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I’ll say the same thing I said to luckow: if you want to jack the front roll stiffness way up like a normal rear wheel drive autocross car, you’re going to have a bad time. If you make the rear axle twitchy on the bump stops to accomplish this or do anything that limits the power you can put down by making you wait with your right foot, you’re going to have a bad time that’s also slow.
I’ll say the same thing I said to luckow: if you want to jack the front roll stiffness way up like a normal rear wheel drive autocross car, you’re going to have a bad time. If you make the rear axle twitchy on the bump stops to accomplish this or do anything that limits the power you can put down by making you wait with your right foot, you’re going to have a bad time that’s also slow.
I think his phrase "aggressive roll stiffness balance" refers to reducing the front roll stiffness, thus moving the balance rearward, not increasing it, as he is down to only 30% stiffer than stock.
Plus, if it wasn't clear, he's talking about a 997, not a Boxster. But jacking up the front roll stiffness could very well be worse for the 997. I think the limits for increasing front roll stiffness are pretty well known for the Boxster/Cayman cars in Street for cars running stockish (or less tall) bumpstops.
In any case, for the 987.1 Cayman base, I don't expect to get it all perfect on the first try. The rear is a very valid concern because the bump stops are not as tall as in the front so there can be a much larger step change when it gets engaged. Especially if you go a lot stiffer as I plan to do. The same was true for my 944 as well, but it didn't pose a problem. Of course, that car had an LSD and the Cayman will not.
If anyone other than BMacIL has properly valved their shocks to account for stiffer bumpstops in Street I don't know about it. I've seen nothing to indicate that the CS Miata people have a clue about this. Someone did a little testing and found a bump stop they could cut down that gave them some roll reduction, then they all just copy it and throw on MCS or Penske shocks with no idea if the damping is correct and start twisting dials until it works ok.
There shouldn't be any twitchiness in the rear any more than if you could upgrade the steel springs. The bumpstops materials available now can produce very linear spring rates if you choose/design wisely. Then the shock valving must be properly matched or it won't work right, that's for sure.
I think his phrase "aggressive roll stiffness balance" refers to reducing the front roll stiffness, thus moving the balance rearward, not increasing it, as he is down to only 30% stiffer than stock.
Plus, if it wasn't clear, he's talking about a 997, not a Boxster. But jacking up the front roll stiffness could very well be worse for the 997. I think the limits for increasing front roll stiffness are pretty well known for the Boxster/Cayman cars in Street for cars running stockish (or less tall) bumpstops.
In any case, for the 987.1 Cayman base, I don't expect to get it all perfect on the first try. The rear is a very valid concern because the bump stops are not as tall as in the front so there can be a much larger step change when it gets engaged. Especially if you go a lot stiffer as I plan to do. The same was true for my 944 as well, but it didn't pose a problem. Of course, that car had an LSD and the Cayman will not.
If anyone other than BMacIL has properly valved their shocks to account for stiffer bumpstops in Street I don't know about it. I've seen nothing to indicate that the CS Miata people have a clue about this. Someone did a little testing and found a bump stop they could cut down that gave them some roll reduction, then they all just copy it and throw on MCS or Penske shocks with no idea if the damping is correct and start twisting dials until it works ok.
There shouldn't be any twitchiness in the rear any more than if you could upgrade the steel springs. The bumpstops materials available now can produce very linear spring rates if you choose/design wisely. Then the shock valving must be properly matched or it won't work right, that's for sure.
I was actually more referencing what you were trying to do, when you mention using the front sway bar to keep the rear loaded. We actually have a 997.2 GT3 and the two caymen… so have a lot of time with both. The cayman is basically a 911 from the front bulkhead forward.
I think his phrase "aggressive roll stiffness balance" refers to reducing the front roll stiffness, thus moving the balance rearward, not increasing it, as he is down to only 30% stiffer than stock.
Plus, if it wasn't clear, he's talking about a 997, not a Boxster. But jacking up the front roll stiffness could very well be worse for the 997. I think the limits for increasing front roll stiffness are pretty well known for the Boxster/Cayman cars in Street for cars running stockish (or less tall) bumpstops.
In any case, for the 987.1 Cayman base, I don't expect to get it all perfect on the first try. The rear is a very valid concern because the bump stops are not as tall as in the front so there can be a much larger step change when it gets engaged. Especially if you go a lot stiffer as I plan to do. The same was true for my 944 as well, but it didn't pose a problem. Of course, that car had an LSD and the Cayman will not.
If anyone other than BMacIL has properly valved their shocks to account for stiffer bumpstops in Street I don't know about it. I've seen nothing to indicate that the CS Miata people have a clue about this. Someone did a little testing and found a bump stop they could cut down that gave them some roll reduction, then they all just copy it and throw on MCS or Penske shocks with no idea if the damping is correct and start twisting dials until it works ok.
