Tires mods usually take the form of Plus one or Plus Two upgrades. I went the other way. I replaced my stock 17" SLites w/ runflats w/ a set of 15" SSR w/ 205-55-15 toyo proxes T1S. If the stock wheel/tires weigh around 50lbs the new SSR combo is only 27lbs. I reduced the 23lbs per corner. What an improvement in acceleration. The car redlines so much faster. You step on the gas and the car jumps. This is my first mod and I wanted to do something that will improve performance, hadling and braking. How much horsepower is gained by reducing the unsprung weight?

 

the reduction in weight is not what is giving you the gains, it is the reduction in radius. there is a smaller moment in the smaller tires, that gives you a bigger 'jerk' than your old 17"'s (the change in moment over the change in time: dj/dt for those real physics nuts) the smaller tires allow you to use more of the torque, more quickly-thus better acceleration, better braking (as long as the tires don't slip)

 

I'm sorry but i would have to slightly disagree. Yes the reduction in diameter is a huge part of it but i went from the 16" Xlites to 16 Rota's at 13lbs and notice a huge difference. I also notice the difference every month since the Rota's are my Autox wheels and when i put them on i smile.

I weighed the Xlites with the runflat tires at 40lbs and the Rota's with Azenis at 34lbs. HUGE Difference!

 

1 lb unstprung is percieved as 4 lbs sprung being removed


Alex

 

That kind of reduction in weight is super dramatic! I did a minus one.

(17x7 -> 16x7.5 SSR comps) and I can feel a dramatic difference too...especialy braking!

 

205-45-17 = 24.3" diameter
205-55-15 = 23.9" diameter

Slightly shorter than stock (close enough to be the same size). Mods should enhance the performance of the car. If your paying for performance parts, the car should go faster, reduce braking distance, and improve handling.

 

First, if you go with 15" rims then you have several good choices for tire size based on the width of the rim you get. You can talk to Alex@tirerack.com to get the details and it would be possible to get 205/50-15 tires to fit and maybe improve on the contact patch if you want to use the tires for performance reasons.

Second, NO horsepower is "gained" by reducing the wheel weight. You keep the horsepower that you had before but you improve the power to weight ratio in a very meaningful way. You shift to smaller diameter wheels which helps gearing and loose a lot of unsprung weight.

More related threads-
https://www.northamericanmotoring.co...lighter+wheels
and
https://www.northamericanmotoring.co...lighter+wheels

For street driving I have aftermarket 17x7" rims and non runflat tires for 38 pounds each compared to stock S-lytes and runflats for 48 pounds each.

For autocross I use 15x5.5" stock holeys and Kumho victoracer 205/50-15 tires at 30 pounds each. It makes a very big difference to loose that much weight. Speedo is affected though.

 

Your right. The car still has the same hp with the new wheels. I wonder if wheel horsepower will be the same if you compare the stock wheels w/ the SSRs 15". I have not seen any thread with the dyno comparison of different wheelsets. Dyno runs are usually done on bolt on parts.

 

somedays it just don't pay to read what you said...
if you ignore the my first sentence in my previous post, or substitute:"it is the combination of a reduced radius and reduced weight that gives you a smaller moment" my post will then make a little more sense.not a whole lot more, just a little

 

For thread on dyno results and lighter wheels see-
https://www.northamericanmotoring.co...lighter+wheels

 

Lets see a pic of this mod.

 

Pics. Springs next mod.

 

Sorry, having hard time trying to figure out how to upload pics. My pics are too large. I'll take another pic in the day using smaller setting.

 

Pics.

 

pics.