Wheel time weight savings and performance gains - Page 1 - 911/Carrera GT - PistonHeads UK

Worth a watch ... possibly not what I expected ... Edited by ChrisW. on Monday 20th October 20:38

Thanks for sharing. I do enjoy all of Jon's testing (yes I'm a nerd) but they sometimes generate more questions than answers. As suggested in the comments, he needs to repeat this with a lower powered car.

Apologies in advance for the long post ... For a car used primarily on the road, he rather misses the point, unsprung weight impacts more on ride, handling and roadholding. The more unsprung weight the damper has to control, the worse the ride, handling and roadholding, and this is more noticeable on/around bumpy corners and especially mid corner bumps (something the UK now has more than its fair share of) Much is made of wheel weight, but careful selection of tyres is just as important, as the same size tyre can vary in weight considerably between different manufacturers and even between their different product ranges. I think on a 245 section 19" tyre, there could be easily be a 2kg difference. Wheels are another matter altogether, and whilst the OE wheels are invariably heavy, they also tend to more durable than the majority of aftermarket wheels. But a 3-5kg reduction in weight is entirely possible. So a 7kg reduction in unsprung weight per corner isn't to be sniffed at. But then we come to the oft discussed topic of brakes ... Ten + years ago I bought a BMW 1 M Coupe, shortly after I joined a tour which consisted of roughly 12 cars heading down to Cannes via some excellent roads including the Gorge du Verdon and the Gorge du Tarn (and on our day off, a trip up the Col de Turini) I found the car's standard brakes wanting, they got increasingly noisy under duress and eventually the pedal turned to blancmange. They recovered after a 20 minute coffee stop, but had lost their edge and the time before the pedal went mushy when under duress, decreased from thereon. Once home I started scouring the usual websites for a set of AP Racing front brakes, and some months later I found a set of used AP six pot calipers and 378x 32mm thick strap drive discs along with all the necessary mounting hardware. The stock 1M Coupe brakes (shared with E92 M3) utilised a single piston, sliding caliper manufactured in cast iron, this required a carrier to mount the caliper to the hub carrier, again made from cast iron. Combined weight 5.1kg. The front discs measured 360mm and were 30mm thick. The discs utilised an aluminium hub/bell and a cast iron disc which floated on radial pins (the same arrangement as Porsche used on the 997.2 GT3 steel discs) The discs weighed 10.3kg each. So the combined disc, caliper and carrier weighed 15.4kg. The alloy six piston AP calipers weighed 2.8kg The 378 x 32mm thick discs weighed 6.4kg The discs utilised an aluminium bell which would have weighed no more than 1kg, and the caliper mounting bracket would have weighed no more than 600g. Total weight of the AP set up 10.3kg. A saving of 5kg. If one were to choose a set of Apex wheels as used in Jon's test, the fronts (same width and diameter) would weigh 3.3 kg less than the OE wheels. With careful tyre choice, a 10-10.5kg reduction in unsprung weight per corner would have been possible, or put another way, a 42kg decease in unsprung weight across the front and rear axles. On a car weighing 1650-1700kg that maybe doesn't sound much, but ... Get your bicycle out of the garage, remove the front wheel, hold both ends of the axle in between your thumb and forefinger and then get your wife, son, daughter, next door neighbour to spin the wheel/tyre as fast as the can. Now try and "steer" the wheel/tyre combination left or right, and feel the resistance you experience. Do the same again with the wheel tyre stationary ... That wheel and tyre may weigh 2-3kg. The equivalent on a car will weigh (in the case of Jon's "Stupidly heavy wheel" 35kg (and that doesn't include the brake disc, the hub, the caliper, the wheel studs/nuts etc etc) So the total weight is probably closer to 50kg. Imagine the forces involved when that 50kg is rotating at 50mph. Little wonder Jon stated the "Stupidly heavy" wheels dulled the steering response and feel, and rest assured they have a similar effect on the roadholding and handling, as the less unsprung mass the damper has to control, the better the roadholding and ride. Just the fitment of the 5kg lighter per corner AP front brakes on the 1M Coupe was transformational. The previously slightly overly stiff front suspension that had formerly hammered over cats eyes, now floated over them instead. The steering was lighter, more feelsome and the turn in considerably more aggressive, all of which drastically increased my confidence in the car’s abilities on bumpy high speed corners. All the above came with a simply massive increase in braking efficiency, which in turn made the car more confidence inspiring. Cup 2's whilst not a perfect all year round tyre, are amongst the lightest tyres for their size. Apex make excellent, sensibly priced, lightweight AND more importantly, strong wheels suitable for road and track use. Carefully chosen brake upgrades can make for drastic unsprung weight reduction. Add all three up, and the difference in ride, handling, roadholding, steering weight, feel and feedback can be truly staggering on the UK's incredibly poorly maintained road network.

the gyroscope effect is based on conservation of angular momentum, being moment of inertia times angular velocity. Moment of inertia is a product of mass times the radius squared. So lighter/heavier tyres have a lot more effect on that than the wheels. Hence the best ever tyre produced for a road car is the Avon ACB10, sadly more of the realm of the Caterham than the 911. Doubly brilliant because of its ultra lightweight construction and lovely breakaway characteristics being a crossply. Not that great in standing water! I'm getting off topic now.