There shouldn't be any twitchiness in the rear any more than if you could upgrade the steel springs. The bumpstops materials available now can produce very linear spring rates if you choose/design wisely. Then the shock valving must be properly matched or it won't work right, that's for sure.
I know a certain top 3 AS Z06 driver who went to Penskes this year and I've been helping him sort out this kind of thing. The Konis he had previously run had a much taller bumpstop, and there was a lack of understanding that on the front, he was jacking down on them and relying on the balance created by that taller stop. Now he'll be playing with front bar again because the stiffer position would actually work.
I know a certain top 3 AS Z06 driver who went to Penskes this year and I've been helping him sort out this kind of thing. The Konis he had previously run had a much taller bumpstop, and there was a lack of understanding that on the front, he was jacking down on them and relying on the balance created by that taller stop. Now he'll be playing with front bar again because the stiffer position would actually work.
Was he running tall Koni bumpstops on the Z06? Or some Porsche?
1-Porsches are generally conservatively classed in SCCA autocross, especially in Street, for various reasons. This usually means they are "also-rans" in most classes, SS being the possible exception. This has been getting slightly better, I'd say, in the last 5 to 8 years, but is still evident. Porsche autocrossers complain about this state of affairs, but there are specific reasons given for it which can be argued, but this is not the thread to do it in.
2-AFAIK, Porsche is the only manufacturer of sports cars that resolutely used a strategy of employing bumpstops as auxiliary springs at each corner of every car since at least the 944 in the 1980's, carrying through to the 986/987/996/997 cars in the 2000's. (I don't know about the later cars.) It was the invention of Silastic bumpstop material (first used by Koni?) that made this possible, I think. The scheme requires long bumpstops contacted early in the compression direction with somewhat linear spring-rate properties. Later, Mazda did it on the front (and possibly the rear) of the Miata; Toyota did it only on the front of the MR2 Spyder. Maybe Mazda and Toyota used this strategy elsewhere as well, but I don't know about it. I think this is the primary design feature that Porsche used to produce cars that rode well down the highway but were remarkably firm in the corners. It is simple and cheap but requires properly valved shocks that can handle the step-change in spring rate.
3- The Street rules classify the bumpstops as a replaceable part of the strut/shock, while limiting the contact point to be the same as stock. In other words, they are free, but within limits. The key becomes the length of the stock bumpstops, with the shock body length and total length also coming into play. When it comes to tuning with bumpstops and valving shocks for same, maybe nobody knows as much as Shaik at Fat Cat.
So, when you are faced with the deficiencies of strut suspensions (from an autocross standpoint) combined with purposely limited front camber range, using tuned bumpstops on Street-class Porsches becomes a key design capability that can turn also-rans into competitors, IMHO. Any of these cars should be able to achieve Street Touring levels of roll stiffness (and pitch stiffness) without having to resort to a huge FSB and the trade-offs that entails. You don't need as much static negative camber if you don't lose as much in the corners. The natural transient superiority of mid-engined and rear-engined cars gets magnified. This is not easy to put numbers to, but it should make the cars faster. It certainly made my 944 faster in ES, though I didn't get time to perfect it due to the sunset rule, financial limitations and knowledge/learning-curve limitations.
Is it enough to make the 987.2CS a winning car in AS? I don't know. That car (and the 981CS) is giving up both power and grip (about a 7% theoretical grip deficit) to the C6Z06. If a car has a deficit in both grip and power to another car it's probably a lost cause except on an extremely transitiony course. Can it make the 987.2 Cayman base and 987.1CS in BS winners? I think so, on anything except a power course, especially when allied with the grip advantage they already have over the other cars. For instance, the 987.1CS has a whopping 10% theoretical grip advantage over the '23 Supra manual and even more over the M2.
And if the 987.1 Cayman base goes to CS, I think it can make it a competitive car there, for the same reasons: it has a small, but significant grip advantage (3.5%) over the ND2 already with only a slight acceleration deficit though with a huge narrowness deficit.
As for the 997 in AS and the 996 in BS, I just don't know. The cars are too different. You're the expert there.
1-Porsches are generally conservatively classed in SCCA autocross, especially in Street, for various reasons. This usually means they are "also-rans" in most classes, SS being the possible exception. This has been getting slightly better, I'd say, in the last 5 to 8 years, but is still evident. Porsche autocrossers complain about this state of affairs, but there are specific reasons given for it which can be argued, but this is not the thread to do it in.
