103 replies to this topic

[Tom Hampton]

A bit surprised with the 1.6 but I don’t have any quantitative info the the the G-locs to judge just how much difference there is between 12 & 10. Don’t they publish that stuff?
Even more surprised by the NB combo despite having less experience on that platform. Danny, is that closer to what you run?

Stopping is essentially just straight-line cornering, the G’s/acceleration is just longitudinal rather than lateral. That IS the point of the classic friction circle after all. We’ll never see a circle because our engines make no power, but for cornering, braking and any combination of those the idea is the same, get all four contact patches operating very close to their limits of adhesion while maintaining control. If you rely more than necessary on the fronts then you are leaving something on the table, PERIOD, I don’t really care how you drive. Someone offer evidence, or even a good rational explanation why that’s wrong. I’m listening. There's a lot to be said for modulation and trail-braking friendly compounds but those fall under “while maintaining control” and don’t change the fundamentals.

It’s also worth noting that the flatter your car stops (meaning less nose-dive) the less, and less abrupt, the weight transfer off the fronts returning to the rear as you get off the brakes, making the transition approaching and entering corners smoother. Again, the friction circle, and keeping the work split as evenly as possible between all four tires as much as possible. It starts with physics and trying to outsmart it is almost always a losing battle. TO BE CLEAR, I’m not saying that any of the above mentioned combinations don’t accomplish that with a given setup on a given car, but I am saying that any comments about the rears not mattering much are wrong, and with a few extreme exceptions there is no need to vary that balance by track.

Hmm....I'm trying to understand the suspension dynamics you are postulating.  As far as I understand, final weight transfer (not rate of transfer, but ultimate "static loading") is a function of CG-height, spring-rate, and force (cornering or stopping).  Dive (or squat) can be controlled to a degree with anti-dive(or squat) suspension geometry, but we don't have that.   So, I don't see how changing the brake bias to more aft causes less dive, without resulting in less total braking force. Dragging a car down from the rear-axel doesn't change the weight transfer/dive characteristics.
 
For sure, if the rear tires have more grip to give at their given downforce...then more rear-bias makes sense.  But, that increase in total braking force should result in MORE dive, not less.

[Steve Scheifler]

Hmm....I'm trying to understand the suspension dynamics you are postulating. As far as I understand, final weight transfer (not rate of transfer, but ultimate "static loading") is a function of CG-height, spring-rate, and force (cornering or stopping). Dive (or squat) can be controlled to a degree with anti-dive(or squat) suspension geometry, but we don't have that. So, I don't see how changing the brake bias to more aft causes less dive, without resulting in less total braking force. Dragging a car down from the rear-axel doesn't change the weight transfer/dive characteristics.

For sure, if the rear tires have more grip to give at their given downforce...then more rear-bias makes sense. But, that increase in total braking force should result in MORE dive, not less.

I’m talking about nose dive, ass up. If the entire car “dives” evenly then yes, that’s excellent and exactly what I’m looking for. They call it a belly flop at the pool but it’s considered good form at the track. The fact the the front springs compress further as the rears extend is testimony enough that weight is transferring forward, and therefore asking the fronts to do more of the work. If that’s OK, why have rear brakes at all?

[Steve Scheifler]

But I guess you are saying that for a given rate of decel the same amount of weight transfer will occur regardless, though potentially at a different rate depending on other factors. That seems correct but doesn’t account for the dynamics of the real world where sudden changes can result in overshoot or upsetting other things like rear tire adhesion, or briefly getting hard into bump stops where compliance drops to nil.

But perhaps the only important question to answer is just that, “why have rear brakes at all?”. The correct answer should be applicable to why you need the right compound.

[Tom Hampton]

I’m talking about nose dive, ass up. If the entire car “dives” evenly then yes, that’s excellent and exactly what I’m looking for. They call it a belly flop at the pool but it’s considered good form at the track. The fact the the front springs compress further as the rears extend is testimony enough that weight is transferring forward, and therefore asking the fronts to do more of the work. If that’s OK, why have rear brakes at all?

 
So am I---dynamic rake, as it were.  As far as I know, FOR A GIVEN AMOUNT OF STOPPING FORCE, you can't manage that with brake bias.  the car is going to pitch the same amount whether that is from 100% rear brake or 100% front brake, or anywhere in between.  On our cars, you can manage it by raising or lowering the CG, same as mid-corner body roll.  But, with body roll we have the ARB to play with. 
 

