xkrs vs mercedes sc63 amg evo track battle
#21
03-13-2014, 03:18 PM
Veteran Member
The last time I constructed end-links and a custom sway bar for my '05 XK I used those spherical end links purchased right at my local hardware store, Orchard-supply. Basically, you need 4 end links for the two sides (8 for front and rear) and a continuous thread that those links screw into. They are solid and have zero slop. Race cars use them all the time. Here is a google search to show what I am talking about. The one negative about those solid end-links is that when they wear, they tend to develop a clicking sound that is not appropriate on street cars. My XK and other street cars I constructed with those links never developed the sound, likely because I never put very high mileage on them.
https://www.google.com/search?q=sphe...m=122&ie=UTF-8
Last edited by axr6; 03-13-2014 at 03:24 PM.
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MaximA (03-13-2014)
#23
03-13-2014, 07:16 PM
Veteran Member
Traction from R rated tires...
Interesting article about how sticky the R-rated tires are on the Camaro Z/28. This gives an idea how much extra traction those "R" tires provide for the XKR-S GT over other XKR variants with normal tires.
https://autos.yahoo.com/news/camaro-...161308591.html
I have not ran R-compound tires for many years and do not know how far their development progressed. As a word of caution before you run out and install them on your street cars be aware that they, at least the older R compounds I used, did not work well under street driving condition. They required the tire to be heated up before providing good traction. Without heat in the tires they were worse than a cheap tire from a discount store. The fact that manufacturers are increasingly installing those tires as standard equipment on street machines seems to suggest that they overcame the cold compound problems.
https://autos.yahoo.com/news/camaro-...161308591.html
I have not ran R-compound tires for many years and do not know how far their development progressed. As a word of caution before you run out and install them on your street cars be aware that they, at least the older R compounds I used, did not work well under street driving condition. They required the tire to be heated up before providing good traction. Without heat in the tires they were worse than a cheap tire from a discount store. The fact that manufacturers are increasingly installing those tires as standard equipment on street machines seems to suggest that they overcame the cold compound problems.
Last edited by axr6; 03-13-2014 at 07:26 PM.
#24
03-13-2014, 08:08 PM
Senior Member
I had Toyo R888's R-Comp tires on my Lotus and they were superb on the street and the track. They warmed up very quickly and offered traction like nothing else I had tried. The big problem was standing water. The tires were scary in the rain, but since it wasn't a daily driver it worked out well.
While R-Comp tires definitely do not belong on a daily, most of the newer ones should work well on the streets if its not too cold outside. I honestly wish the XKR-S came with them stock. I think that would have made a good difference in how it handled in testing.
One of the best "mods" I have done to my car is removing the toe and ditching the Pzeros.
While R-Comp tires definitely do not belong on a daily, most of the newer ones should work well on the streets if its not too cold outside. I honestly wish the XKR-S came with them stock. I think that would have made a good difference in how it handled in testing.
One of the best "mods" I have done to my car is removing the toe and ditching the Pzeros.
#29
03-14-2014, 08:09 AM
#32
03-14-2014, 08:47 AM
Veteran Member
#33
03-14-2014, 09:38 AM
Should be an excellent starting point for tracking, and to determine any desired adjustments from there.
I hope the Bilstein damper rates are suitable for you. I do question the suitability of active dampers for track use, and wonder what, if any, trade-offs are involved. Good luck with your set-up, and we'll be interested to follow your quest to the track day Holy Grail...Jaguar Style!
Bruce
#34
03-14-2014, 09:39 AM
Veteran Member
When discussing those spring rates with MaximA, he indicated that they might have been "wheel rates" instead of pure "spring rates". That would take into account the length of the suspension arms and the location of the springs on those arms. At least, that is how I understood the discussion. So, I have to say, I do not have full confidence in knowing the pure spring rates.