2-AFAIK, Porsche is the only manufacturer of sports cars that resolutely used a strategy of employing bumpstops as auxiliary springs at each corner of every car since at least the 944 in the 1980's, carrying through to the 986/987/996/997 cars in the 2000's. (I don't know about the later cars.) It was the invention of Silastic bumpstop material (first used by Koni?) that made this possible, I think. The scheme requires long bumpstops contacted early in the compression direction with somewhat linear spring-rate properties. Later, Mazda did it on the front (and possibly the rear) of the Miata; Toyota did it only on the front of the MR2 Spyder. Maybe Mazda and Toyota used this strategy elsewhere as well, but I don't know about it. I think this is the primary design feature that Porsche used to produce cars that rode well down the highway but were remarkably firm in the corners. It is simple and cheap but requires properly valved shocks that can handle the step-change in spring rate.
3- The Street rules classify the bumpstops as a replaceable part of the strut/shock, while limiting the contact point to be the same as stock. In other words, they are free, but within limits. The key becomes the length of the stock bumpstops, with the shock body length and total length also coming into play. When it comes to tuning with bumpstops and valving shocks for same, maybe nobody knows as much as Shaik at Fat Cat.
So, when you are faced with the deficiencies of strut suspensions (from an autocross standpoint) combined with purposely limited front camber range, using tuned bumpstops on Street-class Porsches becomes a key design capability that can turn also-rans into competitors, IMHO. Any of these cars should be able to achieve Street Touring levels of roll stiffness (and pitch stiffness) without having to resort to a huge FSB and the trade-offs that entails. You don't need as much static negative camber if you don't lose as much in the corners. The natural transient superiority of mid-engined and rear-engined cars gets magnified. This is not easy to put numbers to, but it should make the cars faster. It certainly made my 944 faster in ES, though I didn't get time to perfect it due to the sunset rule, financial limitations and knowledge/learning-curve limitations.
Is it enough to make the 987.2CS a winning car in AS? I don't know. That car (and the 981CS) is giving up both power and grip (about a 7% theoretical grip deficit) to the C6Z06. If a car has a deficit in both grip and power to another car it's probably a lost cause except on an extremely transitiony course. Can it make the 987.2 Cayman base and 987.1CS in BS winners? I think so, on anything except a power course, especially when allied with the grip advantage they already have over the other cars. For instance, the 987.1CS has a whopping 10% theoretical grip advantage over the '23 Supra manual and even more over the M2.
And if the 987.1 Cayman base goes to CS, I think it can make it a competitive car there, for the same reasons: it has a small, but significant grip advantage (3.5%) over the ND2 already with only a slight acceleration deficit though with a huge narrowness deficit.
As for the 997 in AS and the 996 in BS, I just don't know. The cars are too different. You're the expert there.
Ok yeah I follow you. I have executed that on the rear in particular with a firmer but slightly shorter stop and packers (same total length as OE stop). On the front I've gone the opposite way and gone shorter but that is also with a GT3 front bar. In theory another angle to go on either the 987 or 997 is big, firm front stops and an adjustable rear bar. That could prove to be faster but have to be mindful of the open diff with that strategy.
Ok yeah I follow you. I have executed that on the rear in particular with a firmer but slightly shorter stop and packers (same total length as OE stop). On the front I've gone the opposite way and gone shorter but that is also with a GT3 front bar. In theory another angle to go on either the 987 or 997 is big, firm front stops and an adjustable rear bar. That could prove to be faster but have to be mindful of the open diff with that strategy.
If the 997 is anything like the 993 I worked on, it needs what might seem to be a huge (numerically, in terms of lb/in) rear stiffness increase.
If the 997 is anything like the 993 I worked on, it needs what might seem to be a huge (numerically, in terms of lb/in) rear stiffness increase.
If the goal is to make it nearly undriveable at a nationals-style course like 2023 East, this would be the way . Moving closer to OE front/rear roll stiffness distribution - but still higher front - made the car faster overall but far more on a knife edge in high speed transient elements. 15th on day 1 (West) with quite solid, albeit not stellar, driving. East overall exposed weaknesses in the platform despite offering opportunities for a narrower, shorter car to have some chances to gain ground (walloms). My driving was subpar as well, and I significantly underperformed on East. It's still no A street car but that ship has sailed.
Looks like next Fastrack on Nov 20th may reveal all regarding the SS->AS->BS->CS moves...
07-07-2023, 04:07 PM
. Moving closer to OE front/rear roll stiffness distribution - but still higher front - made the car faster overall but far more on a knife edge in high speed transient elements. 15th on day 1 (West) with quite solid, albeit not stellar, driving. East overall exposed weaknesses in the platform despite offering opportunities for a narrower, shorter car to have some chances to gain ground (walloms). My driving was subpar as well, and I significantly underperformed on East. It's still no A street car but that ship has sailed.