LVDell

Thanks for the calrification Larry. I don't disagree with you. I just had my physics fouled up. My thought process was from feel DURING trailbraking rather than straight line into trailbraking. I thought (obviously wrong now) that the car would pitch forward less with more rear bias thus leaving more grip on the rear tires. My choice of wording saying there would be more "squat" was a poor choice but I still thought I understood the physics of the brakes a bit better. Does what I just explained make sense? It seems to be garbled in my head.

Larry Herman

I understand where you can get confused, but taking any strange suspension geometry out of the equation, your car will pitch forward more with more rear brakes because it is stopping harder, resulting in more weight transfer.

Now when you turn in while on the brakes, the greater braking force (and even less weight) on the rear tires will cause them to have less lateral grip.

LVDell

Thanks again Larry. Damn....I really do have a lot to learn and will be the first to admit my mistake when wrong. Now I understand a bit more clearly the braking issues. I envy your boys, they have a great teacher

mfennell

And remember that the pitching is an effect caused by weight transfer, not the transfer itself. A kart still transfers weight under braking and turning but (relatively) doesn't pitch or roll at all.

Larry Herman

True, it is the compliance of the suspension that determines the amount of body movement.

Chris L.

Dang...did not think my little thread would generate such a discussion on physics.

I am just a confused MD looking for some reassurance on these PFC pads my shop wants me to try. Thank goodness you guys are around to help.

RJFabCab

Larry, I follow your thinking and here's my take on Chris' situation to further this excellent discussion.

I agree that a tire has a limited amount of traction force that must be divided between accelerating, braking, and cornering. This limit is load dependent and variable due to such dynamics as tire deflection, weight transfer, and suspension interaction as you mention.

In general, I'd say that you can slide a tire by 1) pushing hard on it or 2) by causing weight to transfer off of the tire.

Chris' description of the 'bite' up front from the RS14 makes me think that weight transfer is one of the root issues of his over rotation while trailbraking. His braking forces are creating enough weight transfer to the front axle creating a 'loose' rear end due to the relative unloading of the rear axle. This combined with a little steering input, even when trailing off the brake, leads to the over rotation. The static coefficient of friction remains the same, but the forces required to slide the rear are reduced due to the unloading. The extreme example of this that comes to mind is Chris doing an endo in the parking lot.

So, my thinking is that going with RS19 up front and keeping the RS14 in the rear, as he described, would help shift some of the braking bias to the rear. Thus, under braking the front axle is now working a little less hard creating a smoother forward weight transfer, less pitching and 'endo effect', which would help keep the rear more evenly loaded under braking allowing for more cornering grip and less over rotation.

LVDell

My brain now officially hurts

mfennell

These are really the same thing if you think about it. Both are "pushing too hard for the available traction". The traction available at each wheel is constantly changing and as you transfer weight off, the available traction goes down.

Absent any weird alignment issues, my gut feeling is that this is not the problem. The front tires are being asked to turn and brake. The rears have it pretty easy, relatively speaking, if we agree that the brakes are biased too much towards the front.

Lots of things can cause a loose car on corner entry. Aggressive downshifting gives a lot of engine braking, which is just like rearward brake bias except you can't modulate it. An unexpected one is that releasing the brake too quickly (often coinciding with an agressive turn-in) can cause an oversteer condtion:
This is somewhat counterintuitive but I had this problem when I raced my little SpecRX7. I was snapping off the brake as I bent into the corner, causing myself all kinds of excitement. Learning to transition off the brakes more smoothly allowed me to set the car up more neutrally, not get loose on corner entry, and go faster.

With more aggressive rear pads but still using exactly the same braking markers, the weight transfer will be unchanged. It doesn't matter how the braking force is distributed, the loading will be the same. When you turn in under braking, the rears will have less traction available for cornering and the car will be more willing to rotate.

More likely, shifting the bias back will let you brake deeper and harder, transferring more weight to the front. They will be able to work harder before hitting ABS and the rears will have even less load on them.

Better brake distribution = stronger braking = more weight transfer = fronts work harder and less weight on the rears.

Remember that the car pitching forward is the reaction, not the action.

I edited this several times. I hope I added clarity rather than removed it.

RJFabCab

I agree that the available traction in both instances is being exceeded, but the 2 conditions are very different. For example, setting off front ABS under hard straight line braking is exceeding the limit under a loaded axle ('pushing the tire') versus something like acute oversteer resulting from suddenly lifting off the brake at speed as you enter a corner (transferring weight off of the tire).

Assuming the same rate of deceleration, if the rear brakes are now doing more of the braking duties, the weight transfer may be largely unchanged, however, the front axle distribution of this weight transfer should be lessened. One of the advantages of the 996's heavy rear end and fat tires is to create more even distribution to all four contact patches under hard deceleration.

I understand your perspective. I'm coming from the 'loading' end of Chris' issue. If you take the bike example and travel at a fast rate of speed, there should be less 'endo effect' if you brake both front and rear wheels versus just using the front brake even though the rate of deceleration is the same. Adhesion limits are load dependent.

I agree that shifting the bias back will allow deeper and harder braking.

These are all good points. It really depends on what's creating Chris' issue. Is the rear getting light under hard braking and turn in due to unloading or will biasing the brakes to the rear worsen the situation by stealing from some of the cornering grip of the rear due to the increased braking forces?

mfennell

In both cases, you tried to impart a force to the tire that it was not able to handle. In the braking case, it is highly loaded so it can take a LOT more force. In the turn-in case, the lightly loaded tire is unable to handle a small amount of force.

