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Discussion Starter #1
Guys...spend some time studying this chart. I'm happy to answer any questions about it.

It's a friction circle or g-g diagram I produced from some recent Autocross data logs. The course had mostly important right hand turns hence the bias to that side.

The 0,0 point in the middle of the funny shape is when there is no acceleration in any direction. Stopped or straight forward or backward at constant speed would be at that point. To the left = left turns and to the right = right turns. Below the equator and there is some braking going on, above it and there is some gas pedal action. Get it? Where I put the red stars is very interesting. Notice that you can corner the Elise harder with slight braking than with none!! When driving to the limit, you want to spend as much time as possible on the perimeter and transition rapidly through the core.

This is with the stock "touring" tires. Folks that is a pretty good performance envelope. You can see what I mean when I say that the Elise is a killer trail braking machine!!

 

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Discussion Starter #3 (Edited)
Here is another run...this one more marked up so you can see the path the car took. Note that at A and B the car could do the highest cornering g-s while trail braking. Pretty good grip for the "touring" tires...

Note that in the slalom section in the middle (the wiggley straightaway) the cornering gs were not high...this was an easy slalom which you could accelerate through pretty easily...
 

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VERY interesting.

What are you using to log this data?
 

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I wonder if the Vette owners have mined similar data? If we could use this to the fullest extent, I think it would be a huge advantage for us at auotXs.:)
 

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Discussion Starter #7
>>>VERY interesting. What are you using to log this data?<<<

I'm using a datalogger that uses accelerometers, and GPS for those plots. Other things logged at the same time include driver inputs and car stuff.

>>>What do the different shapes (circle, triangle, square) in the friction diagram represent. I didn't see a key.<<<

The different shapes are separate data points from different runs. You can pretty much ignore them...there are about five different autocross runs (same course) on this plot. The thing to notice is that we are seeing what the car can do..the performance envelope in which the car lives. In a perfect autocross run you'd want to be at the perimeter as much as possible, and pass throough the interior as quickly as possible.

If you tested the car on more courses or on a figure 8 setup you'd fill in more of the outer edges. This course had many important right handers so there is more activity on the right, especially the lower right quadrant. The complete shape would be pretty much round with a flattish top defined by max accel in 2nd gear and some first gear start-off stuff. The round shape has "jowels" for the Elise. This is where I am seeing that the car is a killer trail braker. Because that represents cornering and braking at the same time. You can corner a bit harder when you transfer some weight to the front tires than steady state cornering in the Elise. Most cars don't do this to this degree.
 

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Discussion Starter #8
>>>I wonder if the Vette owners have mined similar data? If we could use this to the fullest extent, I think it would be a huge advantage for us at auotX<<<

Well....I've logged SS Z06 vettes on Hoosiers and on Kumho 710s to0....lots of cool things can be seen. One thing the diagram does not show is the car size element. For example a Z06 needs to slow down more for certain types of tight turns since it's so large by comparison. This offsets some of their accel advantage. The gate size is the same for both cars. On some slaloms the Vette and Elise can do similar speeds in MPH. Yet the Vette needs to generate higher g-forces simply to be able to fit through the course. The Vettes are better at accelerating hard out of high speed sweepers. Vette likes open...Elise like tight. The Elise and the Vette accel similarly in 1st gear. Subjectively when you watch the cars one at a time the roaring Vette appears much faster in 1st gear...but it isn't. If anything the advantage goes to the Elise. It appears that the Elise outhandles the Vette...but will this be enough to make up for the sweepers and straights where we can't be in the best gear or above 6000 Rs...?
 

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Not to poke holes - but I think the highest G's are generally at Turn-In and at Track-Out regardless of trailbraking.

I use trailbraking when I want to carry lots of speed through a corner and I need the car to rotate. Trailbraking helps me find the "limit" and I know I can control it by simply easing off on the brake.
 

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Discussion Starter #10
>>>Not to poke holes - but I think the highest G's are generally at Turn-In and at Track-Out regardless of trailbraking. <<<

Classic friction circle theory says that tires have roughly the same potential grip in any direction. So you can combine lateral and longitudinal forces in various combinations to be anywhere on the edge of the circle. The top of the circle is flattened out since we can't accelerate as hard as we can brake/corner.

In the case of the Elise I see that the highest pure lateral forces comes with some amount of simultaneous braking. I think this is related to things like the 38/62 weight balance aspect. On most cars I have logged (Miatas, S2000s, Z06, BMW...) you tend to have much less of this effect if at all.

>>>I use trailbraking when I want to carry lots of speed through a corner and I need the car to rotate. Trailbraking helps me find the "limit" and I know I can control it by simply easing off on the brake.<<<

It appears that the way to ride the edge of the friction circle on the Elise is to use the brakes very well so that you enter the corner optimally for exit. Done right in many corners this can allow earlier applcation of throttle and earlier time to full throttle. If you are not doing lots of TB I think you may be leaving something on the table in many types of turns. Not all of course. Many cars can come out of turns harder accelerating than us due to having more motor. Power to weight wise the Elise is about like a low 300 HP Vette with a narrow power band.
 

