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Braking vs turning G-forces


turbogrill
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Hi,

 

When we brake we typically reach about 0.8 G. But when we corner we reach about 1.2 G.

Is it common to have less grip when braking vs turning?

 

We are running pretty soft suspension, could it be that the weight transfer during braking makes the front tires work much more than the rears?

But due to the swaybar all 4 tires are at work during cornering?

 

We can easily lockup the tires and have ABS. No aero. Modest camber ( -2)

Car is a 2006 Miata with 245 RE71s if that matters.

 

 

edit: breaking vs braking

Edited by turbogrill
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Yes, but not that much difference. Usually it's within about 10% of the cornering force. And you should be over 1.0 braking with RE71Rs, most street tires/cars are 0.8-0.9 gs. Something odd there.

 

What did you use to get your data? And how did you test?

Edited by mender
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6 hours ago, mender said:

Yes, but not that much difference. Usually it's within about 10% of the cornering force. And you should be over 1.0 braking with RE71Rs, most street tires/cars are 0.8-0.9 gs. Something odd there.

 

What did you use to get your data? And how did you test?

 

I am using a Race Capture Pro, it uses the inbuilt accelerometers. This is looking at data from the races.

 

 

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Those numbers are reasonable. Most tires in the realm we play in have higher lateral capabilities than longitudinal on the car. Typical g-g plots are ovals wider than they are tall... the friction circle is actually an ellipse.

 

Could be weight transfer or camber in addition to the tire's inherent capabilities due to tread and carcass stiffness distributions, footprint shape, etc. 

If the car's ABS is calibrated to a lower threshold than the tire's capability, you won't get all the longitudinal grip that is on the table either. These cars came from the factory with much lower grip tires than we run.

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You've likely got your bias out of whack and are locking just the fronts, meaning the rears aren't contributing as they should.  That and the different caliper sizing doesn't match the factory ABS tune.  It's a pretty common problem with aftermarket brake kits.  I've said it before in another thread, you'll likely do better if you go back to your stock brakes.

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Doesn't sound like we are terrible off, car brakes pretty decent compared to others. So I think I can move on and focus on other more important things (like having a car that runs).

 

The NC Miata has some kind of electronic smart brake bias, not sure how it works. I could pretty easy add brake pressure sensors for all 4 wheels, it's just the cost. Probably going to cost at least $500 for all the sensors and adapters. Not sure if it's worth it. But would be interesting to see.

 

Stock brakes are pretty good, the Spec MX-5 runs them and that series is a few seconds faster than ours. Benefits of wilwoods is easier to change pad and a little less weight. We use the same rotor but a little larger piston. (Our stock caliper broke in a crash, so instead of buying an OEM caliper for $250 we bought a wilwood for $140).

 

We currently run DTC60/30 and also the wilwood BP40 but pad wear is a little high, going to try ST43s I think. So will gather some new data and also improve the logging as well.

 

 

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8 minutes ago, Burningham said:

 

If your ABS is kicking in you aren't easily locking up the tires, right?  

 

True :) 

I meant that the brakes can easily lockup the wheels but the ABS will kick in. I should remove the ABS fuse and see what happens... 

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5 hours ago, turbogrill said:

You don't sustain the max deaccelration long, it's very peaky. Could obviously be the track as well, are these heavy braking zones (like at COTA)?

Would be nice to see the g vector as well, gives a better picture of how well the driver is using the tires.

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1 hour ago, ross2004 said:

I think that's what this channel is supposed to represent.

Capture.JPG

That's better. With the other two traces, one can do a bit of analysis. Let's start with the bit at 0.4.

 

In the Y axis, we can see that the driver brakes heavily for the upcoming turn then gets off the brakes as he transitions into the corner. Pretty good peak gs during the braking phase, then a dip and then another peak in the cornering phase. The dip in between appears to indicate that either the driver or the car isn't happy about trail braking on that particular corner. Looks like some time lost there.

 

If the tires squeal at the upper limit, one would hear them as the braking force peaks, then a big drop in sound in the trail braking/transition, then back to squealing near the apex of the corner (I'm assuming).

 

Coming out of the corner, it looks like the driver either missed the apex a bit or got on the gas too much and the tail wagged a bit, seen by the wavy trace in the x axis with a relatively steady Y axis trace. (Edit: FWD so no tail wagging but still had some sawing at the wheel during corner exit)

 

At 0.9, the g sum trace hovers around the 1.0 mark for a while, showing a more consistent use of the tires but short of the peak values. The driver appears to have that corner mostly figured out, just needs to push a bit more. Again, either the car or driver isn't quite happy during trail braking as the top of the trace is a bit ragged.

 

1.3 shows a tentative approach to a longer corner, with mild turn-in, then moderate braking while turning and what looks like a pinch at the apex, and finally aggressively back on the gas while releasing the wheel too soon. A fair bit of time available from smoothing out that corner.

 

Hard to know for sure without having a track map but that's what I see. Would also be better to have the time in seconds across the bottom of each chart to see how long each phase is taking and also a finer grid to be able to line up the traces more accurately. (Edit: also helps to know what the car is: FWD would be noticeably different from a RWD or mid-engine)

 

Comparing this trace with one from a better driver can help separate driver issues from car issues and show how much can be gained from working with each.

Edited by mender
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Unfortunately we got Volkswagen'd before I had a chance to drive, which was a real bummer. I'd done a good bit of work to the car an was anxious to have a go. My slowest driver (referenced) who is usually about 3 seconds off our FTD, was 8/10ths quicker than our fastest lap last year. 

VIR_South-Course_2018.jpg

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8 minutes ago, collinskl1 said:

If I'm looking at the right map - VIR South, I see this:

 

Distance - Turn

0.4 - T1

0.75 - T3a

0.8 - T4

0.9 - T5

1.2 - T7

1.4-1.5 - T8 and T9

So turn 5 at 0.9 appears to be a pretty straightforward corner but can't tell if there are elevation changes. I'll assume it's fairly flat.

 

The part at 0.9 should show the driver's style. In addition to my above comments, I think the driver is using a late apex and is getting on the throttle early and hard. That's usually a safe way to exit corners without losing too much time but he could carry more mid-corner speed and practice feeding in throttle as he unwinds the wheel.

 

A few laps of that while concentrating on keeping the tires howling at a consistent pitch and volume from the start of braking through to track-out would be the place to start. I'm assuming that there's a reasonable amount of run-off there so a safe place to push things.  Once he's dialed in that corner, he can work on the others.

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