Unsprung weight vs Traction?

[Special Guest]

I’d like to get input from you much smarter than I people. Laugh if you must, but my weird brain gets in the weeds thinking about 5 pounds here or there in a vehicle weighing over 4000lbs. Specifically, in an AWD vehicle, where would the line of benefit fall when fitting wider tires, although adding unsprung weight but increasing the car’s footprint, better traction, let’s say in trying to get the best 1/4 mile time?

I know there are A LOT of other factors, and I like discussing those. Yes, I was literally debating 10” wide wheels instead of 10.5” because they would be lighter(in the same style) and, since I don’t like the stretched tire look, felt more comfortable staying with the stock 275/30/21 on the 10”. But then I wonder if, as the power increases, would the extra width of a 285 or 295 make a difference that outweighed the extra weight at the worst position.

Curious how you all navigate this when choosing wheels and if there is any data showing real world differences.


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[Hurricane17]

If you aren’t currently spinning the tires, making them wider is not going to benefit you in straight line traction. You need to have a loss of traction in order to have a need to increase it.

[paul-g]

so one deciding faster is what is car used for? do you plan on have a track set of wheels and tires? then with out spending gobs of money get a set of 18 or 19" wheels that are light, and stick a sticky tire on it. if you don't want that again look for a light set of 21" wheel (will be pricey) and put stickier tires on it. sure you can go wider why not, as long as they fit right. as for the weight between 285-295 i bet it's marginal at best, make sure when you do your 1/4 times have less than 1/4 tank that should offset that extra weight. again keep same size tire buy something stickier...unless your doing some crazy mods and adding a ton of HP i doubt your spinning much if any

[Special Guest]

If you aren’t currently spinning the tires, making them wider is not going to benefit you in straight line traction. You need to have a loss of traction in order to have a need to increase it.

So following this train of though… I guess my question is referring to the comparison between the additional weight in the tire needed to be big enough to no longer break traction vs the time lost in not hooking up with smaller lighter weight tires. Sorry if I’m not explaining clearly. My uneducated guess is that slippage would always be worse than the added unsprung weight, unless you add some crazy amount of weight per wheel in order to not slip. But then I wonder if it is the added traction making the connection, or the added weight making the wheels not able to spin fast enough to slip. And I wonder if there is a calculation to help decide.


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[Hurricane17]

So following this train of though… I guess my question is referring to the comparison between the additional weight in the tire needed to be big enough to no longer break traction vs the time lost in not hooking up with smaller lighter weight tires. Sorry if I’m not explaining clearly. My uneducated guess is that slippage would always be worse than the added unsprung weight, unless you add some crazy amount of weight per wheel in order to not slip. But then I wonder if it is the added traction making the connection, or the added weight making the wheels not able to spin fast enough to slip. And I wonder if there is a calculation to help decide.


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Weight doesn’t help with traction. Extra weight is extra weight, no matter where it’s at.

[Special Guest]

so one deciding faster is what is car used for? do you plan on have a track set of wheels and tires? then with out spending gobs of money get a set of 18 or 19" wheels that are light, and stick a sticky tire on it. if you don't want that again look for a light set of 21" wheel (will be pricey) and put stickier tires on it. sure you can go wider why not, as long as they fit right. as for the weight between 285-295 i bet it's marginal at best, make sure when you do your 1/4 times have less than 1/4 tank that should offset that extra weight. again keep same size tire buy something stickier...unless your doing some crazy mods and adding a ton of HP i doubt your spinning much if any

I get what you’re saying , though I also put it into the other factors category. But to try and answer, in the scenario in my head, I would say the purpose is best 1/4 mile time. I’m also focusing on the same brand and type tire only in different sizes to take the tire compound itself out of the equation since it would be the same in all sizes. To your last point, let’s say you add a lot of horsepower and start slipping, how many millimeters need to be added to the tires to connect again? And where is the curve where the added horsepower negates the negatives of the added unsprung weight? Sorry, this is how it bounces around in my head and I can’t find a calculator, which I think would be helpful since buying and trying tires would be so expensive for me, at least.


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[Special Guest]

Weight doesn’t help with traction. Extra weight is extra weight, no matter where it’s at.

