That S4 manifold of mine ....
02-09-2015, 09:21 AM
#16
Pro
Thread Starter
Okay, you caught me.
I've been carefully and secretly amassing all of the requisite tools and supplies to undertake this brazen step toward performance nirvana.
Now.....the only remaining question....do I start with the crepe paper streamer or the elmer's glue...
It's too overwhelming....perhaps I'm not worthy....
I've been carefully and secretly amassing all of the requisite tools and supplies to undertake this brazen step toward performance nirvana.
Now.....the only remaining question....do I start with the crepe paper streamer or the elmer's glue...
It's too overwhelming....perhaps I'm not worthy....
Polystyrene cups for coffee.
If it makes you feel any better my 'garage' resembles a mad inventor's shamble.
02-09-2015, 09:33 AM
#18
Pro
Thread Starter
Hi Jerry.
I measured the differences in my driveway, before and after the change. Same pressures.
Mind though, the 'old' 45 rubber was very well worn, while the 50 rubber was new.
When driving in steady 60 mph freeway traffic my speedo registered a 'needle width' above 60 mph.
Very fortunate outcome. Whew.
Cheers.
02-09-2015, 09:39 AM
#19
Rennlist Member
Hi Jerry.
I measured the differences in my driveway, before and after the change. Same pressures.
Mind though, the 'old' 45 rubber was very well worn, while the 50 rubber was new.
When driving in steady 60 mph freeway traffic my speedo registered a 'needle width' above 60 mph.
Very fortunate outcome. Whew.
Cheers.
I measured the differences in my driveway, before and after the change. Same pressures.
Mind though, the 'old' 45 rubber was very well worn, while the 50 rubber was new.
When driving in steady 60 mph freeway traffic my speedo registered a 'needle width' above 60 mph.
Very fortunate outcome. Whew.
Cheers.
Have you looked at the raw data at tirerack. They provide rotations/mile.
02-09-2015, 09:50 AM
#20
Pro
Thread Starter
02-09-2015, 09:17 PM
#21
Rennlist Member
First of all the wheel does not roll around a circle. It rolls around a point and that point is the center of the axel. The distance from that point to the surface upon which the wheel is rolling is the rolling radius. Pie has nothing to do with it.
On the other hand, if you double the rolling radius you will get something like a "rolling diameter," and then if you multiply that by pie you will get the rolling circumference, which, I think is what you are actually talking about.
Now I see how you got 7 percent. The difference between a tire with new tread (about 10/32 inch) and a nearly worn out one (with say about only 3/32 left) is probably just about quarter of an inch. That, together with the difference in tire sizes of about half an inch in rolling radius, will increase the percentage I got by calculation of 3.9 percent; so 7 percent make some sense.
On the other hand, if you double the rolling radius you will get something like a "rolling diameter," and then if you multiply that by pie you will get the rolling circumference, which, I think is what you are actually talking about.
Now I see how you got 7 percent. The difference between a tire with new tread (about 10/32 inch) and a nearly worn out one (with say about only 3/32 left) is probably just about quarter of an inch. That, together with the difference in tire sizes of about half an inch in rolling radius, will increase the percentage I got by calculation of 3.9 percent; so 7 percent make some sense.
02-12-2015, 12:15 PM
#22
Pro
Thread Starter
First of all the wheel does not roll around a circle. It rolls around a point and that point is the center of the axel. The distance from that point to the surface upon which the wheel is rolling is the rolling radius. Pie has nothing to do with it.
On the other hand, if you double the rolling radius you will get something like a "rolling diameter," and then if you multiply that by pie you will get the rolling circumference, which, I think is what you are actually talking about.
Now I see how you got 7 percent. The difference between a tire with new tread (about 10/32 inch) and a nearly worn out one (with say about only 3/32 left) is probably just about quarter of an inch. That, together with the difference in tire sizes of about half an inch in rolling radius, will increase the percentage I got by calculation of 3.9 percent; so 7 percent make some sense.
On the other hand, if you double the rolling radius you will get something like a "rolling diameter," and then if you multiply that by pie you will get the rolling circumference, which, I think is what you are actually talking about.
Now I see how you got 7 percent. The difference between a tire with new tread (about 10/32 inch) and a nearly worn out one (with say about only 3/32 left) is probably just about quarter of an inch. That, together with the difference in tire sizes of about half an inch in rolling radius, will increase the percentage I got by calculation of 3.9 percent; so 7 percent make some sense.
The rolling radius is probably better described as 'instantaneous radius', and is the only thing that determines the rotational speed of the wheel for a given road speed.
As I type I'm thinking of a wheel equivalent being a solid wheel with it's radius equal to the instant radius of the tyred wheel.
Not saying anything you don't already understand, but once that vertical slice of rubber above the contact patch leaves the road it does all sorts of wobbling, stretching etc but it has no further bearing on the road speed.
The instantaneous diameter (and circumference) could be anything, so to speak, (hence my reference to the balloon tyres on Mickey's car).
It's a situation where the 'lower' vertically measured radius can differ to the 'higher' vertically measured radius. As in the expanding Top Fuel dragster tyres of the '70's. Academic consideration for us but important to tyre designers.
When I changed wheels (and tyres) on the 928 I made it a point to measure the rolling ie instantaneous radius of a rear wheel, before and after.
Specifically because of the stringent speed laws operating here ( 1km/hr over can get me booked) I needed to know, with some degree of accuracy, the change I could expect in my speedo reading.
Law Enforcment view here has changed from "ok - we'll make allowance for your speedo error" to "speedo error is your problem, and we don't give a rats **** as to the cause - you fix it". Don't want to get booked? Simple - if in doubt, stay under the posted limit according to your speedo reading.
See, the problem is that if I drive at VDO indicated speed with 45 profile rear rubber, I'm 8% slower that the other freeway traffic. Not good.
So if the speed limit is 100km/hr (60mph) I need to mentally correct for the speedo error and calculate "my" speedo reading so I sit level with the other traffic, so: (108/100)*100 = 108kmh. So if I drive with my speedo showing 108 I'll stay with the traffic flow and not be booked.
For 110 it's a bit harder: (108/100)*110 = umm errr ... c'mon ... 108*11 = 1188 ... ok 118.8 indicated is what I need. Gets tiring.
Yes I know - Smartphone GPS and all that - but not quick enough on fast twisting roads.
So, if I know I've corrected 7% of a +8% error, I can be more relaxed when on my favourite remote roads.
BUT ... if you drive a late model car showing a more realistic speedo reading and you fit 50 profile rubber instead of 45, then your speedo reading will have you 8% over the actual speed limit. Errrk.
None the less my basic premise still stands: understand the way in which tyre choice affects your car, and quantify the change where you can.