However, what I CAN say is that the XKR as well as my other Jags feel too stiffly sprung by my unscientific, "feel of the pants" method, as well. That includes driving down the road and feeling every little bit of surface coming through the chassis, something that may be desirable on a pure race car but, not necessarily on a comfort oriented GT or luxury car. It also makes the car feel nervous at speeds, instead of calm and composed. A nervous suspension tends to break your concentration, as well as creating a constantly varied degree of tire adhesion.
My other, often used, non-measurable way to tell stiffness was to simply place my 190 lbs body weight over the trunk or the front end of the car and "feel" how much the the springs compress. Even my race cars at 300lbs/inch compressed more readily at the rear than any of my Jags. So, from all available evidence I gather that the Jags are using rather stiff springs, particularly on the rear end. Combine that with the low profile tires that have next to zero flex in the sidewall and you got a very stiffly sprung car.
The issue is an obvious lack of traction, as listed by nearly every single test driver on the various tracks. We can not just brush them aside, there is an apparent problem here. So, what we are trying to find out is the "cause" for that. The car has plenty of power to go fast, has a solid chassis, so, what is the cause?
The answer: the suspension. Next; is it all a tire issue for the lack of traction? I don't think so because one can have some cheap tires and still have predictable handling. Yet, testers complain that the car is twitchy at the limit, hard to keep from spinning. When I go through the chain of components and possible causes, still, the overly stiff rear springs pop up as a red warning. Particularly since MaximA reported that those rates are higher in the rear than the front. That is a double red warning.
Now, it maybe something else but, that is my best guess from my years of experience. I certainly would love to try out some softer springs at the rear, combined with higher rate bars and I would be shocked if that combo would not result in a much more livable, all around GT.
However, what I CAN say is that the XKR as well as my other Jags feel too stiffly sprung by my unscientific, "feel of the pants" method, as well. That includes driving down the road and feeling every little bit of surface coming through the chassis, something that may be desirable on a pure race car but, not necessarily on a comfort oriented GT or luxury car. It also makes the car feel nervous at speeds, instead of calm and composed. A nervous suspension tends to break your concentration, as well as creating a constantly varied degree of tire adhesion.
My other, often used, non-measurable way to tell stiffness was to simply place my 190 lbs body weight over the trunk or the front end of the car and "feel" how much the the springs compress. Even my race cars at 300lbs/inch compressed more readily at the rear than any of my Jags. So, from all available evidence I gather that the Jags are using rather stiff springs, particularly on the rear end. Combine that with the low profile tires that have next to zero flex in the sidewall and you got a very stiffly sprung car.
The issue is an obvious lack of traction, as listed by nearly every single test driver on the various tracks. We can not just brush them aside, there is an apparent problem here. So, what we are trying to find out is the "cause" for that. The car has plenty of power to go fast, has a solid chassis, so, what is the cause?
The answer: the suspension. Next; is it all a tire issue for the lack of traction? I don't think so because one can have some cheap tires and still have predictable handling. Yet, testers complain that the car is twitchy at the limit, hard to keep from spinning. When I go through the chain of components and possible causes, still, the overly stiff rear springs pop up as a red warning. Particularly since MaximA reported that those rates are higher in the rear than the front. That is a double red warning.
Now, it maybe something else but, that is my best guess from my years of experience. I certainly would love to try out some softer springs at the rear, combined with higher rate bars and I would be shocked if that combo would not result in a much more livable, all around GT.
#35
03-14-2014, 09:45 AM
Veteran Member
That's a 26% increase in rates for both front and back. That doesn't seem extreme at all, and should maintain the stock handling balance which I found to be quite good, and manageable at the limit on track. The car was quite easy to catch when the rear stepped out, and understeer was basically non-existent with my driving style. I generally threshold brake to turn-in followed by light trail-braking and slightly late apex.
Should be an excellent starting point for tracking, and to determine any desired adjustments from there.
I hope the Bilstein damper rates are suitable for you. I do question the suitability of active dampers for track use, and wonder what, if any, trade-offs are involved. Good luck with your set-up, and we'll be interested to follow your quest to the track day Holy Grail...Jaguar Style!
Bruce
Should be an excellent starting point for tracking, and to determine any desired adjustments from there.