Here’s where my ignorance announces itself. I’m not thinking of the extra weight as helping with traction, but actually hurting performance to the point where the car can no longer spin the heavier tires hard/fast enough to break traction because of the added weight? Again, I can be completely wrong on how this actually works.


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[Hurricane17]

I get what you’re saying , though I also put it into the other factors category. But to try and answer, in the scenario in my head, I would say the purpose is best 1/4 mile time. I’m also focusing on the same brand and type tire only in different sizes to take the tire compound itself out of the equation since it would be the same in all sizes. To your last point, let’s say you add a lot of horsepower and start slipping, how many millimeters need to be added to the tires to connect again? And where is the curve where the added horsepower negates the negatives of the added unsprung weight? Sorry, this is how it bounces around in my head and I can’t find a calculator, which I think would be helpful since buying and trying tires would be so expensive for me, at least.


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Racing is expensive. Welcome to the world of racing. It’s all trial and error typically, especially if someone else hasn’t already done it before you. Luckily there are forums that you can learn from now.

As to your original question, I think you’re getting down too far in the weeds with your thoughts. A sticky tire that’s narrow, will usually outperform a wider tire that isn’t sticky. Tire compound is typically the best way to get more traction “on the cheap”.

[A665]

Yes, I'm laughing at the idea of worrying about a few pounds on a car that weighs 4,000+ pounds.

[MattyMarkey]

Yes, I'm laughing at the idea of worrying about a few pounds on a car that weighs 4,000+ pounds.

You always have the best comments that really add to the conversation and that people value...

For most people, it's not worth diving this deep, but it clearly is keeping him up at night. People have seen straight-line data gains - yes, marginal - when reducing unsprung weight; whether that be upgrading brakes or lighter wheels. Wider wheels on these cars don't always mean increased traction and performance, especially for a straight line. There have been some cases where wider wheels can actually increase the chance of wheel hop.

Curious to hear your specific knowledge and expertise on this topic though, A665

[A665]

OP said we can laugh if we want, and I find this subject matter amusing. I think some people might be more satisfied racing with a completely different vehicle than a C7.
Carry on.

[Special Guest]

As to your original question, I think you’re getting down too far in the weeds with your thoughts.”.

Thanks, and you couldn’t be more correct. Pondering 2 extras lbs at the circumference, an added 6mm diameter, 10mm of thread, keeps me up at night, all the while fully aware that the biggest factor in my wheel decision will come down to if I like how they look.

That being said, the less crazy is that I do have goals to increase power to the point where other members have had traction issues, so I’m trying to calculate and plan for that issue now on a daily drive vehicle, making my solution width instead of slicks.


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[Special Guest]

Yes, I'm laughing at the idea of worrying about a few pounds on a car that weighs 4,000+ pounds.

You are not alone. I was talking with the wheel manufacturer and he stopped me and said, “Dude you’re talking about 5 pounds in a 4000 pound car.” I’m laughing at it too, but the curiosity hasn’t diminished.


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[Special Guest]

There have been some cases where wider wheels can actually increase the chance of wheel hop.

Curious to hear your specific knowledge and expertise on this topic though, A665

Thank you for this! I’d never heard this which adds even more to the equation. I have something else to look up now. Especially since I’ve been down almost two months with a bad rear diff. Wheel hop, I’ve read and been told, can cause all sorts of drive train catastrophes.


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[Alabama]

For most people, it's not worth diving this deep, but it clearly is keeping him up at night. People have seen straight-line data gains - yes, marginal - when reducing unsprung weight; whether that be upgrading brakes or lighter wheels. Wider wheels on these cars don't always mean increased traction and performance, especially for a straight line. There have been some cases where wider wheels can actually increase the chance of wheel hop.

Way beyond me; I only know (maybe) what I read. I thought the goal for both acceleration and handling is to start with traction. First make sure one has maximum traction. Then try to minimize unsprung weight without sacrificing any traction. Lower unsprung weight helps keep the tires planted when the road is bumpy. I also thought while it's not intuitive, unsprung weight is not really related to the total weight of the car; that's why folk spend big bucks on very light (alloy vs. steel) wheels even when the total car weighs much more. Does any performance car use steel wheels anymore?

