IcemanG17

https://rennlist.com/forums/racing-a...rformance.html

I posted a thread in the racing forum about "sprung" weight and performance.... Since I am debating buying new larger wheels for the 928 estate to improve its handling (not that its bad now, but always could be better)

Speedtoys

Its not just weight..but where the weight is in the package.

A heavier wheel with more of it's mass near the hub..-vs-..the same weight wheel with most of its mass at the outside edge, will perform better, because it's, and apologies if I get the terminology wrong..moment is closer to the hub, and it'll accelerate faster. Less rotational parasitic loss to deal with to accelerate the wheel.

Of course, the tire weight slapped around the wheel makes a difference as well. The same 25lb tire on a 16" wheel accelerates faster than a 25lbs tire out on an 18" wheel..where both wheels have the same moment of inertia..as more tire mass lives farther out on the 18" wheel.

As a whole unit, a heavier unsprung package takes longer..with more energy required..to get the suspension to react. You have to battle the vertical inertia of the package. The suspension will react slower and feel rougher..in comparison to a lighter corner package of unsprung hardware. One could argue, that a super lightweight corner package will require a more pliable and manageable suspension package as well...I think.

My last fun car project, I removed about 75lbs of rotating mass from the front damper, to the lugnuts out back. It was a -very- noticeable difference in acceleration. Dyno readings were impressive before and after..considering zero powerplant changes were made.

Of course, it's most felt when the difference between your starting speed and ending speed greater, and at lower speeds where you have less parasitic drag to contend with as well.

Brett928S2

Hi

Although I appreciate the weight vs performance logic as noted in the above post there is another angle to consider...

When I changed from standard 16 x 7 slots to 18 x 7.5 (front) and 18 x 10 (rear) i could LITERALLY drive around people (also in 928s) on a VERY sharp race track right angle corner or drive up the inside of them....this was because I had a LOT of extra width and rubber on the track, so it made a massive difference in lap times....

All the best Brett

Speedtoys

Oh yes, of course, but what you did was to obtain a performance goal, the next step is to gain as much efficiency as you reasonably can..within that goal.

seagul extra

There are four types of weight. sprung weight, unsprung weight, rotating sprung weight, and unsprung rotating weight. By far the most harmful type of weight is rotating unsprung weight and the futher away the weight is from the centre of rotation the worse. I have watched you and your car in some of the vidios you have posted here and your performance seems to be very good considering the amount of power you are working with. I don't know what you are running for rubber but I might suggest you may get more from using slicks with the most grip possible that fit your 16" wheels. Maybe BF Goodrigh G Force slicks?

IcemanG17

Currently I am running "R" comps (street legal slicks)....I run BFG R1's in 245/45-16 on 16x8 wheels...... I have looked into real non street legal slicks, but they all are wider and require wider wheels to work properly....so I would need larger wheels regardless..... I guess I could run slightly wider 255/50-16 wheels....but I doubt they would perform much better being pinched down on the 8" rim?

seagul extra

My bad, I ment to sugest the R1's. I am currantly using and older set of Yokohama DOT slicks right now, I think they are called RD 800's . I will be replacing them with a set of BFG R1's when they wear out. It would be nice if you could "borrow" a set of the larger rims and tires and do a back to back comparision. My gut feeling is that you are running a good set up right now and it might be hard to better your lap times with the bigger tires because of the effect on gearing and the extra spool up effect of the larger tires. The larger tires will also likely have a negative effect on braking as they will reduce the leverage of your brakes.

mark kibort

Why do you think that? breaking power is the same formula as acceleration power. it has nothing to do with diamter or gearing. actually, a major effect of larger diameter tires and weight, is more controlable stoping power. the leverage and greater inertia have little to do with stopping characteristics.



Where do you get this information from. you didnt test this on a dyno, did you . what were the results??

Here is the net net of wheel weight. the increased wheel weight has almost the exact same effect, as if 1.4 x the weight was in the car sitting next to you. (2x if on the tire) . What this means is 10lbs of wheel weight, would be worth about 1.5 HP as far as acceleration gain. As a note, going from a larger rim. (aluminum alloy) and replacing the diameter that would usually be rubber (heavier than aluminum) can sometimes yeild a lighter overall package.

You will not feel 1.5hp gains on your car, and this is pretty easy to prove. 10lbs per wheel , would be more like 60lbs and that would have the effect of near 6+hp and that is significant, but its only 6hp. something you are not likely to really feel on the street.

as far as unsprung weight, and handling. yes, that is all about shock dampers and springs. in rough track environments, or very turney courses, it can be a significant factor, but generally, with most tracks being very smooth (not like Sebring for example) and not having a lot of autox "S" turns, its not a very large handling factor.

Tire size would have a more dominant effect.

To your last paragraph, its felt most not when you have the greatest change in speed, but at the lowest speed range where your acceleration time is lowest and forces is greatest. for longer acceleration times, like 7 seconds, like 3-4 th gear, the effects of the greater inertial are not really measurable and noticable.

mark kibort

Ive raced you basic set up for years, before doing all the mods possible. larger tires were a pretty significantly positive change and dont worry about squeezing on 255s on to 8" rims. they will fit fine and you will be faster, especially with DOTs on them. try and get some 9s and 10s and run 275/305s front and rear and you will not be disapointed. need more room up front, pull out the fender as I have done., and make that 935-looking fender mod

mk

Speedtoys

Look..I got the results from my own work on one of my older Supras back about 2002'ish.