I hope the Bilstein damper rates are suitable for you. I do question the suitability of active dampers for track use, and wonder what, if any, trade-offs are involved. Good luck with your set-up, and we'll be interested to follow your quest to the track day Holy Grail...Jaguar Style!
Bruce
As far as the active damping I don't really feel it much at speed but it tends to soften up rough roads. I could always pull the plug on the shocks and see what kind of difference it makes on the track.
Last edited by MaximA; 03-14-2014 at 09:48 AM.
#36
03-14-2014, 09:47 AM
Veteran Member
Albert,
The rates are actual spring rates, Matt was talking about the wheel rate reducing the spring rate due to the leverage of the location and angle of the springs. Sorry if the last post about this wasn't clear.
Next week I'm going to work on the swaybar end links to see if I can make them more "usable"
The rates are actual spring rates, Matt was talking about the wheel rate reducing the spring rate due to the leverage of the location and angle of the springs. Sorry if the last post about this wasn't clear.
Next week I'm going to work on the swaybar end links to see if I can make them more "usable"
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axr6 (03-14-2014)
#37
03-14-2014, 09:51 AM
Veteran Member
Albert,
The rates are actual spring rates, Matt was talking about the wheel rate reducing the spring rate due to the leverage of the location and angle of the springs. Sorry if the last post about this wasn't clear.
Next week I'm going to work on the swaybar end links to see if I can make them more "usable"
The rates are actual spring rates, Matt was talking about the wheel rate reducing the spring rate due to the leverage of the location and angle of the springs. Sorry if the last post about this wasn't clear.
Next week I'm going to work on the swaybar end links to see if I can make them more "usable"
#38
03-14-2014, 10:52 PM
However, what I CAN say is that the XKR as well as my other Jags feel too stiffly sprung by my unscientific, "feel of the pants" method, as well. That includes driving down the road and feeling every little bit of surface coming through the chassis, something that may be desirable on a pure race car but, not necessarily on a comfort oriented GT or luxury car. It also makes the car feel nervous at speeds, instead of calm and composed. A nervous suspension tends to break your concentration, as well as creating a constantly varied degree of tire adhesion.
My other, often used, non-measurable way to tell stiffness was to simply place my 190 lbs body weight over the trunk or the front end of the car and "feel" how much the the springs compress. Even my race cars at 300lbs/inch compressed more readily at the rear than any of my Jags.
The issue is an obvious lack of traction, as listed by nearly every single test driver on the various tracks. We can not just brush them aside, there is an apparent problem here. So, what we are trying to find out is the "cause" for that. The car has plenty of power to go fast, has a solid chassis, so, what is the cause?
And I recall it being the XKR-S that was considered tail-happy, not the XKR. The XKR-S suspension has different everything.
The answer: the suspension. Next; is it all a tire issue for the lack of traction? I don't think so because one can have some cheap tires and still have predictable handling. Yet, testers complain that the car is twitchy at the limit, hard to keep from spinning. When I go through the chain of components and possible causes, still, the overly stiff rear springs pop up as a red warning. Particularly since MaximA reported that those rates are higher in the rear than the front. That is a double red warning.
Now, it maybe something else but, that is my best guess from my years of experience. I certainly would love to try out some softer springs at the rear, combined with higher rate bars and I would be shocked if that combo would not result in a much more livable, all around GT.
Now, it maybe something else but, that is my best guess from my years of experience. I certainly would love to try out some softer springs at the rear, combined with higher rate bars and I would be shocked if that combo would not result in a much more livable, all around GT.
Driving style has a lot to do with a driver's ability to control a car's tendencies. I've tracked a mid-engine car that's somewhat prone to snap oversteer, and a high powered front engine that can over-steer pretty easily. Both have led me to the cornering strategy that I described, and that is the fastest line through a corner with a car like the XKR as well.