[MattyMarkey]

Way beyond me; I only know (maybe) what I read. I thought the goal for both acceleration and handling is to start with traction. First make sure one has maximum traction. Then try to minimize unsprung weight without sacrificing any traction. Lower unsprung weight helps keep the tires planted when the road is bumpy. I also thought while it's not intuitive, unsprung weight is not really related to the total weight of the car; that's why folk spend big bucks on very light (alloy vs. steel) wheels even when the total car weighs much more. Does any performance car use steel wheels anymore?

It could be cause it’s late, but I’m not totally picking up what you’re putting down. It’s likely just me lol.

Reducing unsprung weight will likely always help with acceleration, until you start increasing power. That’s when traction comes into play. A wider contact area doesn’t necessarily mean more traction, since you also want more pounds per square inch; which thinner tires have. Tire compound and ratios matter greatly when you increase width. The weight of the wheels, tires, brakes and sprung weight can help compensate for more contact area.


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[Alabama]

Reducing unsprung weight will likely always help with acceleration, until you start increasing power. That’s when traction comes into play. A wider contact area doesn’t necessarily mean more traction, since you also want more pounds per square inch; which thinner tires have. Tire compound and ratios matter greatly when you increase width. The weight of the wheels, tires, brakes and sprung weight can help compensate for more contact area.

I'm on your side; many factors that inter-relate. The main point I was getting at is that reducing "unsprung weight" is an issue specific to how the weight of those components affects how that part of the suspension works, not as a percentage of the total car weight. Suppose I have a car weighing 4,580 pounds total, 3,500 in "sprung weight" and 1,080 in "unsprung weight". If I reduce 20 pounds at each corner, the total weight goes from 4,580 to 4,500, a difference of 1.75%. But, the difference in unsprung weight is 7.5%. As I understand it, that's the difference folk care about.

[MattyMarkey]

I'm on your side; many factors that inter-relate. The main point I was getting at is that reducing "unsprung weight" is an issue specific to how the weight of those components affects how that part of the suspension works, not as a percentage of the total car weight. Suppose I have a car weighing 4,580 pounds total, 3,500 in "sprung weight" and 1,080 in "unsprung weight". If I reduce 20 pounds at each corner, the total weight goes from 4,580 to 4,500, a difference of 1.75%. But, the difference in unsprung weight is 7.5%. As I understand it, that's the difference folk care about.

I am with you now! Sorry you had to explain that a bit more - it was definitely a me thing

[exorcet]

You are not alone. I was talking with the wheel manufacturer and he stopped me and said, “Dude you’re talking about 5 pounds in a 4000 pound car.” I’m laughing at it too, but the curiosity hasn’t diminished.


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Well, 5lb of unsprung weight is equivalent to dropping 60 pounds of static weight. Across 4 wheels dropping a total of 20lb unsprung, that's equivalent to 240lb static weight. Every 100lb of static weight is approximately a 1/10 second reduction in quarter mile times. So these "just 5lb in 4000lb car" wheels theoretically are the difference between a 12.1 second quarter mile and an 11.86 second quarter mile.

Speed costs money. How fast do you want to go? Squeezing out those last few tenths gets exponentially expensive. Most people are happy with just an ECU tune and getting 80% of potential gains at 20% of total expenses. I understand the C7 may not be the ideal car to throw a shit ton of money into chasing those 1/10th of a second, but people also build jet engine school buses setup to wheelie down the entire quarter mile. So..it's your money, spend it however it makes you happy.

[paul-g]

the really big questin you need to ask yourself is.....what am i after. if your after 1/4 mile i think you need to to take that direction and really devote alot of time and money (more so money) and make that your goal. weight, power lots of things. if this is your everyday driver BUT you want it to be faster that's a whole other avenue and there can be money spent better than worrying about weight. it is an easy thing to think "same power if i can reduce weight it will go faster" ya maybe but weight reduction is very expensive for small gains, HP is a cheaper avenue and very large gains.

for me since this is everyday car in my mind is "what can i do to get the most out of my car and still get decent mileage but hurt a lot of feelings" well right now Stage 1 tune and TCU tune and to my request it does, car is totally different and you'd be surprised on what that little can do. Now for me stage 2 would be great BUT removing front bumper cover and all that is a lot of work and again is the juice worth the squeeze....yes but just not right now. when i get bored with it is when that changes.

