Another 21" Setup
02-24-2015, 07:17 PM
#16
Three Wheelin'
02-24-2015, 07:22 PM
#17
Rennlist Member
I sure like the look but would be terrified to drive down the street.
02-25-2015, 04:19 PM
#21
Thus there are solid engineering reasons wheel diameter is growing as power grows from OEMs, and it's worthwhile both developing the tires and re-tuning the car's suspension for them in some cases. Race car wheels are stuck at 18 (or smaller) mainly because of rules designed to limit costs.
In most aftermarket cases, on the other hand, it's all about looks. The larger OD here is going to either raise the car's CG or throw the kinematics slightly, while the only rubber Tire Rack has listed in these sizes are PZeros. Both of these will be a step backwards from OEM for the car's designed purpose.
02-25-2015, 11:53 PM
#22
Three Wheelin'
Just as tire contact patch width largely determines lateral grip, tire contact patch length impacts longitudinal grip. To increase length you make the OD larger: dragsters have tall tires for a reason. However tall sidewalls like a dragster reduce a tire's response rate, impacting handling. Thus if you want to have more longitudinal grip without sacrificing response you need a bigger wheel. Of course you need more power to push it: you wouldn't put a Veryron's wheels on a go-kart or vice-versa.
Thus there are solid engineering reasons wheel diameter is growing as power grows from OEMs, and it's worthwhile both developing the tires and re-tuning the car's suspension for them in some cases. Race car wheels are stuck at 18 (or smaller) mainly because of rules designed to limit costs.
In most aftermarket cases, on the other hand, it's all about looks. The larger OD here is going to either raise the car's CG or throw the kinematics slightly, while the only rubber Tire Rack has listed in these sizes are PZeros. Both of these will be a step backwards from OEM for the car's designed purpose.
Thus there are solid engineering reasons wheel diameter is growing as power grows from OEMs, and it's worthwhile both developing the tires and re-tuning the car's suspension for them in some cases. Race car wheels are stuck at 18 (or smaller) mainly because of rules designed to limit costs.
In most aftermarket cases, on the other hand, it's all about looks. The larger OD here is going to either raise the car's CG or throw the kinematics slightly, while the only rubber Tire Rack has listed in these sizes are PZeros. Both of these will be a step backwards from OEM for the car's designed purpose.
02-26-2015, 11:10 AM
#23
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Just as tire contact patch width largely determines lateral grip, tire contact patch length impacts longitudinal grip. To increase length you make the OD larger: dragsters have tall tires for a reason. However tall sidewalls like a dragster reduce a tire's response rate, impacting handling. Thus if you want to have more longitudinal grip without sacrificing response you need a bigger wheel. Of course you need more power to push it: you wouldn't put a Veryron's wheels on a go-kart or vice-versa.
Thus there are solid engineering reasons wheel diameter is growing as power grows from OEMs, and it's worthwhile both developing the tires and re-tuning the car's suspension for them in some cases. Race car wheels are stuck at 18 (or smaller) mainly because of rules designed to limit costs.
In most aftermarket cases, on the other hand, it's all about looks. The larger OD here is going to either raise the car's CG or throw the kinematics slightly, while the only rubber Tire Rack has listed in these sizes are PZeros. Both of these will be a step backwards from OEM for the car's designed purpose.
Thus there are solid engineering reasons wheel diameter is growing as power grows from OEMs, and it's worthwhile both developing the tires and re-tuning the car's suspension for them in some cases. Race car wheels are stuck at 18 (or smaller) mainly because of rules designed to limit costs.
In most aftermarket cases, on the other hand, it's all about looks. The larger OD here is going to either raise the car's CG or throw the kinematics slightly, while the only rubber Tire Rack has listed in these sizes are PZeros. Both of these will be a step backwards from OEM for the car's designed purpose.
It's been asked a few times on this forum but I've never been satisfied that I know if an MPSC 325 x 20 inch tire would work without big problems on the 991GT3. I understand it will slightly upset the grip balance between front and rear, but if you don't track your car is it really going to screw things up (will it clear rear suspension; will the rolling diameter which will be about a 1/2 inch greater throw off ABS or negatively impact rear wheel steering, or throw off speedo, tach, etc.???).
Thanks Pete and sorry for the compound question!
02-26-2015, 03:50 PM
#24
It's been asked a few times on this forum but I've never been satisfied that I know if an MPSC 325 x 20 inch tire would work without big problems on the 991GT3. I understand it will slightly upset the grip balance between front and rear, but if you don't track your car is it really going to screw things up (will it clear rear suspension; will the rolling diameter which will be about a 1/2 inch greater throw off ABS or negatively impact rear wheel steering, or throw off speedo, tach, etc.???).
Thanks Pete and sorry for the compound question!
Thanks Pete and sorry for the compound question!