I thought Randy Pobst was rather positive about the XKR's handling, and while he might not choose it as his track weapon of choice among a dozen other cars he's testing, and the weight alone would exclude the XKR from top honors, that viewer shouldn't interpret that to mean it isn't a fine track day car. I didn't choose the XKR because it was the most capable, nor did Max either I'm sure, but I have enjoyed mastering it to the extent that one can over 5 or 6 track day, and Max will as well as he also enjoys modding it. Even if he puts on the widest R-compounds that will fit (305's?), I don't believe any amount of suspension mods will significantly reduce power on oversteer.
Just my opinions based on lots of track experience, and some very relatable suspension mods to the rather similar Supra TT. Hopefully both of our perspectives will help max as he explores the limits of adhesion with his XKR.
Bruce
#39
03-15-2014, 08:47 AM
Veteran Member
Interesting to note the Pilots have a minimum rim width of 10.5" & and the Corsas have a minimum of 10".
#40
03-15-2014, 11:24 AM
Veteran Member
...
That's the damper valving and stiff sidewalls, not overly stiff springs.
Yeah, that's dampers again. You're more likely to be able to bench press the rear of a Jag than compress the spring with a damper in there. Much easier when the damper is blown...seriously, ask me how I know!
Trust me Bruce, I can tell the difference between damper resistance and spring resistance.
Loss of rear tire traction under throttle with warm tires at track speeds only happens by power on oversteer, not in a straight line. It also happens primarily at lower vehicle speeds and with high throttle in lower gears. It's a simple matter of an engine with huge torque multiplied by low gear ratios, and the over-zealous electronic throttle that makes it a challenge to smoothly modulate as Max mentioned here, and I did in one of my track review threads. When it's driven at the limit in testing it suffers no traction issues under braking either. In addition to a torque curve that requires restraint to master, I believe the damper valving isn't optimized to keep the tires planted, and there's no adjustment for that. And then you add the stiff low profile tires that can't flex as much under acceleration to maintain grip and you needn't look any further.
And I recall it being the XKR-S that was considered tail-happy, not the XKR. The XKR-S suspension has different everything.
Bruce, as long as you have been posting about the performance aspects of the XKR you tend to "whitewash" or deny that it has handling issues. Yet, you claim that you do not take your car to 10/10ths where the problems become obvious. On the other hand, just about every single pro testers complain about the car being unpredictable near the limit. Likely, exactly because of that, the lap times shown for the variations of the XKR are much slower than they should be for the power/weight ratios. While I recognize that it is a GT car, still, a Ford Mustang or a Camaro should not put several seconds on a 510HP XKR on a race track. If they do, there is a reason for that. Torque alone does not explain the traction issues. Those same testers are NOT complaining about other cars in the test with similar power and torque values.
I'm concerned that changing the front to rear spring rate ratio with a softer rear spring could have the result of inducing under-steer, as would changing the sway bar ratio with a stiffer rear bar. The springs are easy and inexpensive to swap out to try, and perhaps Max will try that if he finds the need to change the car's overall balance. Disconnecting the rear bar for testing would also make for an interesting experiment that would affect rear traction, and lead to selecting a new bar.
Indeed, if you only lowered the rear spring rates, you would increase understeer. But, do note, that I called for simultaneous reduction in rear spring rates AND an increase in the rear roll bar size. If you dropped the rear spring rates to, say, 350 lbs/inch than you should be able to tune the perfect near-neutral front-to-rear balance with a rear bar, say, close to 7/8", as where I would start. That would provide multiple benefits; better rear tire bite, less body roll and definitely a more comfortable street ride. The problem Max and all of us are facing is the lack of availability and the difficulty of installation for swaybars. Without doing both, yes, you will mess up the handing even more.
Driving style has a lot to do with a driver's ability to control a car's tendencies. I've tracked a mid-engine car that's somewhat prone to snap oversteer, and a high powered front engine that can over-steer pretty easily. Both have led me to the cornering strategy that I described, and that is the fastest line through a corner with a car like the XKR as well.