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Stan could you please describe trailbraking for me??

You say the Elise is a great trailbraking car. Would love to know what it is.
Cheers
 

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Stan said:
>>>VERY interesting. What are you using to log this data?<<<

I'm using a datalogger that uses accelerometers, and GPS for those plots. Other things logged at the same time include driver inputs and car stuff.
well, i knew it was a data logger ... just curious which one, what software, and what-not.
 

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One of the problems I guess I have is that in order to trailbrake effectively you need to be able to moderate the brake pedal very well.

In my car (motorsport brake package), I can brake much later than almost any car and I can do it all day long. So, in extreme braking areas I don't feel that I control the trailbrake like I can in a place where only moderate or a tap of the brakes in required.

Maybe it comes with more seat time, but the timing to trailbrake while threshold braking (no ABS either) is very precise and if you screw it up - around you go.

Based on weight distibution and contact patch differences front to rear it would make sense that if you brake a bit and unload the wider rear tires and move weight forward that lateral G's would be maxed out. Executing is a different thing.....:D
 

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Re: Stan could you please describe trailbraking for me??

Maverick1 said:
You say the Elise is a great trailbraking car. Would love to know what it is.
Cheers
Very basically, carrying braking INTO the corner as compared to braking in a straight line before the corner. And it is not something I recommend for people to do unless they really know what they are doing. It is a trade off and the benefits and penalities exist.

For most people in most corners, trail braking the Elise will not be faster. Brake earlier, accelerate to the apex point... will be faster.
 

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Also, trail braking an Elise into a 3rd gear corner on a racetrack is very different from trail braking into a 2nd gear or slower corner on an autocross course.
 

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Very simply, trail braking is an advanced driving technique.

the SAFEST way to get around a race track is to brake in a straight line, apex late and power out. This gives a lot of margin for error.

Trail braking can often lead to over rotation ending in spins, offtrack excursions, etc.
 

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Stan said:
Notice that you can corner the Elise harder with slight braking than with none!!
More accurately, you were driving the car closer to its traction-budget limits while turning in than you were in mid-corner. It might follow that your results reflect the car's ultimate capacity, but, by itself, your data does not establish that.

When driving to the limit, you want to spend as much time as possible on the perimeter and transition rapidly through the core.
I bought my g-analyst in 1987, when they first came out, and I've collected this kind of data on a pretty wide variety of cars since that time. In my experience, data of this sort is potentially useful in two ways: for evaluation of the effectiveness of vehicle modifications, and for identifying obvious underutilization of a car's capacity. If you're interested in the former, you need to set up repeatable tests under controlled conditions (e.g., a marked skidpad or a straight section for acceleration runs.)

Your data is useful for the latter purpose. However, once you've identified the really major areas of underutilization (e.g., not using all the car's braking capacity), it becomes increasingly difficult to improve your times significantly just by analyzing recorded accelerometer data. Many of the things you need to change in order to improve your times aren't readily apparent in that kind of data.

This is with the stock "touring" tires. Folks that is a pretty good performance envelope. You can see what I mean when I say that the Elise is a killer trail braking machine!!
If your conclusion turns out to be correct - which has yet to be established - then it would imply that the front tires, which gain some grip under braking, are being underulitized in steady-state cornering. IOW, a bigger front swaybar will be helpful.
 

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Discussion Starter #19 (Edited)
>>Very simply, trail braking is an advanced driving technique<<<

Sure. And especially on a high speed track. Think of it as a new "feature" the car has built right in for when you are ready to develop skills in those areas. And that this capability can help deal with some issues you may discover when exiting a corner.

Note also that the worst corner exit understeer you can experience happens at lower speeds. When TB is safest. And that at higher speeds our cars have lesser of that corner exit concern in the first place.

You can safely do TB to a low degree though. It takes talent, skill, practice, and magic to be able to consistently do it at the edge of the friction circle though.

Another way to think of trail braking as being similar in some ways to what you already do coming out of a turns...you add power gradually as the car can take it and as the steering lock is reduced. You don't suddenly floor it or whatever. In this case you are cornering and using positive acceleration in greater amounts as the cornering forces taper off. For trail braking you are cornering and using lesser amounts of negative acceleration. Pretty similar concepts.

Check out the messy sketch I made below. That is supposed to be a circle. The red trace starts out accelerating straight...then braking straight....when all braking has ended, cornering begins. When cornering is done, acceleration begins. Easy and safe and slow.

The Blue trace shows some nice trail braking where first the car is accelerating on a straight. Then comes hard braking which is then gradually reduced as cornering gradually increases. At peak cornering there is no braking and then the driver adds power as the turn changes back to a straightaway at it's exit. Most of us can easily do the accelerating out of a turn thing.

The green trace shows a lesser (and safer) amount of trail braking. Some cars can get very hard to control if they are taken near the trail braking limit. The Elise seems to handle very well in this area though, so that is some icing on the cake.
 

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Discussion Starter #20
>>>well, i knew it was a data logger ... just curious which one, what software, and what-not.<<<

Sorry had to rush off to a meeting. It's a race technology DL1 logger using their software. www.race-technology.com
 
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