[Mark.in.tulsa]

Slightly on topic what is the rough delta between factory rims and "light weight alloy" rims that still look good.

[MattyMarkey]

Slightly on topic what is the rough delta between factory rims and "light weight alloy" rims that still look good.

Depends on the OEM wheel and light-weight wheels, but probably anywhere from 2-8lbs per wheel.

My 21” Rotors weigh ~34lbs. There are forged 20” OEM wheels that are around 28-29lbs


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[sepheroth86]

I think the RS7 blade wheel is one of the heaviest.

[Alabama]

Depends on the OEM wheel and light-weight wheels, but probably anywhere from 2-8lbs per wheel.

My 21” Rotors weigh ~34lbs. There are forged 20” OEM wheels that are around 28-29lbs

This is a good illustration for OP's question and the folk who are side stepping the issue by focusing on horsepower. APR markets a forged wheel for my 2014 S6 (20 x 9 which is I believe the OEM size) that they claim weighs only 22 pounds. Suppose I'm traveling and hit some west coast potholes, damaging my wheels so I need new ones. If I like the appearance of two competing wheels that are about the same price and only differ functionally in weight, if I care about OP's issue I should buy the wheel that weighs less.

[Special Guest]

Slightly on topic what is the rough delta between factory rims and "light weight alloy" rims that still look good.

I had a bent stock 21" wheel and weighed it when I took it in to get fixed. 59.9 lbs with Perelli 275/30 tire. I'm waiting for wheels to arrive to do a weight comparison between the stock, forged aluminum, and forged magnesium. Problem is, although they are all 21s, they are all different widths and designs.

Stock Wheel+Weight.jpg

[Special Guest]

the really big questin you need to ask yourself is.....what am i after. if your after 1/4 mile i think you need to to take that direction and really devote alot of time and money (more so money) and make that your goal. weight, power lots of things. if this is your everyday driver BUT you want it to be faster that's a whole other avenue and there can be money spent better than worrying about weight. it is an easy thing to think "same power if i can reduce weight it will go faster" ya maybe but weight reduction is very expensive for small gains, HP is a cheaper avenue and very large gains.

for me since this is everyday car in my mind is "what can i do to get the most out of my car and still get decent mileage but hurt a lot of feelings" well right now Stage 1 tune and TCU tune and to my request it does, car is totally different and you'd be surprised on what that little can do. Now for me stage 2 would be great BUT removing front bumper cover and all that is a lot of work and again is the juice worth the squeeze....yes but just not right now. when i get bored with it is when that changes.

Knowing how much thought I'm putting into a two pound difference in tire weight, I won't burden you with the thought process with the questions you mention. Trust me, the weight/traction thought is a small part of the overall.

Well, 5lb of unsprung weight is equivalent to dropping 60 pounds of static weight. Across 4 wheels dropping a total of 20lb unsprung, that's equivalent to 240lb static weight. Every 100lb of static weight is approximately a 1/10 second reduction in quarter mile times. So these "just 5lb in 4000lb car" wheels theoretically are the difference between a 12.1 second quarter mile and an 11.86 second quarter mile.

Speed costs money. How fast do you want to go? Squeezing out those last few tenths gets exponentially expensive. Most people are happy with just an ECU tune and getting 80% of potential gains at 20% of total expenses. I understand the C7 may not be the ideal car to throw a shit ton of money into chasing those 1/10th of a second, but people also build jet engine school buses setup to wheelie down the entire quarter mile. So..it's your money, spend it however it makes you happy.

You're speaking my language. Not only the huge effect that unsprung weight has, but even more so when the weight is in the tires since that is weight farther out from the center where increases are even more detrimental. I don't have a problem with speed being expensive since I can just say no and not spend the money. That being said, I still search for some sort of guideline where sprung weight, unsprung weight, rotational mass, horsepower, torque, etc are put into an equation where we can decide if the extra millimeters in tire width are worth the extra weight. Yes I know tire compound has a lot to do with it, but assuming the different sizes are of the same compound, I remove that part.