Yes, driving style has a lot to do with the ability to control a car. But, if a car is too stiffly suspended it will lack tire adhesion under acceleration and no matter how great a driver you are you will not be able to exit the turns with sufficient power. In all appearances, that is exactly where the XKRs are falling down.
I thought Randy Pobst was rather positive about the XKR's handling, and while he might not choose it as his track weapon of choice among a dozen other cars he's testing, and the weight alone would exclude the XKR from top honors, that viewer shouldn't interpret that to mean it isn't a fine track day car. I didn't choose the XKR because it was the most capable, nor did Max either I'm sure, but I have enjoyed mastering it to the extent that one can over 5 or 6 track day, and Max will as well as he also enjoys modding it. Even if he puts on the widest R-compounds that will fit (305's?), I don't believe any amount of suspension mods will significantly reduce power on oversteer.
Just my opinions based on lots of track experience, and some very relatable suspension mods to the rather similar Supra TT. Hopefully both of our perspectives will help max as he explores the limits of adhesion with his XKR.
While we are having this debate on the ultimate handling limits of the XKRs, I also will say that those that do not actually either street-race or track race their XKRs at or near 10/10ths will NOT have an issue here. The car is perfectly fine and enjoyable as a GT car.
My decades of driving history includes lots of all-out banzai canyon racing of which I only lost a single race, due to selecting the wrong tire (R-compound) for the rear that day. In track racing my GT car was basically unbeatable for all the years I owned it and tuned it. I finally sold it and went open wheel racing because it became BORING to win all races by huge margins. My car was THAT superior. (BTW - just found out from my racing buddy this week that the new owner that I sold my GT race car to, stored the car outside for years and now the car is rusted and basically ruined. What a crying shame) The last year I raced it I won 14 out of 15 SCCA races, plus the Western States Road Racing Championship and, lost only one when my shifter handle broke while I was leading.
My philosophy in setting up both street and racing cars was to install as soft a springs as possible, just stiff enough to keep the lowered street or racing suspensions off the bump stops. Combine that with stiff roll bars on both ends to ensure basically no body roll in corners. Worked great for me against all competition. Thus, I have proven that the softer springs/stiffer swaybar approach is a winner but, I am not at all convinced that the same could be said for stiffly suspended cars.
Will I change the suspension for my XKR? Not at all certain about that. Due to my changed driving habits where I no longer take street challenges, nor do I track-race any more, I do not foresee driving my XKR at, or near the limit. At sane, fun speeds (open for translations...) the car feels just fine and quite enjoyable and I may just stick with it, as it is. Time will tell...
Bruce
Yeah, that's dampers again. You're more likely to be able to bench press the rear of a Jag than compress the spring with a damper in there. Much easier when the damper is blown...seriously, ask me how I know!
Trust me Bruce, I can tell the difference between damper resistance and spring resistance.
Loss of rear tire traction under throttle with warm tires at track speeds only happens by power on oversteer, not in a straight line. It also happens primarily at lower vehicle speeds and with high throttle in lower gears. It's a simple matter of an engine with huge torque multiplied by low gear ratios, and the over-zealous electronic throttle that makes it a challenge to smoothly modulate as Max mentioned here, and I did in one of my track review threads. When it's driven at the limit in testing it suffers no traction issues under braking either. In addition to a torque curve that requires restraint to master, I believe the damper valving isn't optimized to keep the tires planted, and there's no adjustment for that. And then you add the stiff low profile tires that can't flex as much under acceleration to maintain grip and you needn't look any further.
And I recall it being the XKR-S that was considered tail-happy, not the XKR. The XKR-S suspension has different everything.
Bruce, as long as you have been posting about the performance aspects of the XKR you tend to "whitewash" or deny that it has handling issues. Yet, you claim that you do not take your car to 10/10ths where the problems become obvious. On the other hand, just about every single pro testers complain about the car being unpredictable near the limit. Likely, exactly because of that, the lap times shown for the variations of the XKR are much slower than they should be for the power/weight ratios. While I recognize that it is a GT car, still, a Ford Mustang or a Camaro should not put several seconds on a 510HP XKR on a race track. If they do, there is a reason for that. Torque alone does not explain the traction issues. Those same testers are NOT complaining about other cars in the test with similar power and torque values.
