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Camber recovery. Interesting.
Do we need camber recovery on a track car? I suspect this is more to do with a street car so we do not ware out the inside of the tires. It would seem that the important point on a track car is the tires are at there best angle to the track and stable at the limit in a corner. Stability from getting the best possible dampening and little change in camber with compression and or front wheel angle (caster created camber gain.) But that is just what I am guessing. |
Is it just me or do these race cars look like there do not have much in the way of camber recovery? Expecially up front.
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Distortion from the wide-angle lens used to take the picture
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Camber recovery = gain = ability of suspension to make the wheel lean less than the car does; think it is caused by compression & effects of compression via the linkage geometry
I bet Bill V. can explain it... |
Interesting how many responses this thread has generated. I AX campaigned a supercharged miata that would 0-60 in 4.2 secs. The usual shocks, sways, springs, chassis stiffing mods. It was a lot of fun, but still occasionally got smoked by a well driven NA miata (and several Scoobies). I suspect it was the nut behind the wheel problem.
Our 911 feels totally superior in road feel and corner lean. The turbo and peaky torque adds a much more complicated dimension, but I would never go back. I don't race the Porsche (DE or AX) so my comments are only for the sporty touring feel. Both of my cars weighed roughly the same with fuel (2700 lbs). The 911 is much more satisfying and I can assure you that a forced induction miata is a load of fun. |
Camber recovery is dependent on suspension geometry. It is how much the camber changes relative to the body due to suspension travel in bump/rebound, as one gets in roll. This therefore compensates for the roll and keeps the tire at a more optimum camber.
911st, the camber recovery IS a mechanism that keeps the tire properly oriented to the track surface, which you are so concerned about. |
There's no such thing as a 50/50 Miata. The last time I had my car on the scales, the weight limit was 2350lbs for a 1.6L like mine (with 180lb. driver factored in). My weights were-
Lf-638 Rf-621 Lr-545 Rr-547 Now the minimum weight is 2275 with 180lb driver for the 1.6L. 1.8L's have to run 2400lbs., and the 99 and up cars have to run 2450lbs. |
I see the pictures of the white Miata #377, and it sure looks cute. It's probably faster than my '78 911, and way faster than my Ford Ranger. But it better not pick a, autocross fight with this car:
http://forums.pelicanparts.com/uploa...1285254779.jpg http://forums.pelicanparts.com/uploa...1285254801.jpg http://forums.pelicanparts.com/uploa...1285254820.jpg |
Racerfink,
Thx for the real info. Still I suspect the Miata still has a lot of tire per axel compaired to many 911's. Example, your front axel is about 10% more up front and about 48% less rear axel weight than the assumed 2700 lb 911. Assuming the Miata runs 205's the 911 would need 225 ft and about 275 to 305's rear to have about the same amount of tire where most SC body track cars run about 225/255's. Again this is a bit simplistic but my hope is it helps convey the point. TimT, I wondered if a wide lens was used but thought that would have bent the long straight lines in the picture and to my eye that did not seem to be the case. All good stuff. Just trying to learn and having some fun!!! |
What about toe change? The rear of a torsion bar 911 will have toe change when the suspension moves. Is this a factor?
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RSRs usually run a lot of static camber because they have little suspension movement due to low ride height and stiff springs. This means not much motion available for camber recovery (or whatever you want to call it), even with very aggressive camber gain kinematic ratios.
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from the factory late RSRs only run -3*35' static camber in front -3*10' in back late GT3 Cups run 4* f/r earlier versions ran as much as -4*45'/-4* 20' depending on the track, less for high speed venues like Daytona |
Well, a lot compared to a street car, which most people are used to looking at. I did not mean RSRs have a lot of camber recovery, just that the way to get the camber to be optimal in a corner is to use static camber since the suspension does not move much so the roll due to tire deflection can only be compensated for with static camber, or caster if steering input is sufficiently large.
I do not think that the effect is entirely due to the wide angle lens. |
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Given enough time & money you can make a 9eleben faster than a Miata on the track, but it probably won't "handle" better. |
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- 2nd question: I usually think of the advantage of progressive springing as applying to street cars. They introduce a non-linearity that the driver must compensate for (of course, humans deal with non-linear phenomena all the time), but... what is the advantage on the track of progressive springing? |
Every miata at laguna seca gets killed on the straights by the 911s. Both of the "fast straights" are up hill too. Put slicks on and do some suspension tuning and you'll have no problem taking them on. I drove my 911 at laguna and a friends miata (both had slicks) and I much prefer a 911. I feel like I should have a wig and a skirt on to drive the miata, though it was fun.