[Alabama]

That being said, I still search for some sort of guideline where sprung weight, unsprung weight, rotational mass, horsepower, torque, etc are put into an equation where we can decide if the extra millimeters in tire width are worth the extra weight. Yes I know tire compound has a lot to do with it, but assuming the different sizes are of the same compound, I remove that part.

If you find it, please let us all know. My layperson sense is that the final equation does not exist; eventually the problem becomes trial and error. Maybe the engineers know differently?

[Special Guest]

If you find it, please let us all know. My layperson sense is that the final equation does not exist; eventually the problem becomes trial and error. Maybe the engineers know differently?

Will do. I sent a detailed email to Michelin looking for help with fitment of their product and they said they couldn’t help. However, I’m still searching. Wondering how to get the contact info for the R&D people.


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[hunter_killer]

Hopefully I’m not going to far off topic here. On the traction notion how does one combat keeping traction with air suspension that doesn’t quite keep the car planted down? IE the front end of the car picks up quite a bit. Giving some weight transfer to the back but then the front tires don’t get as much weight/ force and then you loose some traction/ torque steer. I don’t think I’ve seen anyone mention limiting straps although it seems a bit extreme.


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[Special Guest]

Hopefully I’m not going to far off topic here. On the traction notion how does one combat keeping traction with air suspension that doesn’t quite keep the car planted down? IE the front end of the car picks up quite a bit. Giving some weight transfer to the back but then the front tires don’t get as much weight/ force and then you loose some traction/ torque steer. I don’t think I’ve seen anyone mention limiting straps although it seems a bit extreme.


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So I don't have an answer, but what you bring up was part of my thought process. I didn't know that this was an air suspension issue, but in my research I did learn that the rear is heavily favored. This is when I learned that the wheels and tires all have to be the same size, well at least the same width and overall diameter within a small range of about 5mm maybe? But this adds to my issue of unsprung weight. With my probably flawed logic, seems there would be added unnecessary unsprung weight at the front since, with the AWD, we can't run a narrower tire leaving the bulk of the width, and weight in the rear.

[hunter_killer]

Running any kind of drag pack would be tough since everything basically needs to be equal so yeah just another obstacle aside from a stickier tire and small/ or light wheels. Like mentioned tire compound will help a lot but another thing about traction and power is breaking axles with this platform when/ or if you do hook up. I’ve seen some cars launch pretty even but I’ve seen other squat hard on the rear and lift the front up way to high in my eyes (my car included in the latter). So on my buddy’s E55 they’ve figured out the best way to get those cars to launch with messing around what setting to set the car up with so it’s firm but not too firm and soft enough to put some force down on the rear without getting too soft. I thought maybe there was something along those lines with this platform being it’s air ride all the way around too.


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[Special Guest]

Running any kind of drag pack would be tough since everything basically needs to be equal so yeah just another obstacle aside from a stickier tire and small/ or light wheels. Like mentioned tire compound will help a lot but another thing about traction and power is breaking axles with this platform when/ or if you do hook up. I’ve seen some cars launch pretty even but I’ve seen other squat hard on the rear and lift the front up way to high in my eyes (my car included in the latter). So on my buddy’s E55 they’ve figured out the best way to get those cars to launch with messing around what setting to set the car up with so it’s firm but not too firm and soft enough to put some force down on the rear without getting too soft. I thought maybe there was something along those lines with this platform being it’s air ride all the way around too.


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I had heard about axles breaking, but what I was told, and what was mentioned in some thread here, was that lack of traction is more likely the cause. That things should be fine if you hook up correctly, but slippage and wheel hop are what cause the problems. I haven’t experienced it, so I have no idea what is true. When I get my car back, and the mods to a respectable level, I want to test out the rear vs front and see how it differs with DRC. I haven’t started logging yet. Does logging show the distribution of power/torque being sent to each wheel? Or maybe a percentage?


