CO track thread

[blefevre]

Stupid work computer froze. Can I rotor share hps and pfc06?

[Bimmerman325i]

Geez, I come on here in the morning to brain dumb and prepare for engineering at work. Now it's engineering here and at work. Bah, time to work I guess.

It's kinda sad that this e-spat is the most engineering I've done this semester. Oh well, month and a half till graduation.

[Bimmerman325i]

Quick question. When you did this, did you take the weight of the tire and make it a point mass at the OD of the wheel or maybe the OD of the tire? Or did you make that part of the "ALL wheel mass is located at the wheel radius"?

Sorry, was early and wasn't explaining as best I could. I assumed m_rim + m_tire is a point mass at r_wheel. Not the best analysis, but as you say below, best possible with a quick and dirty non-ugly-integral method. More accurate would be m_rim point mass at r_rim + m_tire point mass at r_tire (same as r_wheel), but parallel axis theorem and I had words at o'dark thirty.

Not the best assumption, but in order to do it the correct way you would have to assume uniform density throughout the wheel (and thickness, which is silly) and then have some crazy integral with the bounds being the wheel radius and the wheel OD. That seems impossible with information you have, so your methods are about the best that you can do for now. But, just think about that in reality, your basic momentum analysis would show that more of your outcome would be affected (effected? I hate english) by the OD of the tire because in reality the wheel weight is not a point mass at the wheel radius. Some of that weight and therefore inertia and therefore torque to stop it is located away from the wheels radius.

Agreed, but think about it for a moment. If my simplistic analysis makes OD a bigger factor than it normally should be compared to wheel weight(which is still the more dominant factor), in reality wheel mass is going to have an even greater effect than the tire OD.

Anyways, not saying you are wrong in any way, you did the best you could for the available resources/information/knowledge, but I think in reality the OD of the tire would have more of an impact than you think. Also, as your conclusion stated that adding mass is the biggest impact (which I'm sure we all agree, unsprung mass...), did you take into consideration the weight of different tires? That plays a huge roll in the torque needed to stop because that is a large mass at the wheels OD! Lots of interta!

I did. I used the quoted 22lbs for the 225/45/15 RS3 and the 24 lbs for the 235/40/17 Z1 SS.

The OD does have some small effect on required braking force and energy, but it is at least 3x smaller than the effect from a change in mass. I'm not arguing that OD is irrelevant (anymore), but I'm arguing that it isn't the factor to fret over.

[blefevre]

Geez, I come on here in the morning to brain dumb and prepare for engineering at work. Now it's engineering here and at work. Bah, time to work I guess.

[MisterMiniS]

Quick question. When you did this, did you take the weight of the tire and make it a point mass at the OD of the wheel or maybe the OD of the tire? Or did you make that part of the "ALL wheel mass is located at the wheel radius"?

ALL wheel mass is located at the wheel radius.

Not the best assumption, but in order to do it the correct way you would have to assume uniform density throughout the wheel (and thickness, which is silly) and then have some crazy integral with the bounds being the wheel radius and the wheel OD. That seems impossible with information you have, so your methods are about the best that you can do for now. But, just think about that in reality, your basic momentum analysis would show that more of your outcome would be affected (effected? I hate english) by the OD of the tire because in reality the wheel weight is not a point mass at the wheel radius. Some of that weight and therefore inertia and therefore torque to stop it is located away from the wheels radius.

Anyways, not saying you are wrong in any way, you did the best you could for the available resources/information/knowledge, but I think in reality the OD of the tire would have more of an impact than you think. Also, as your conclusion stated that adding mass is the biggest impact (which I'm sure we all agree, unsprung mass...), did you take into consideration the weight of different tires? That plays a huge roll in the torque needed to stop because that is a large mass at the wheels OD! Lots of interta!

[Bimmerman325i]

So I did a basic momentum analysis, and it pointed out that both Sam and I are right. Changing OD DOES have a small effect, but changing mass has a 3 times greater effect with respect to total system momentum (car + wheels). This is using a 2800lb car, 70mph constant speed, and the different tire ODs and weights (tirerack source) and rim weights, and the assumption that ALL wheel mass is located at the wheel radius.

[Roland H]

I have distraction problems, bad.

Truth.

[iamsam]

I'll have to think about this some other time, i am getting no HW done.... sheesh

I have distraction problems, bad.

[Bimmerman325i]

I wasn't talking about rotational inertia of the wheel at all. It seems to me rotational inertia of the wheel itself is insignificant in braking. Try lifting the rear on jackstands, do 100mph, tromp on the brakes. How long does it take to stop the wheels from spinning?

What property of a spinning wheel would increase the amount of time it takes to stop? Rotational inertia.

You don't think you're talking RI, but you are. It's the main thing that matters for braking and accelerating, and is the reason lightweight wheels are so highly sought after. Effects from OD are far less in comparison.