But I guess you are saying that for a given rate of decel the same amount of weight transfer will occur regardless, though potentially at a different rate depending on other factors. That seems correct but doesn’t account for the dynamics of the real world where sudden changes can result in overshoot or upsetting other things like rear tire adhesion, or briefly getting hard into bump stops where compliance drops to nil.

Yes, that's what I'm saying. 

 

Dynamics (and rates) are managed with spring rates, and dampening---which unless you have the super special Fat Cat shocks...or have removed a round from the front coils...Again, not brake bias.  I suppose it would also be affected by initial bite, and rate of pedal pressure application---in relation to suspension spring/dampener rates. But, that's only the initial 1/3 second or whatever it takes to apply full pedal pressure. 

 

If the point is now being changed to say that maximizing the rear braking force (within the traction circle limits) provides a more manageable handling/braking characteristic...maybe that's true---I'm not good enough to say.  But, its not due to a difference in "nose-dive".  Better balance across the car F/R/L/R is usually easier to manage (more predictable) when things get weird (surface bumps, bump-stops, contact, water, oil, dirt, etc).   So, sure I'd expect a car with more even braking F/R would be less surprising when the car gets "upset" by those unexpected events. 

[Sphinx]

I think we are talking about the wrong issue.  The issue is always the downward forces the tires exhibit.  Oversteer is when the rear tires are unable to push against the ground and slide.  Understeer is the same but for the front tires.  So, the ultimate question is what is the maximum downward force you can generate at the front during braking that doesn't cause the rear tires to slide.  And if the rear tires will be touching the ground, we don't want them to lock up but instead to generate braking force.  The total force downward is the weight of the car.  And there's a point right before lockup where the fronts will slow down the car, not lockup, but their combined forces are less than the weight of the car.  And it's the rear brakes' job to manage those remaining pounds, without sliding or locking up, yielding control.  Unless the front brakes are massive enough to lift the rear off the ground, then there's always work to be done by the rear brakes.

 

https://www.allenber...eight-transfer/

[Ron Alan]

  Unless the front brakes are massive enough to lift the rear off the ground, then there's always work to be done by the rear brakes.

 

 

Does size matter?

 

Doesn't ABS keep your massive brakes from overpowering your little rubber patch? Size relates more to temperatures created and wear...IMO 

[Johnny D]

I think we are talking about the wrong issue.  The issue is always the downward forces the tires exhibit.  Oversteer is when the rear tires are unable to push against the ground and slide.  Understeer is the same but for the front tires.  So, the ultimate question is what is the maximum downward force you can generate at the front during braking that doesn't cause the rear tires to slide.  And if the rear tires will be touching the ground, we don't want them to lock up but instead to generate braking force.  The total force downward is the weight of the car.  And there's a point right before lockup where the fronts will slow down the car, not lockup, but their combined forces are less than the weight of the car.  And it's the rear brakes' job to manage those remaining pounds, without sliding or locking up, yielding control.  Unless the front brakes are massive enough to lift the rear off the ground, then there's always work to be done by the rear brakes.

 

https://www.allenber...eight-transfer/

 

I'm late to the party and haven't read really anything so bear with me if i'm off or repeating something.

 

If you have over or under you can adjust your suspension.

http://mazdaracers.c...ta-setup-guide/

You have to have a good setup to begin with or you'll always be chasing it.

Bent parts, old tires, blown shocks can screw things up too.

 

I'm not sure how your rear brakes get involved in this.

 

You should know how to Threshold brake and your brake should be well maintained, bleed, lubed, etc.

Also over and under is a moving target so you can't say X will be what you need.

 

J~

[gerglmuff2]

so we have reached the point of the thread where respected racers butcher freshman level physics i see. 

the circle of a thread's life is complete. 

[Tom Hampton]

No.  That's exactly the issue we are discussing---and the point that I was making.  However, I find that articles way of describing the physics awkward---its not wrong, but it is awkward. 

 

Nevertheless, the point is that the rotational torque applied to the vehicle is simply a function of the total G-force of braking and the height of the CG.  Lower G-force => lower rotational torque => less weight transfer to front tires.....or.... CG closer to roll-center => lower rotational torque => less weight transfer to front tires.  The moment arms and resulting forces and torques (and amount of nose-dive / rear-end lift) are identical whether those braking forces are applied by the rear brakes or the front brakes or both.  For a given ride-height and deceleration rate, the amount of weight transfer to the front tires is fixed---and independent of front/rear braking forces (bias---either from compound selection, or hydraulic pressure...doesn't matter). 