I'm happy to agree to disagree here though.


The first part of the sentence conflicts with the second. Maybe you meant to say the distribution of the braking forces? In a straight line, changing around the brake distribution will not affect the weight transfer*. The front axle "receives" the same amount of weight.

* There suspension geometry has a slight effect on the CG and therefore weight transfer as we muck around with the brake bias that we will consider to be in the noise (which is to say: I don't know enough to comment intelligently) for this discussion.

Agreed. The CG is probably pretty low for a street car too.

I don't think the bicycle is a good equivalent. The wheelbase is very short with respect to the size of the wheels so the torque applied to the frame through the wheels could become a significant component. I ride quite a bit and like to stand my mountain bike on its nose with some frequency (if not much skill) but I've never thought to see how the rear brake would effect things. A test is in order...My opinion:
If the car is already oversteering on turn-in, more rear brake will only make it worse.

Wildcard: a difficult to modulate or grabby pad could easily make things worse even if, overall, it provides less ultimate friction. That's one of the theoretical gotchas when you start mixing pads although I never had it happen in my (limited) experience.

Larry Herman

Time to shake it up a little. I think that part of the difficulty here is that you guys are thinking statically, while things are happening dynamically. You have to take time into account. What mfennell quoted "A sudden, sharp reduction in braking effort delivers instant cornering traction to the front of the car by suddenly giving all of the front tires' grip over to cornering force." is true, but only for a moment in time. Once the suspension "figures out" that you have taken your foot off of the brake, and uncompresses, you will get understeer. You have to take the whole dynamic into account, not just freeze a tiny portion of it and use that as a basis for understanding.

mfennell

Fair enough criticism but there are a whole lot of things happening dynamically that aren't necessarily important to understand the problem IMHO. It's probably safe to assume, for example, that the roll centers in these cars don't move around too much, right?

That's a small consolation when I'm looking out my driver's window to see where I'm going.

FWIW, I'll put the quote in, to add context:
(pg 88, Going Faster, Carl Lopez.)

The first time I read that was a eureka! moment for me as it described precisely what I was experiencing. Indeed, in slower corners it sorts itself right out. In faster ones, close to the limit, the experience was a little more attention getting.

Something occurred to me. Chris L. mentioned instability under braking. It seems that if the rear bias is way too low, you could have the back end of the car (with that engine hanging way off the back and tossing its opinion into the fray) trying to pass the front under hard braking. In that case, things are well unsettled before even turning in. Maybe that's what RJ was thinking. [stretching]If it's really low, it may not even overcome the inertia of the engine & transmission.[/stretching] Chris is running slicks, no? He must get a ton of braking regardless.


Thought provoking thread. I must log off now.

Chris L.

Gentlemen...I am truly amazed at the wealth of knowledge that has been exposed via this thread. I truly stand in awe as I read through this.

I must say that I am a simple man...so in simple terms I will try and sum up the whole picture for me.

Go to shop. Shop man say "you better off with PFC pads." Stupid man say, and ask for advice on forum because he

He say to wise men "me likey go fast. Me drive GT3. Me run RS-14s on all four legs. Me have slicks with much aggressive alignment. At end of straight going wicked fast near 155 on front and 165 on back me hit brakes cause no want to end up in tire wall. In a straight line the car begins to wiggle quite a bit from side to side. Not enough to cause me to make extreme corrections, but a handful nonetheless. Me say this problem #1.

I learned from David Murry to apply whatever maximum braking power you need on a scale of 1-10 based on the speed you are carrying, down to a 3. Always be at a 3 when you initiate turn in. Trail the brake from 3-2-1 and off brake for only the briefest portion of time before throttle application occurs prior to the apex as throttle goes 1-whatever on up and gradual unwinding of the wheel occurs.



When stupid man do this back end of beast not so much want to dance with front end, as I am at this magical braking number of 3 so I am not carrying so much speed at this point as far as coming off of the long straights is concerned but rather beast seems slow and lazy. Thus I believe the hard initial bite is causing me to over brake for sure coming into many of these turns. Yes I know I could compensate for this by braking later. I am sure that is part of the progression in my driving, but the Synergy guys are telling me that they feel the PFC pads offer their drivers more progressive braking as opposed to this all or none. This along with me growing bigger b___s may help.

Stupid man make up for slowing beast too much by making beast go faster before next turn. Turn come much quicker than stupid man realize and must kick beast to slow. Here is where I wonder if I am doing what David taught me as well as I should. I am seeing a progressive increase in my corner entry speeds in these corners not coming off of the long straights. Here is where I feel the car "over rotate", at least that is what I call it, and I wonder now whether that might be because of the same hard initial bite of the RS-14s. Or...could it be that I sense a need to apply more brake application than is necessary to reach the magical #3 for turn in? I am trying to be gentle with application and come in at say a 6 or 7-6-5-4-3, but am I really?

Stupid man say thank you again to all ancient warriors who have fought battle in past and now help make me strong.

LVDell

Chris, explain what you mean by the braking David taught you? In the past I went hard on the brakes (threshold) and then came off them progressively so that I was basically 100% off the brakes just after turn in and then start applying throttle to power through the apex. I have since changed my braking strategy to almost that it looks like a "normal curve" (think stats class) whereby I start to brake a little sooner and then quickly move to threashold then back off. This has helped me be much more smooth and take away the tail waggle I feel pushing 160 on the back straight at VIR when I hit the braking zone at full threashold braking like I did in the past.

Did that make sense?