Hey guys,

I just put on some new 19" HRE wheels on my 996TTx50 (Stage 4 w/ RWD conversion) and it seemed to slowdown the throttle response of my car noticeably.

The new 19" front wheel and tires weigh 7 pounds more each and the rears are about 6 pounds more a piece compared to the stock wheels and tires.

With my factory wheels and tires, I could spin the tires a lot when I get on the throttle car hard in the first three gears, but with my new 19" HREs there is almost no wheel spin. Also, the new tires stick better in the corners.

Both the factory 18" wheels and 19" HREs have MPS2 tires.

Is it possible my factory wheels have heat cycled PS2, which could be the culprit? Or is adding 27 pounds of unsprung weight that big of a deal on acceleration?

How much RWHP does 27 pounds of unsprung weight sap from my car? Is there a formula?

Thanks for your help
Rob

 

There is a formula I foget it is either mulitply by 3 or 4 the most important thing is to reduce unsprung weight not add it..

 

Your crazy...(kidding). I couldnt tell when I put 19 Iforged on mine....
JB

 

that amount would make a huge difference, both in acceleration
and in handling over bumps, and also in wear and tear on the
shocks that have to control a much heavier weight up and down...
Joe

 

I had added +1 rims on my old BMW 328i which were 5 lbs more on each corner. I definitely felt the difference in acceleration and how "agile" the car felt. My bad for not checking prior to ordering the set. Big diff is that I only had 225hp on that car while you are over 400 so it shouldn't be felt as much.

Anyways, definitely not the thing to do if you can help it...I always chuckle when I see 22" bling sets on trucks since I cannot imagine adding that much weight to the stock suspension and what it does to an already poorly handling vehicle!

 

My car with 18" tires and stock wheels stuck a lot harder when the tires were new. With a few thousand miles, I notice a lot more wheelspin. I think it has something to do with the stickiness of the tires as they wear...

 

When I added my 19" Champions, the car was quicker and hit .1 bar higher under WOT. Went from .7 to .8 before being tuned.

 

Even at the same weight and the same outer diameter (measured at tread of tires), increasing the size of the wheels will increase the overall inertia. You're moving the weight outwards, so it takes more power to turn them.

 

I am assuming your Champion wheels are the lightweight 1 piece wheels? I would guess if you reduced unsprung weight that the opposite would happen; better acceleration.

Do you know the weight differences?

Thanks

 

Has anyone perfomed before and after dyno runs after installing heavier 19" wheels on their turbo?

 

Rob,

For every 1 lb of increased unsprung weight you add to tires, multiply it by 2 to find out the sprung weight equivelancy. For every 1 lb added to wheels, multiply by 1.5.

For your new setup, I'd say you added the equivelent of 45 lbs or so, sprung weight to the car. About the same as going from a half tank of fuel to a full tank of fuel.

 

Thanks Scott!

That is not as bad as I had originally thought. I think I will keep the wheels now.

BTW, I thought I read years ago that increasing unsprung by 1 pound was like adding 6 or 7 pounds to the car. However, I have no idea where I read this.

Thanks again.
Rob

 

Adding wheel weight is multiply bad because:

1 - Any weight slows acceleration.
2 - unsprung weight hurts handling over any
road irregularities because it takes more time for the
springs to stop the wheel in bump, and return it to
the ground on rebound. It also adds wear to the shocks,
which were calibrated for the lighter wheel.
3 - Wheels are also rotational mass, so it takes more
energy to spin them up, even if you're on a roller, and it
takes more energy to stop them spinning, so it also puts
more load on the brakes. Because brakes are stronger
than the motor, this effect is more felt in acceleration
than braking.
4 - Because your tire overall diameters are staying the same,
you lose the suspension/cushioning that the wider standard
tire wall gave you, so this gives a harsher ride, sends more
stress and shock load to the chassis.

Joe

 

I just went the other way, traded heavier solid spoke (added 5-7 lbs per wheel) for hollows. BIG TIME difference in how fast the car spools up

This is just like the effects of the LWFW.

I've always thought it was like a 4:1 ratio....don't make me yank out the kinematics books

 

This all makes a lot of sense. I guess I should have done my homework before buying my HREs. However, they look absolutely ****!

Thanks for all your feedback