 

Oversteer and understeer are terms associated with lateral acceleration (cornering).  In those cases, its the result of the difference in grip between the front and rear tires...which can be caused by a whole bunch of different things, most not related to this thread (braking force being one of those things while trail braking).  However, we are discussing longitudinal acceleration---ie braking.  As such, those terms under/over steer don't really have direct application. 

 

The braking moments aren't nearly large enough to lift the rears off the ground---like you can do on a bicycle. Besides, that's more limited by the front TIRE GRIP more than the "massiveness of the brakes".  Even in cornering we can't, in general, roll the car up on the two outside wheels. 

[Danny Steyn]

I would suspect that you should also factor into this whether you car, in the neutral position, is raked or de-raked. This would probably affect the rate at which the front suspension compresses, and the rate at which the weight transfers off the rear tires. There are several of us who run very de-raked cars for this, and other reasons. 

[Steve Scheifler]

Tom, I will gladly concede superior technical engineering knowledge to someone formally trained, but I’m not always convinced that all of the relevant real-world details are being factored in. And sometimes I just don’t grasp the explanation. Keeping controllability issues aside, boiling it down to the simplest form it still sounds as if you are in effect arguing that max braking Gs are independent of rear brake participation. I assume that is not your point. If you are just trying to hammer home that weight transfers forward regardless of which end does the slowing, then I’ve already conceded that. I’m not sure that this is a perfect textbook case, but I don’t know that it matters.

So let’s get back to the core issue of whether changing the F/R compound balance matters, whether it impacts max decel and how much each end contributes to the task, and for the curious among us, why. And why changing that balance creates or resolves the issue of premature rear lockup.

[Danny Steyn]

I seem to recall that the former owners frequently recommended that combo.  Perhaps they meant it as a starting point.  

 

This. Happy medium!

[Danny Steyn]

A bit surprised with the 1.6 but I don’t have any quantitative info the the the G-locs to judge just how much difference there is between 12 & 10. Don’t they publish that stuff?
Even more surprised by the NB combo despite having less experience on that platform. Danny, is that closer to what you run?

 

I run a more aggressive combo. It STOPS. But it LOCKS UP when its even slightly wet and pads have to be changed. Lots of time to be made up in the braking zones. 

[Steve Scheifler]

As a reminder, this is where we started:

Other than using a softer rear brake pad, is there any magic here that we should be looking for and do you guys mix/match brands for fronts and rears? For example, I have some 1/2 used Cobalt XR5 and Hawk blacks rears - any reason I can't use them up with Carbotech XP10 fronts?

Back when I raced SpecRX7, the class gurus would say that it didn't matter what you ran on the rear, even stock pads were ok (though back then preferred Blues Front/Blacks Rears).

We're always talking about front pads, thought it might be useful to talk about rears for once.

And this quote from above was the crux of my point, and still is:

“Stopping is essentially just straight-line cornering, the G’s/acceleration is just longitudinal rather than lateral. That IS the point of the classic friction circle after all. We’ll never see a circle because our engines make no power, but for cornering, braking and any combination of those the idea is the same, get all four contact patches operating very close to their limits of adhesion while maintaining control. If you rely more than necessary on the fronts then you are leaving something on the table, PERIOD, I don’t really care how you drive. Someone offer evidence, or even a good rational explanation why that’s wrong. I’m listening. There's a lot to be said for modulation and trail-braking friendly compounds but those fall under ‘while maintaining control’ and don’t change the fundamentals.”

Weight transfer details aside, this is still the original topic and why the large differences in pad “balance” is surprising. Is there that much difference in static camber? Seems backwards since where some people seem to run more is up front. I think there is more going on, or some are getting less from their brakes rhan they could.

[Tom Hampton]

Tom, I will gladly concede superior technical engineering knowledge to someone formally trained, but I’m not always convinced that all of the relevant real-world details are being factored in. And sometimes I just don’t grasp the explanation. Keeping controllability issues aside, boiling it down to the simplest form it still sounds as if you are in effect arguing that max braking Gs are independent of rear brake participation. I assume that is not your point. If you are just trying to hammer home that weight transfers forward regardless of which end does the slowing, then I’ve already conceded that. I’m not sure that this is a perfect textbook case, but I don’t know that it matters.

Sorry, Steve. Wasn't trying to "hammer home" anything. My last post was in response to Sphinx. I probably should have quoted him to make that clearererer.