On the other hand, I went to an all miata event at Laguna and I saw some 350 hp 1.8 or 2 liter turbo/supercharged miatas that could go. I even saw an LS1-chevy V8 miata, under-tuned puting out only 350 from Flying Miata. I can't believe this is even a serious question. It's all in the set up of the car and driver. A miata slammed with slicks and sway bars will beat a stock, street tires, "comfy" ride height 911, for sure with the same driver. |
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Bill!
Check out the braking system on this car. Why would one want a ballance bar and an adjustable PV? Quote:
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Because an adjustable balance bar can only adjust the front to rear proportion. It cannot put a knee in the prop curve. It is still desirable to have a prop valve in the system because it better matches the brake proportions to the weight transfer. More rear bias initially, then at a certain pressure, reduces the rear once weight has been transferred from the rear to the front. A balance bar can't do that. Having both systems gives you full control. |
Thought of that but could not find anything from Tilton that dose that.
Adjustable PV:http://forums.pelicanparts.com/uploa...1285295706.jpg Merlin Line Lock. http://forums.pelicanparts.com/uploa...1285295763.jpg |
Thx Tyson.
I love Pelican. Always something new to learn. |
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Whatever... Does anyone remember the beemers from a few years back that ran -5 deg or so camber? |
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as for the Miata vs the 911, well, apples to apples (either a stock Miata vs a stock , oh, 89 Carreara, or a Spec Miata vs a mildly tuned 911), the Miata will own the 911 in the entry, the corner, and off the corner. Of course the 911 will own the straight....that's just engine. I say this having driven both. The Miata is a great car. Mazda did a wonderful job with it. It just is greater than the sum of it's parts. That said, most guys I see in 911s at track days are in the way in corners. Having horsepower makes it too easy. To learn how to really drive, they should have something that needs to maintain momentum. |
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The graph below illustrates: http://forums.pelicanparts.com/uploa...1285350861.jpg The amount of pressure required to lock the front and rear wheels at the same time depends on how much weight the car is transferring. The harder the car is stopping, the more weight is transfered off the back wheels and the less pressure the back brakes can take relative to the front before they lock up. This relationship is the blue curve in the graph above. A car with no bias valve, like a 930, has a fixed ratio to the front and rear- see the green line. This ratio matches the ideal (locks the front and rear at the same time) at one point on the graph. The rest of the time it's off, and for safety they err on the side of locking the front brakes too early. In wet conditions, however, the front brakes lock much earlier than ideal, so total braking is reduced. A dual master cylinder race car lets you adjust the bias for the conditions, so you can adjust to the red line in the wet, back to the green line in the dry and everything in between. Dual master cylinders are reliable and consistent, but they make for lots of adjustments to stay close to ideal. A good bias valve and brake setup can virtually eliminate the adjustment by being almost ideal over a range of conditions- the purple line. If either the caliper bias or the bias valve itself isn't quite right, however, you're back to adjusting the valve often to compensate for changing conditions. |
In the category I race, the rules require stock master cylinders. So, balance is done with pads choice. Calipers and discs are stock as well. We are allowed a prop valve. So, I set the car up with the basic balance to the rear, then dial it back for the 'standard" conditions with the prop valve. Then, in the rain, I have some adjustment left. And I have some adjustment for the steep downhill where the fronts get a lot of weight transfer.
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Great info guys!!!
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Is the graph in post 66 above why Porsche went to a PV w larger rear pistons with the 3.2 Carrera?