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[hunter_killer]

I had heard about axles breaking, but what I was told, and what was mentioned in some thread here, was that lack of traction is more likely the cause. That things should be fine if you hook up correctly, but slippage and wheel hop are what cause the problems. I haven’t experienced it, so I have no idea what is true. When I get my car back, and the mods to a respectable level, I want to test out the rear vs front and see how it differs with DRC. I haven’t started logging yet. Does logging show the distribution of power/torque being sent to each wheel? Or maybe a percentage?


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I’ll agree to an extent but getting a good hook up will definitely show you what’s your weakest link the in drivetrain. On this it’s the axles. Other platforms it’s driveshafts. Traction puts a lot more constant stress on everything vs wheel hop is sudden. Still it’s a constant battle regardless lol I fight traction and wheel hop, so until I get my car setup better I don’t do much launching anymore. Stock axles have quite a bit of mileage on them too. Sorry I can’t weigh in on logging for showing hp/tq at the wheels but I’d say probably not.


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[widebodyfx]

So I don't have an answer, but what you bring up was part of my thought process. I didn't know that this was an air suspension issue, but in my research I did learn that the rear is heavily favored. This is when I learned that the wheels and tires all have to be the same size, well at least the same width and overall diameter within a small range of about 5mm maybe? But this adds to my issue of unsprung weight. With my probably flawed logic, seems there would be added unnecessary unsprung weight at the front since, with the AWD, we can't run a narrower tire leaving the bulk of the width, and weight in the rear.

Why can't you run a narrower tire? Have seen drag set ups with an 18/19/20 in the back. Staggered widths is done by a few owners. Performance minded chiromikey has a 265 and 305 set up, I run a 275 and 325 set up but that's cause of a fat ass.

[Special Guest]

Why can't you run a narrower tire? Have seen drag set ups with an 18/19/20 in the back. Staggered widths is done by a few owners. Performance minded chiromikey has a 265 and 305 set up, I run a 275 and 325 set up but that's cause of a fat ass.

I was told that it disturbs the AWD system. It wasn’t really explained well, just something about adjusting power based on perceived slippage. It also could be that when I mentioned it, I just asked about different sizes, without differentiating between tire width and overall diameter. They may have just assumed I wouldn’t do the math and pick sizes with the same diameter.


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[Alabama]

I was told that it disturbs the AWD system. It wasn’t really explained well, just something about adjusting power based on perceived slippage.

Speaking of which, to what extent does the electronic limited slip differential help with the traction part of the equation? At one point JXB was marketing the ability to convert the open front diff into an LSD on a C7.

[hunter_killer]

Speaking of which, to what extent does the electronic limited slip differential help with the traction part of the equation? At one point JXB was marketing the ability to convert the open front diff into an LSD on a C7.

PPF Motorsport aka Sean Civitano on FB and I know I’ve read of others blowing up the sports diff from excessive power which would guess over 600 whp/tq and a large amount of launches would do that. Just speaking on longevity but to that even I don’t think that clutch pack can handle power and help with traction after some time. Jay has converted his auto x S4 to a full front LSD and I think he still offers that conversion but a bit spendy and a big job for the most part.


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[Special Guest]

Speaking of which, to what extent does the electronic limited slip differential help with the traction part of the equation? At one point JXB was marketing the ability to convert the open front diff into an LSD on a C7.

Well this is a sore subject… So I’ve been without my car for two months now because of a blown rear diff. I have the Torque Vectoring version however. In my search for a replacement, seconding what hunter_killer said, I reached out to JXB, and he pointed me in the correct direction, but I think they weren’t seeing results better than stock or there wasn’t much demand, or a combination of the two, but that info may have come from the guys that told me he was working on one in the past. Your question is a part of the equation, better differential may mean being able to use narrower, lighter weight tires. All part of what I’m trying to figure out.


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[Special Guest]

I still haven't found an answer that helps, but I did come across some great reads that touch on some of the other factors for consideration and are helping me get a better overall picture. Some of it is pretty basic, but they do get into more depth and explain things well. I personally felt parts II and IV were the most interesting and applicable to my original question, but they all touch on it. I sent them a message to see if they can opine on the subject.

http://www.tuneruniversity.com/blog/...-size-matters/

http://www.tuneruniversity.com/blog/...formance-part/