BertBert said: the gyroscope effect is based on conservation of angular momentum, being moment of inertia times angular velocity. Moment of inertia is a product of mass times the radius squared. So lighter/heavier tyres have a lot more effect on that than the wheels. Hence the best ever tyre produced for a road car is the Avon ACB10, sadly more of the realm of the Caterham than the 911. Doubly brilliant because of its ultra lightweight construction and lovely breakaway characteristics being a crossply. Not that great in standing water! I'm getting off topic now. There are several benefits to reducing unsprung weight. Whilst I don't doubt what you've said is accurate, it would be helpful to all if you state, in layman's terms, what is beneficial about the above you've stated in bold. Faster acceleration. Lighter/more responsive steering. Better damping/damper control. Improved roadholding.

Slippydiff said: Apologies in advance for the long post ... For a car used primarily on the road, he rather misses the point, unsprung weight impacts more on ride, handling and roadholding. The more unsprung weight the damper has to control, the worse the ride, handling and roadholding, and this is more noticeable on/around bumpy corners and especially mid corner bumps (something the UK now has more than its fair share of) Much is made of wheel weight, but careful selection of tyres is just as important, as the same size tyre can vary in weight considerably between different manufacturers and even between their different product ranges. I think on a 245 section 19" tyre, there could be easily be a 2kg difference. Wheels are another matter altogether, and whilst the OE wheels are invariably heavy, they also tend to more durable than the majority of aftermarket wheels. But a 3-5kg reduction in weight is entirely possible. So a 7kg reduction in unsprung weight per corner isn't to be sniffed at. But then we come to the oft discussed topic of brakes ... Ten + years ago I bought a BMW 1 M Coupe, shortly after I joined a tour which consisted of roughly 12 cars heading down to Cannes via some excellent roads including the Gorge du Verdon and the Gorge du Tarn (and on our day off, a trip up the Col de Turini) I found the car's standard brakes wanting, they got increasingly noisy under duress and eventually the pedal turned to blancmange. They recovered after a 20 minute coffee stop, but had lost their edge and the time before the pedal went mushy when under duress, decreased from thereon. Once home I started scouring the usual websites for a set of AP Racing front brakes, and some months later I found a set of used AP six pot calipers and 378x 32mm thick strap drive discs along with all the necessary mounting hardware. The stock 1M Coupe brakes (shared with E92 M3) utilised a single piston, sliding caliper manufactured in cast iron, this required a carrier to mount the caliper to the hub carrier, again made from cast iron. Combined weight 5.1kg. The front discs measured 360mm and were 30mm thick. The discs utilised an aluminium hub/bell and a cast iron disc which floated on radial pins (the same arrangement as Porsche used on the 997.2 GT3 steel discs) The discs weighed 10.3kg each. So the combined disc, caliper and carrier weighed 15.4kg. The alloy six piston AP calipers weighed 2.8kg The 378 x 32mm thick discs weighed 6.4kg The discs utilised an aluminium bell which would have weighed no more than 1kg, and the caliper mounting bracket would have weighed no more than 600g. Total weight of the AP set up 10.3kg. A saving of 5kg. If one were to choose a set of Apex wheels as used in Jon's test, the fronts (same width and diameter) would weigh 3.3 kg less than the OE wheels. With careful tyre choice, a 10-10.5kg reduction in unsprung weight per corner would have been possible, or put another way, a 42kg decease in unsprung weight across the front and rear axles. On a car weighing 1650-1700kg that maybe doesn't sound much, but ... Get your bicycle out of the garage, remove the front wheel, hold both ends of the axle in between your thumb and forefinger and then get your wife, son, daughter, next door neighbour to spin the wheel/tyre as fast as the can. Now try and "steer" the wheel/tyre combination left or right, and feel the resistance you experience. Do the same again with the wheel tyre stationary ... That wheel and tyre may weigh 2-3kg. The equivalent on a car will weigh (in the case of Jon's "Stupidly heavy wheel" 35kg (and that doesn't include the brake disc, the hub, the caliper, the wheel studs/nuts etc etc) So the total weight is probably closer to 50kg. Imagine the forces involved when that 50kg is rotating at 50mph. Little wonder Jon stated the "Stupidly heavy" wheels dulled the steering response and feel, and rest assured they have a similar effect on the roadholding and handling, as the less unsprung mass the damper has to control, the better the roadholding and ride. Just the fitment of the 5kg lighter per corner AP front brakes on the 1M Coupe was transformational. The previously slightly overly stiff front suspension that had formerly hammered over cats eyes, now floated over them instead. The steering was lighter, more feelsome and the turn in considerably more aggressive, all of which drastically increased my confidence in the car s abilities on bumpy high speed corners. All the above came with a simply massive increase in braking efficiency, which in turn made the car more confidence inspiring. Cup 2's whilst not a perfect all year round tyre, are amongst the lightest tyres for their size. Apex make excellent, sensibly priced, lightweight AND more importantly, strong wheels suitable for road and track use. Carefully chosen brake upgrades can make for drastic unsprung weight reduction. Add all three up, and the difference in ride, handling, roadholding, steering weight, feel and feedback can be truly staggering on the UK's incredibly poorly maintained road network. This The only fly in the ointment is that the dampers and spring rates are set up for the heavier wheels and brakes so you don’t get all the benefit you might.