I was talking about the inertia of the car, which translates into turning the wheel through friction with the pavement, which is what the the brakes have to fight against. Wheel mass itself barely enters into it. Assume the wheel is weightless. What are the variables in this scenario? Where the brake caliper is radially with respect to the centerline of the wheel, OD of the tire, car speed, car mass, brake pad compound, etc. Again, assume the wheel is weightless. Which will require harder braking, Ben's car on wheels the size of skateboard wheels, or Ben's car on wheels the size of bicycle wheels, all other variables held constant?

You're right in that OD affects things, but the rotational mass is the dominant factor affecting the energy required in braking (and force required to decelerate the rotating mass).

If the wheels are weightless and all variables are the same, the larger wheel will take more force to stop.

See below post for clarification, I did some math (and more engineering work than I've done all semester), and have edited my post.

[iamsam]

I wasn't talking about rotational inertia of the wheel at all. It seems to me rotational inertia of the wheel itself is insignificant in braking. Try lifting the rear on jackstands, do 100mph, tromp on the brakes. How long does it take to stop the wheels from spinning?

I was talking about the inertia of the car, which translates into turning the wheel through friction with the pavement, which is what the the brakes have to fight against. Wheel mass itself barely enters into it. Assume the wheel is weightless. What are the variables in this scenario? Where the brake caliper is radially with respect to the centerline of the wheel, OD of the tire, car speed, car mass, brake pad compound, etc. Again, assume the wheel is weightless. Which will require harder braking, Ben's car on wheels the size of skateboard wheels, or Ben's car on wheels the size of bicycle wheels, all other variables held constant?

[Bimmerman325i]

FWIW, I ran HP+ on my E30 and loved the pads on the street and the limited track I did (I used too much brake and ended up cooking the fluid), and I just swapped the HT10s on my E36 for HP+s. They are an amazing street and autox pad, but they are limited as a track pad.

[Bimmerman325i]

David, i was comparing MY wheels/tires to Ben's 17s. Mine had a smaller OD.

Also, I am pretty confused at your reasoning. Why are you comparing the OD of a tire to the dia of a rim?

Also, to stop the car, the wheel DOES apply torque to the brakes, or more specifically, the brakes need to supply a torque to the wheels to slow them down. With bigger OD, the torque needed is much higher, even if rotational mass was held constant. If you had reeealy tall skinnies like bike tires (hypothetically), you would need alot more braking power for the same car, and same size brakes, and while retaining same rotational mass. Every action has an equal and opposite reaction...

disclaimer: I didn't do a whole force analysis on this, I am just thinking through this atm with just intuition.

I'll explain more at work.

The OD doesn't have any effect on how easy/difficult it is to slow the car down. The reason I was talking about rim diameter is the very simple rotational inertia equation-- mass further out from the center of the wheel is bad, and requires more force to stop. THAT is the issue. NOT OD.

What you are calling Braking torque is really just countering rotational inertia, and again, OD doesn't matter. Technically, braking torque is the braking force applied at the radius of the rotor. That's it. You are referring to the amount of braking required to counter the rotational inertia, which is very much not braking torque.

Remember basic physics and moments of inertia? That is the issue. OD of the wheel and tire combination does not have an effect on braking (has one on gearing) because most of the weight of the setup is not at the very edge. If you put a 215/40/17 tire on Ben's 17" wheel setup, which is stock tire diameter, the issue would still be there, because it is weight distribution, which affects rotational inertia, and the required retarding force to counter it. OD doesn't come into play whatsoever

I can explain more at work.

[Bishop]

Why not remove shim, track, install shim....

[fsmtnbiker]

HP+ aren't bad, but they're not good at being a street pad or a track pad - You suffer in both venues with them. They dust, squeal, and aren't particularly easy on rotors on the street (although they aren't bad), and they overheat quickly on track. I run HP+ in my M3 mainly because I love the initial bite and very strong torque they generate. They are prehaps the perfect autocross pad.

Basically, it comes down to this: Once you drive with real track pads, you'll understand why everyone who *has* used track pads doesn't ever go without them after that. Having a strong, consistent braking system for 30 minutes at a time is a HUGE confidence boost and allows you to focus on other, much more important aspects of driver improvement.

[iamsam]

How are you going to separate the nylon once its melted to the pad and around the piston?

heh, a razor blade may come in handy here.... I used a razor blade to remove all my old melted pad-quieting goop after BMW driving school to install the nylon shims. maybe putting a thin layer of pad quieting goop under the nylon may make them easier to remove...

They aren't bad on the E30. I wouldn't ever run them on teh E36, but I would consider running them on an E30 if I had one. But honestly, nothing beats the right pad for the right use. Just like snow tires for the snow, and sticky tires for events.

Hey, I can say from experience, I couldn't have hoped for better performance out of my HP+ pads when I was at HPR. But that is my specific situation, take from that what you like.