Id pulled about 75lbs from the rotating mass of the drivetrain. Crank damper, flywheel, clutch, driveshaft, and the tow rear wheels.

The numbers I recall with ease, were 16lbs per wheel (including before/after tire weight & lugnuts), 23lbs driveshaft, 15lbs FW/Clutch, and about 5 main pulley.


"at the lowest speed range where your acceleration time is lowest and forces is greatest. "

Yes..thats what I was thinking, I didnt characterize it correctly, but you are correct.


I recall, with about 2mos in between dyno runs, about a 12hp difference.

Later on, a number of other mods got me over the 300rwhp goal I'd set out to reach on the stock fuel, air, turbo...just worked in efficiencies all around the powerplant. Not bad for a car that was 220Hp crank, to begin with.

RKD in OKC

I had a stock 2001 Boxster S. I changed the wheels to wheels that was about 5 lbs per wheel heavier. It cost me about 3 seconds a lap. I presume the difference in the courses I run and a typical track that the difference would be a little less. I ran the same tires on both sets of wheels.



I changed wheels because the originals were the "highly polished turbo look" wheels and they got scratched up putting new tires on. It was cheaper to get a new set of wheels than replace or refinish the turbo look wheels that got scratched. I ran in a couple of events before I found the Roock USA competitions and put the tires from the turbo looks on the Roock Competitions. The Roock Competitions are HEAVY 2 peice wheels. 38# front, 32# rear. My times were slower with the Roocks, but I kept them because I really really like the look of them on the Boxster.

seagul extra

Iceman, how does your front to rear balance feel with the 245's all around? My Yokohama dot slicks are 245 45 16 on the rear and 225 50 16 on the front. I'm using a set of S4 manhoe covers 7" front and 8" rear. If I can get my hands on another pair of the 8" rims I would consider going to the 245 45's on all corners as well. This would also alow me more options for tire rotation.

mark kibort

The great thing about pulling rotating mass from the engine, wheels etc, is that you can calculate its effect REALLY easily. all you need to know is the size and the inertia and then what the rate of acceleration is. 16lbs per wheel is significant! the effects would be like 24lbs as if it was on the car, so X 4 would be near 100bs as an effect. that could be near 10hp . But again, it depends on the acceleration rate, so it would be most noticable in 1st gear, and much less in the other gears, being almost insignificant in 4th or 5th.
This is why the dynos have such large rollers, as to rule out and remove the inertial effects of mods like these. and if you run a brake dyno, the effects will be "0" because there is no acceleration of the engine on the brake dyno, only speed and torque.

So, you couldnt get any of those results of 12hp from the rotating weight reductions. Its simple physics. (unless you were doing the dyno runs in 1st or 2nd gear) in 4th, where most of us do dyno runs, the acceleration is so slow that the effects would be reduced dramatically. how long does it take to do your dyno? 7 seconds? speeds are 70 to 140mph, which is 800rpm to 1600rpm at the wheel. what is the force or HP needed to accelerate 15lbs over 7 seconds from 800 to 1600rpm? If I was to guess, I would think we are in the 1hp range. (per wheel) BUT, do the calculation for reving the engine, (lightened flywheel) or 1st gear, you see the effects are quite large!

The numbers you reduced were huge. 38lbs from the flywheel effectively (clutch and driveshaft). 60lbs from the wheels and tires. (not as much of an effect, as it is after the gearing). Still, near 100lbs of spinning weight.

I did a dyno stock, and then removed 25lbs of clutch weight going to the dual disc, but the dyno results were not any different due to that. why, because it spun up too slowely. revs in idle were much crisper though!

mark kibort

Would you please stop posting stuff like this. its so silly. you dont gain 3 seconds a lap, even if you driving the "Ring" for changing out a set of rims.

"Real" racers usually find time improvements in the sub second range for MAJOR changes. (like, slicks to DOTs, or major suspension changes, aero, etc) you are seeing a 5% decrease in lap time base on almost nothing.
we can easily applie the Hp-seconds effect of such a change. handling is usually more difficult to quantify, but based on the type of track and the effects of such change, it can be estimated.

I worked very hard on finding 1 second a lap. I got 1.5 seconds for adding the stroker. tenths here and there for set up and 1.5 sec. for a A series hoosier vs a worn Toyo. Ive done the test multple times, on the same track on a car with no changes. add 5llbs per wheel and Ill spin the same lap times !

RKD in OKC

Nope, sorry Mark, won't stop posting my real world experiences no matter how silly or "amateur" you think those experiences may be. And I think you are an ******* for calling my experiences silly and not "real" and am personally offended. I did state there is a difference in our race courses and the typical track, say maybe a lot more lower gear acceleration. So, Bite Me *** Hole.

And by the way, removing 100 lbs, by taking out interior bits, did not measurably decrease lap times in that same Boxster S before the wheel change.