Or is there some other reason? |
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The 911 gains an advantage vs a miata as the power to weight ratio of the cars improves. As power increases traction becomes more and more of a limiting factor. The extra weight over the drive wheels allows the 911 to usefully deploy a much higher power to weight ratio, part of the reason the spec 911 is on the lower end of power to weight ratios for 911 race classes. Quote:
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oops, missed Pete's response |
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Looked up a few things. Spec Miata's run 205/50/15, Spec911 225/245 RA1's. Spec Miata weigh 2285 to 2450 lbs, spec911 run 2400lbs min. Suspect Miata puts out about 120whp and a spec911 about 220whp. Thus about a 19lbs per HP for the Miata and 11lbs per HP for the 911. 2 sec a lap on I am guessing what is about a 2:05 lap time (thus 125 v 123 sec per lap). Thus it looks like the 911 has about a 40% hp advantage, more gross tire per lb (maybe 20% more) and probably more braking reserve per lb than the Miata. However, the 911 is only about 2% faster. What is up with that? It would be cool to see some data acquisition info comparing from the Miata and Spec 911 on the same track and see the speeds in the different part of the tracks. |
I think Infineon would be a better track to compare handling since it is less of a horsepower track than Thunderhill.
I've run a 1:48.47 at Infineon in my Spec-911, and that was carrying an extra 150# of trophy weight, so 2520# with driver. My car dynod @ 205 RWHP. What is the Spec-Miata record at Infineon? |
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Infineon puts more of a premium on putting power down than Thunderhill, which helps the 911's cause. "Handle better" is certainly a slippery question. Thought experiment: take a Spec 911 and Spec Miata chassis but equalize hp at say 150 hp: the miata would likely win at most tracks. But if all you did was up the power of both cars to 450 hp while keeping the chassis and tires the same the 911 would handily beat the miata. The miata can't use much of the extra power while the 911 can. You didn't change the suspension or tires- which car "handles better"? |
Spec 911's have run 2:00.xx at Thunderhill. I think in ideal conditions one of us is going to break into the 59's there.
So that's about a 6 second delta at both Infineon and Thunderhill. |
The Spec Miata is an interesting case, and I think one aspect that is in it's favor is the incredible numbers of them, and the resulting development.
And as a starting package, the SM "kit" (dampers, springs, etc) is pretty darn good, on top of a car that is pretty darn good. Whats interesting to me is that, when you "upgrade" a Spec Miata to Improved Touring allowances, the result is faster, but not by a whole lot. A friend has a well prepped ITA Miata, and holds ITA track records all over the NE. (IT allows 7" wide wheels, which get Hoosier A or R comps, some more engine allowances (mid 130 at the wheels, max), and free damper and spring/bar setups, essentially, at a race weight of 2380) Yet, at a track like Lime Rock, where laps are a minute and change long, it only beats the top SMs of the Runoffs champ level guys by maaaaybe a second or so. I've driven them, and the cars are telepathic, you think, they do. I find them super easy to get to 98%. Finding the nth degree in speed takes time, and experience. Tyson, I haven't driven a spec 911, but I have to think it's a much trickier machine to carry speed with. I imagine you've been in both, is that the case? |
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I'm sure you're right on the THill times Tyson. I saw 2-3 seconds same day on a couple time sheets, but those were 2:02s for the 911 spec, not 2:00.
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Thus while it's true that the high powered miata spec will carry more speed through the corner, it's largely true to say that's because it can't accelerate with the 911. And while it's also true that at some point on the straight the high-powered miata will hook up and be able to deploy the extra power, the time down the straight is disproportionately effected by acceleration at the beginning of the straight. So as power to weight climbs even a miata (or 928, or 951, etc) wants be be a "slow in, fast out" car. They are limited, however, by having ~50% weight over the back wheels, which caps their peak acceleration off the corners regardless of the power available. A 911's peak acceleration will be more than 25% greater, all else being equal, due to where the weight is, so it's that much closer to the ideal "slow in, fast out" car. At low power to weight ratios this doesn't come into play, as you're rarely traction limited. The faster the car, however, the more you want the weight in the back and the less mid-corner speed is the deciding factor. |
So simplistically we have a two cars that are close on racing weight but the 911 has about a 40% HP advantage, maybe a 20% GROSS tire advantage, better brakes and it is only about 2-4% faster than the Miata.
I am still guessing the 911 is a bit rear traction challenged compared to the Miata. I think 205/255's would get the 911 to about the same traction per axel. (The Miata I am guessing has about a 1200 lb axel weight w 205 8.4" wide tires. The 911 rear axel is about 1440 lb with 245 9.6" wide tires.) The Miata also has less frontal area to try to push down the straights. It would be cool is some G readings for the Spec911 and Spec Miata. |
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