There might be some exceptions with a competition car- the increased footprint with a matching front could increase grip, if balanced correctly, more than it hurts performance with worse feel, ride height, etc. But it'd take time to dial in and it's not clear you'd be ahead in the end. For the street there are simply too many drawbacks:
The rolling diameter difference you mentioned will hurt braking distances and potentially acceleration. The ABS expect the rear wheels to be turning 1.5% slower than the front- more than this and it believes the rear wheels are nearing lockup. The taller OD means the rear as turning 3% slower, so the ABS will kick in sooner and prevent the rear tires from sharing the full braking load, generally resulting in subtly but measurably longer stops. Under throttle you'll see the opposite- the traction control will let the rear tires slip more before it decides to pull the plug.
Moving to the handing balance, the car will tilt towards understeer. You'll also change the roll center heights fractionally even if you did both front and rear, contributing to a small handling balance change. Finally the taller sidewall on the narrower rim will be a little floppier, resulting in the rear not following the front quite as faithfully and overshooting slightly, etc. Thus it will probably feel less precise if you're sensitive.
All of these effects are subtle, but each results in the car not working quite as the designers intended. In this case the designers are very good, so they will have gotten things close to ideal in the first place. GT3 customers were lucky enough to get a tire and a car designed together, which has many advantages. As time goes on better tires will become available, but initially you'll be hard pressed to find anything that improves one area without sacrificing others. If I was looking for more grip I'd probably try the Trofeos (which sacrifice wear and perhaps noise/ feeling) instead of trying to change sizes. A lot goes into matching the car to the tire, and it's easy to screw it up.
Last edited by Petevb; 02-27-2015 at 05:00 PM.
02-27-2015, 12:16 AM
#25
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I wouldn't personally put the 325/30 20s on my car, particularly without changing the fronts as well, because I believe it will negatively impact performance in most areas.
There might be some exceptions with a competition car- the increased footprint with a matching front could increase grip, if balanced correctly, more than it hurts performance with worse feel, ride height, etc. But it'd take time to dial in and it's not clear you'd be ahead in the end. For the street there are simply too many drawbacks:
The rolling diameter difference you mentioned will hurt braking distances and potentially acceleration. The ABS expect the rear wheels to be turning 1.5% slower than the front- more than this and it believes the rear wheels are nearing lockup. The taller OD means the rear as turning 3% slower, so the ABS will kick in sooner and prevent the rear tires from sharing the full braking load, generally resulting in subtly but measurably longer stops. Under throttle you'll see the opposite- the traction control will let the rear tires slip more before it decides to pull the plug.
Moving to the handing balance, the car will tilt towards understeer. You'll also change the roll center heights fractionally even if you did both front and rear, contributing to a small handling balance change. Finally the taller sidewall on the narrower rim will be a little floppier, resulting in the rear not following the front quite as faithfully and overshooting slightly, etc. Thus it will probably feel less precise if you're sensitive.
All of these effects are subtle, but each results in the car not working quite as the designers intended. In this case the designers are very good, so they will have gotten things close to ideal in the first place. GT3 customers were lucky enough to get a tire and a car designed together, which has many advantages. As time goes on better tires will become available, but initially you'll be hard pressed to find anything that improves one are without sacrificing others. If I was looking for more grip I'd probably try the Trofeos (which sacrifice wear and perhaps noise/ feeling) instead of trying to change sizes. A lot goes into matching the car to the tire, and it's easy to screw it up.
There might be some exceptions with a competition car- the increased footprint with a matching front could increase grip, if balanced correctly, more than it hurts performance with worse feel, ride height, etc. But it'd take time to dial in and it's not clear you'd be ahead in the end. For the street there are simply too many drawbacks:
The rolling diameter difference you mentioned will hurt braking distances and potentially acceleration. The ABS expect the rear wheels to be turning 1.5% slower than the front- more than this and it believes the rear wheels are nearing lockup. The taller OD means the rear as turning 3% slower, so the ABS will kick in sooner and prevent the rear tires from sharing the full braking load, generally resulting in subtly but measurably longer stops. Under throttle you'll see the opposite- the traction control will let the rear tires slip more before it decides to pull the plug.
Moving to the handing balance, the car will tilt towards understeer. You'll also change the roll center heights fractionally even if you did both front and rear, contributing to a small handling balance change. Finally the taller sidewall on the narrower rim will be a little floppier, resulting in the rear not following the front quite as faithfully and overshooting slightly, etc. Thus it will probably feel less precise if you're sensitive.
All of these effects are subtle, but each results in the car not working quite as the designers intended. In this case the designers are very good, so they will have gotten things close to ideal in the first place. GT3 customers were lucky enough to get a tire and a car designed together, which has many advantages. As time goes on better tires will become available, but initially you'll be hard pressed to find anything that improves one are without sacrificing others. If I was looking for more grip I'd probably try the Trofeos (which sacrifice wear and perhaps noise/ feeling) instead of trying to change sizes. A lot goes into matching the car to the tire, and it's easy to screw it up.
