveline.
If it is a rotating part (Such as a tire/rim combo),It will take more hp to accelerate a heavy rotating mass to a specific velocity than a lighter rotating mass. The same holds true for deceleration.
In racing we try to minimize unsprung weight because it hurts handling. When the weight isn't supported by the springs, the shocks and tires control that weight. On bumpier tracks, a large amount of unsprung weight can become difficult to control, and reduce tire contact to the racing surface, which will reduce grip. Also, since the tires are controlling the unsprung weight it can, in certain situations, cause heat build up, affecting tire performance. Also, really light race cars (mine is 2500 pounds, so it's considered light), are very sensitive to unsprung weight, so we attempt to minimize it whenever possible.
Turn a bike upside down, take the back tire/tube off & put the bare rim back on.
Turn the pedals.
Now put the tire & tube back on, & do the same thing.
See how much harder it was to turn the pedals?
Also the unsprung weight (tires, suspension) is supposed to react fast to the bumps in the road (or off road). The lighter these parts are the less force is required to move them and the faster they can react to the bumps resulting in a safer and smoother ride. Additionally you need to consider the rotational energy. The heavier they are, the harder it is to change the axis of rotation (angle, up/down).
Above from various sources on the interwebs
---------- Post added at 03:48 PM ---------- Previous post was at 03:46 PM ----------
And this is from an article on the subject (and probably much more relevant):
Unsprung Weight and Inertia
Unsprung weight and polor moment of inertia. Sounds like physics mumbo-jumbo. However, this is key to understanding the pros and cons of plus sizing your wheels and tires.
Unsprung Weight
Unsprung weight is the term used to describe weight that is not damped on the vehicle. The wheels, tires, brakes, and some suspension components are unsprung, whereas everything else is sprung. This buzz word is often thrown around with the implication that reducing a car's unsprung weight will make the car faster.
This is untrue. (Rotational inertia is what impacts speed.)
Decreasing a car's unsprung weight will increase its sprung-to-unsprung weight ratio, and that directly leads to improved ride quality. When your tire hits a bump in the road it sends a shock upwards into the chassis that must be absorbed. If it is a 3G shock, then the the chassis must absorb three times its unsprung weight. This jolt will cause transfer into the chassis, which causes the unpleasant feeling caused by hitting a speed bump. Cars with stiffer springs and harder bushings transfer this jolt more directly, which is why they seem harsher.
If the car has 80lbs of unsprung weight per tire, than a 4G shock could send 320lbs of force upwards per tire. That's 640lbs per axle hitting you from below to be damped in just one or a few inches of suspension travel, which can be a lot. This is why some sports and most racing cars use forged suspension parts- it reduces unsprung weight, so the suspension does not need to counteract as large of a force. Dropping the unsprung weight by 25% (admittedly a difficult thing to do) would decrease the upward force the springs need to counteract by 160lbs per axle, which in turn can allow the use of slightly firmer springs (reducing body roll) without a degregation of the stock ride quality. This is also the most significant drawback of solid rear axles suspensions- they add the entire weight of the differential and driveshafts as unsprung weight- that's usually 80lbs or more!
The easiest way to decrease unsprung weight for the tuner is to use lighter wheels and tires. While a steel 16" wheel can weigh 22lbs or more, a cast or forged 16" wheel could be found 6lbs or 9lbs lighter, respectively. Lightweight tires, such as those from Continental or Toyo (or Hoosier for racing slicks) can further reduce unsprung weight. Saving just 8lbs of unsprung weight is an improvement of 10% or more on most cars, which can make a marked improvement in ride and responsiveness.
Rotational Inertia
While often considered to be synonamous with unspring weight, rotational inertia is a different term altogether. It is possible to have a heavy wheel that has little inertia, or a lightweight wheel with lots of inertia. A wheel and tire with a lot of inertia takes a greater armount of torque to slow or accelerate, making the car sluggish.
Have a pencil in front of you? Try this: hold the pencil upright by its eraser. Now spin the pencil in your fingers as if you were trying to make a dot on a sheet of paper. It takes almost zero effort to spin the pencil this way, right? Now hold it the pencil's center, between your finger and thumb. Rotate the pencil back and forth, as if you were shaking it to hear loose parts. Feel how it takes more effort to rotate it this way? That is because the mass you are rotating is further away from its center of rotation. Ever notice how an ice skater or karate man tucks their leg in to rotate faster? It's the same concept. If you don't feel the difference, try the same experiment with a larger object such as a broom handle or a baseball bat. Spinning a broom handle like a propeller will take more effort than turning it like a giant drill.
Likewise, the further a wheel and tire's mass is from the axle, the more torque will be required to accelerate it. If a car has 16" wheels and those wheels are replaced with 17" wheels of identical weight, the amount of inertia that wheel carries will probably have risen between 7 to 8 percent. Going up another inch would add another 7 to 8 percent, and so forth. It adds up quickly.