I'm concerned that changing the front to rear spring rate ratio with a softer rear spring could have the result of inducing under-steer, as would changing the sway bar ratio with a stiffer rear bar. The springs are easy and inexpensive to swap out to try, and perhaps Max will try that if he finds the need to change the car's overall balance. Disconnecting the rear bar for testing would also make for an interesting experiment that would affect rear traction, and lead to selecting a new bar.
Indeed, if you only lowered the rear spring rates, you would increase understeer. But, do note, that I called for simultaneous reduction in rear spring rates AND an increase in the rear roll bar size. If you dropped the rear spring rates to, say, 350 lbs/inch than you should be able to tune the perfect near-neutral front-to-rear balance with a rear bar, say, close to 7/8", as where I would start. That would provide multiple benefits; better rear tire bite, less body roll and definitely a more comfortable street ride. The problem Max and all of us are facing is the lack of availability and the difficulty of installation for swaybars. Without doing both, yes, you will mess up the handing even more.
Driving style has a lot to do with a driver's ability to control a car's tendencies. I've tracked a mid-engine car that's somewhat prone to snap oversteer, and a high powered front engine that can over-steer pretty easily. Both have led me to the cornering strategy that I described, and that is the fastest line through a corner with a car like the XKR as well.
Yes, driving style has a lot to do with the ability to control a car. But, if a car is too stiffly suspended it will lack tire adhesion under acceleration and no matter how great a driver you are you will not be able to exit the turns with sufficient power. In all appearances, that is exactly where the XKRs are falling down.
I thought Randy Pobst was rather positive about the XKR's handling, and while he might not choose it as his track weapon of choice among a dozen other cars he's testing, and the weight alone would exclude the XKR from top honors, that viewer shouldn't interpret that to mean it isn't a fine track day car. I didn't choose the XKR because it was the most capable, nor did Max either I'm sure, but I have enjoyed mastering it to the extent that one can over 5 or 6 track day, and Max will as well as he also enjoys modding it. Even if he puts on the widest R-compounds that will fit (305's?), I don't believe any amount of suspension mods will significantly reduce power on oversteer.
Just my opinions based on lots of track experience, and some very relatable suspension mods to the rather similar Supra TT. Hopefully both of our perspectives will help max as he explores the limits of adhesion with his XKR.
While we are having this debate on the ultimate handling limits of the XKRs, I also will say that those that do not actually either street-race or track race their XKRs at or near 10/10ths will NOT have an issue here. The car is perfectly fine and enjoyable as a GT car.
My decades of driving history includes lots of all-out banzai canyon racing of which I only lost a single race, due to selecting the wrong tire (R-compound) for the rear that day. In track racing my GT car was basically unbeatable for all the years I owned it and tuned it. I finally sold it and went open wheel racing because it became BORING to win all races by huge margins. My car was THAT superior. (BTW - just found out from my racing buddy this week that the new owner that I sold my GT race car to, stored the car outside for years and now the car is rusted and basically ruined. What a crying shame) The last year I raced it I won 14 out of 15 SCCA races, plus the Western States Road Racing Championship and, lost only one when my shifter handle broke while I was leading.
My philosophy in setting up both street and racing cars was to install as soft a springs as possible, just stiff enough to keep the lowered street or racing suspensions off the bump stops. Combine that with stiff roll bars on both ends to ensure basically no body roll in corners. Worked great for me against all competition. Thus, I have proven that the softer springs/stiffer swaybar approach is a winner but, I am not at all convinced that the same could be said for stiffly suspended cars.
Will I change the suspension for my XKR? Not at all certain about that. Due to my changed driving habits where I no longer take street challenges, nor do I track-race any more, I do not foresee driving my XKR at, or near the limit. At sane, fun speeds (open for translations...) the car feels just fine and quite enjoyable and I may just stick with it, as it is. Time will tell...
Bruce