Printable Version of Topic

Click here to view this topic in its original format

914World.com _ 914World Garage _ using larger (Dia.) wheels/tires

Posted by: Tom Apr 28 2010, 09:30 AM

Have we ever discussed the effects of changing the wheel/tire sizes and the effects on acceleration, both in starting and stopping? I ended up using 195/55/15's on mine and I noticed that the speedo was off about 10% and I expected that and because the overall diameter was smaller, i thought acceleration would be affected slightly also. What I didn't take into account was the rotating mass effect. So, I guess my starting and stopping both have been slightly improved, maybe handling is improved slightly also.

Link to some info: http://www.the-welters.com/racing/rotational.html

I some times wonder how those pickups with 22" wheels brake in an emergency!
Tom

Posted by: Tom Apr 28 2010, 03:18 PM

I see I didn't get the link right, sorry- my computer skills are low, but you can google (the-welters.com/racing/rotational.html)
And read what it says.
Tom

Posted by: RJMII Apr 28 2010, 03:29 PM

That's some pretty interesting reading. I've been looking at different wheels and was contemplating the different aspects of using other styles of wheels. Thank you for posting this.

on your link, just put a space between the : and the h. (the 'info:' is too close, that's why it didnt' recognize it as a link).

Posted by: SirAndy Apr 28 2010, 03:40 PM

Fixed the link ...

Andy

Posted by: charliew Apr 28 2010, 03:42 PM

I have some hollow spoke big 11j18 tt wheels with worn 295/30 tires and they are only a little heavier than some 8.5x17 996 boxter new wheels and 255/40, I think tires. I just checked the weights out.

Posted by: Tom Apr 28 2010, 03:50 PM

Sure got me to thinking why the 914-6 came with 14 inch rims. Maybe Porsche meant these to be a little quicker in the start/stop areas as well as handling better.
This makes a lot of sense to me as I was a gyro tech in the navy. The heavier the rotor, or, the father away from the center of the roataing mass the weight is, the more the gyroscopic effect will be. Motorcycles use this principal to get the bike to lean over. In gyro talk it is called precession. Bikers call it counter steering!
Tom

Posted by: charliew Apr 28 2010, 04:01 PM

Maybe I can blame not paying attention to some loose asphalt on counter steering.

Posted by: Rotary'14 Apr 28 2010, 04:28 PM

I just read some stuff about this very subject today!

FYI:
http://www.caranddriver.com/features/10q1/effects_of_upsized_wheels_and_tires_tested-tech_dept

-Rob

Posted by: Tom Apr 28 2010, 05:22 PM

Thanks Rob, That is why I started this thread, so others who knew more about this could post their knowledge.
Tom

Posted by: kg6dxn Apr 28 2010, 06:33 PM

I switched from 205/50/15's in the rear to 245/45/16's. I did not notice any change in start/stop. Could be my car...

Car:
73 914
Chevy V8
Tranny regeared for V8
930 Turbo brakes (all 4)

As far as SUV's with 22's; The biggest mistake those guys make it not upgrading the brakes. With all that extra rotating mass, they will go through brake parts way too fast and never be able to stop as quickly.

Posted by: charliew Apr 28 2010, 11:31 PM

I had 20's on my harley ford truck and I'm pretty sure the tire diameter was still the stock size. It stopped fine.

Posted by: Tom Apr 29 2010, 08:46 PM

Reading the info on post #8, maybe there is not too much difference in performance due to wheel/tire sizes as long as the overall circumference is the same or near to stock. Even some sizes seem to improve braking slightly over stock, although some decrease in MPG.?
My education continues, I never knew Harley made Ford trucks.
Tom

Posted by: GeorgeRud Apr 29 2010, 08:53 PM

I think that Porsche used the 14" wheels on the 914-6 because they probably had too many left over from the "comfort packages" that they were marketing for the 911s. They were pretty good at using what was available back then as money was tight.

Posted by: Porcharu Apr 29 2010, 09:19 PM

I'm pretty sure that the popular 'inertia dynos' have correction factors for various wheel and tire combos - ie big ass wheel packages put down less power. I drive by an SUV everyday that has 26" wheels - looks ridiculous.

Posted by: strawman Apr 29 2010, 11:11 PM

Several years ago, I built my '85 Toyota MR2 SCCA C Street Prepared car using 13"x9" Keizer wheels to both reduce the rotating mass and to lower the effective gearing. It was certainly more lively in comparison to the skinnier/taller stock wheels and street tires that I used to drive to events. A lot of the Miata guys used the same logic, until a couple of years ago when Hoosier came out with some WIDE 15" tires -- then almost everyone abandoned the 13-inchers. The newer logic is that the slightly greater rotating mass was worth the extra width of the taller tires. That being said, I'll be running 16x9 and 16x7 wheels on my Suby-powered teener.