No, I absolutely am NOT arguing that max braking G is independent of rear pads. As long as the rear tires are on the ground, they have some non-zero level of participation in braking. So, pad selection will matter to use whatever rear tire grip is available.

That said, ever driven a car without front brakes? They don't stop so good. Even on the street under benign weight transfer scenarios with big fat sticky tires---I've tried. Makes lots of squealy noises, not so many decal-G's.
 

So let’s get back to the core issue of whether changing the F/R compound balance matters, whether it impacts max decel and how much each end contributes to the task, and for the curious among us, why. And why changing that balance creates or resolves the issue of premature rear lockup.

 

Assuming a max braking of 1.2g, and a CG height of 14 inches, you have a forward weight transfer of roughly 400 lbs (less if the CG is lower, more if its higher). I don't have a setup sheet nearby, but even at 50/50 static weight distribution, that's 1550/2300 = 67% front, 33% rear...in round numbers. 

 

That CG is just a rough guess based on a stock height of about 17", and knowing that we lower the car, and remove a lot of weight above that mark...and add weight below it.  In a quick search I saw guesses as low as 12" or so.  AT one point, kent Carter threatened to actually measure it.  But, never saw a published result. 

 

[Tom Hampton]

Weight transfer details aside, this is still the original topic and why the large differences in pad “balance” is surprising. Is there that much difference in static camber? Seems backwards since where some people seem to run more is up front. I think there is more going on, or some are getting less from their brakes rhan they could.

I guess the other question is how much difference is there really in the compounds (within a manufacturer)?  When considering all the other factors: brake pad surface area, proportioning valve effects, smaller rear caliper pistons, etc...balanced with the lower rear-braking potential...how does all that work out?  I don't know...and its probably easier (and more accurate) to test than calculate.    

 

I suppose if I were at the pointy end, and were worried about every 0.01s, I might go out and try a couple more aggressive compounds to get even F/R lockup (assuming that's optimal).

 

But, as a less skilled driver, I'm probably a little safer having some excess rear-grip available under heavy braking, at the marginal expense of 0.1g of theoretical braking potential.  In other words, I probably have more to lose than I do to gain---as do most people who are asking the manufacturer for advice instead of testing themselves. 

 

Besides, as one of like two 1.6 drivers left in the state, I've already got a 125+ lbs advantage on everyone else in Texas in the braking zone. 

[Steve Scheifler]

All of which leaves me scratching my head about certain real world observations, tests and results. As mentioned previously, on both of our primary 1.6 cars, prepared as identically as we are able, we have “solved” rear lockup problems by switching from a less aggressive rear pad to the same compound as the front. Also with them setup the same and driving in back to back sessions the red #28 gives the unmistakable impression of stopping faster and flatter than blue #82. At Road Atlanta a couple years ago I was actually laughing out load as I hammered the brakes approaching 10a and the car just seemed to dig in. There was something decidedly different about it compared to the other one and the impression was that it remained flatter. Whatever the actual cause or difference, it was not subtle. Those are the kinds of things that make me wonder if the complexities of the system are not fully accounted for by the textbook theories.

BTW, trying to solve braking issues on #28 prior to that we had replaced a number of parts, the last being the proportioning valve.

[Steve Scheifler]

Speaking of differences in compounds, what’s in your calipers?

Danny, for you I recommend the 2s in front and the 3s in back.

https://www.cobaltfr...cles.asp?ID=252

[Tom Hampton]

Curently running dtc 60/30.  I do not have ANY rear lockup issues.  And, I don't lose in threshold braking zones to those I race against.  Nor do I seem to lose ground to those closer to the front of the pack on the occasions I end up near them---high speed finesse corners is where I lose time.  I brake a good 1/2 marker later than the NBs, sometimes more depending on their driving style.  As I noted, I'm the only 1.6 left (more or less) so, I don't get to compare apples. 

 

I used to run PFC-97, all around.  That was several years ago, in my previous car, and I weighed 55 lbs heavier.  However, I don't recall ever having rear lockup then either.  Back then I had inside front lockup problems...was always trying to adjust cross, or try and manage the steering wheel to keep enough weight on the inside.  Never the rear.  

 

So, I find your experience quite odd.  Its literally the last thing on my mind at the track.  

[Steve Scheifler]

So, no rear lockup with same PFCs F/R (I hated those pads BTW) yet you stagger compounds with DTCs. Assuming all else being equal doesn’t that seem illogical? Isn’t it reasonable to at least wonder whether you could brake even better with more rear bite?

For you I would recommend Cobalt 3/3 as a starting point.