That's assuming the larger wheels weigh the same, which is often impossible. With a larger diameter wheel comes exponentially more surface area needed to create the outer edge of the rim, which is the worst possible place to add weight. Increasing wheel diameter AND increasing weight (even if only a modest amount) will produce rather noticable drawbacks- you can loose 1-2 car lengths (or more) in a 1/4 mile race. Going with wider wheels raises the amount of metal required too, although it does so only linearly.
However, the largest contributer on the entire car to rotational inertia is the tire. Tires are even further out from the axle than the wheel, and usually weigh more too. Some street tires weigh as much as 4 pounds less than their competitors through the use of lightweight materials. Hoosier racing tires, until recently, were made of fiberglass belts instead of steel for the purpose of weight savings (until new regulations prohibited this). That mere four pounds per tire extra will require about the same amount of force to spin as it would take to carry a date riding shotgun! A small difference in tire weight can make a large difference in rotational inertia.
This is a compelling reason to run smaller diameter tires, as larger diameter tires have more inertia per pound and are heavier due to the increased amount of rubber. The weight and inertia savings of going to a skinnier tires is comparitively smaller than decreasing diameter.
Of course, going to a skinnier, softer wheel and tire combination can sacrifice handling (detailed in another article), so it's up to the tuner to find the proper balance for their car.
Do the Math
Andy Welter has created a spreadsheet for calculating the gains or losses of various wheel and tire combinations, found here:
http://www.mazda6tech.com/files/rotational.xls
Kevin K has revised the original spreadsheet to more correct specifications. Remember that the weight calculation is PER wheel/tire.
http://www.mazda6tech.com/files/wheel_inertia.xls
Article is from: http://www.mazda6tech.com/index.php?...d=16&Itemid=50
Last edited:
Red
FRF Addict
So going to 37's is not worth the loss in performance?
Cleave
FRF Addict
either way you're losing acceleration, but with the lighter 37s, you're going to have better handling characteristics and more ability to soak up what the road throws at you, as well as most 37s having a wider footprint than most 35s, which will help with traction and handling as well
MagicMtnDan
FRF Addict
OP
OP
A.I.I.Raciing
Dezert 8MYMNEY
I think I should have been more clear, the reason for my ? is in regards to my Sport Trac that has very little power right now compared to a Raptor
So the theory of doing a gear change when going to a bigger tires isn't so much the factor, it's the added weight of the tires.
My brain hurts now, I had never thought about how changing wheel size would effect the truck so much, I went from a 15" wheel to a 17" and the tire size went from tiny to a 35", so I probably added 70lbs at each corner. But I did change gears from 3.55 to 5.00
I changed gears in the 9" and went with a 5.0, but never considered the weight of the General's when deciding on what to run and this is why I sold them. Now that I'm looking for new tires I'm starting to consider 37's if I can find a light tire
So the theory of doing a gear change when going to a bigger tires isn't so much the factor, it's the added weight of the tires.
KaiserM715
Kaiser Söze
It is both. Switching to a taller tire is the equivalent of switching to a taller gear (numerically lower). A lower gear (numerically higher) will be needed to to get the power band back where it was. I will use the Raptor as an example. The original combo is 35s with 4.10 gears. To have the powerband exactly where it was designed, 4.30s would be used with 37s. If you were planning on running heavier 37" tire, one may consider going even lower to something like 4.56 for the power to "feel" the same. In addition, a numerically lower gear will have "snappier" acceleration (drag racers all go to a lower gear to get better acceleration).
Note: the Raptor has the benefit of a lot of power and torque, so most folks don't bother with the expense of a gear change, with minimal effect.
In your case on the SportTrak, in addition to the height, there is a big difference in weight / rotating inertia with your current setup vs stock. Looking at some quick numbers, the difference in tire size would lead you to run a gear somewhere in between 4.56 (35's) and 4.88 (37's). Add in some more gear to make up for the increase in rotating weight and vehicle weight along with the stock engine power, 5.00ish sounds about right.
---------- Post added at 08:26 AM ---------- Previous post was at 08:25 AM ----------
Also, more sidewall = more suspension / better ride quality (tires are part of the suspension).
I'd definitely go with the 37's. your straight acceleration may be the same (I bet the 37's will accelerate faster because of so much less weight) but the other pros for 37's make it better.
You've already done the hard part and put the correct gearing in, now put the lighter tire on. Probably get better fuel mileage too. What tire is it? My guess is it's Projects with that weight.
You've already done the hard part and put the correct gearing in, now put the lighter tire on. Probably get better fuel mileage too. What tire is it? My guess is it's Projects with that weight.
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