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      07-07-2008, 08:27 AM   #23
Mickb
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Originally Posted by footie View Post
Mick,

I can assure you that you should feel the ride improvement for a start and the way the car behaves in both steering and way the car finally lets go of grip should also be different. How much this is notice is up for debate and depends on the quality of the driver but the facts are that there will be a difference no matter how small.

The thing about acceleration differences I can't see how but I am remaining open minded of this subject.
Footie

I know the theory, and in lots of places you are talking about how things "should" feel or operate based on what you have read on the boards and short test drive. Of course there is some difference, I just think people are exaggerating it way out of proportion.

I actually own one of the cars and have put a couple of thousand km on it. It came with 18's and I have since switched to 19's ( I had originally hoped to keep the 18s as track rims but my bbk wont fit under it), and I am telling you that there is nothing in it.

Granted, I did not take one out on a track, then switch wheels then repeat the process to isolate differences but IMO all the people above who are claiming night and day differences are talking crap.

I could possibly grant you that perhaps the 18's had a smidgen more sidewall compliancy but that could be my imagination.

Regards

Mick
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      07-07-2008, 09:43 AM   #24
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Mick,

I too am under the impression that what ever there is will be small and it would take a very skilled driver to notice the subtle differences, but the fact remains that they are there no matter how small.

I have also driven numerous cars with different wheel sizes and to be honest with you, there is more of a difference from changing tyre brands than you will notice changing rim size. The most notice thing apart from ride comfort when changing up rim size was how the car turns into the corner, with the bigger rim and lower tyre profile the turn in will be quicker, factional yes but it is there all the same. The other thing that can be noticed is the steering, lower profile means more tyre deflection from the surface which is less noticeable with a higher profile tyre.

This assumption that acceleration or topspeed will lessen due to bigger rim size is something I can't work out, only if the width increase can the performance decease but I still think the width would need to be quite big to be really noticed.
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      07-07-2008, 10:21 AM   #25
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Quote:
Originally Posted by footie View Post
Mick,
This assumption that acceleration or topspeed will lessen due to bigger rim size is something I can't work out, only if the width increase can the performance decease but I still think the width would need to be quite big to be really noticed.
I don't have this problem. I understand full well that if the bigger rim size is accompanied by additional weight, or the distribution of the weight of the rim is more unfavourable then there is a performance penalty, in much the same way as any weight increase in the car. The difference in unsprung weight is amplified due to the rotation factors other have alluded to above.

I seem to remember a discussion paper (of either Porsche or Honda) on the effect of PCCB's where they approximated that a reduction in unsprung weight was the equivalent of a 4x reduction in sprung weight. i.e. lets say ceramic rotors saved you 5lbs per corner this would be the equivalent of taking out 20lbs from the car (eg seats or exhaust). This is obviously weight reduction for dummies and doesnt take into account weight distribution etc

All I would argue is that the difference between the OEM 18's and 19's is insignificant in this regard and I bet if you put a vbox on the same car and tested 0-60's etc you wouldnt even get .1 of a difference in the outcome.

Mick
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      07-07-2008, 01:07 PM   #26
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Quote:
Originally Posted by Mickb View Post
I don't have this problem. I understand full well that if the bigger rim size is accompanied by additional weight, or the distribution of the weight of the rim is more unfavourable then there is a performance penalty, in much the same way as any weight increase in the car. The difference in unsprung weight is amplified due to the rotation factors other have alluded to above.
When I was saying I can't work it out I meant when both versions 18" and 19" weight almost the exact same. Now if the weight is greatly increased with the 19" then I could understand it more.

Quote:
Originally Posted by Mickb View Post
I seem to remember a discussion paper (of either Porsche or Honda) on the effect of PCCB's where they approximated that a reduction in unsprung weight was the equivalent of a 4x reduction in sprung weight. i.e. lets say ceramic rotors saved you 5lbs per corner this would be the equivalent of taking out 20lbs from the car (eg seats or exhaust). This is obviously weight reduction for dummies and doesnt take into account weight distribution etc
I think this is a misunderstanding of what is meant by this 4 times sprung weight, I believe it's the improvement in direction changes and steering feel they are talking about instead of improvements in actual performance.

Might be wrong but it's how I understand it to be.

Quote:
Originally Posted by Mickb View Post
All I would argue is that the difference between the OEM 18's and 19's is insignificant in this regard and I bet if you put a vbox on the same car and tested 0-60's etc you wouldnt even get .1 of a difference in the outcome.

Mick
That's basically the same as I posted some time ago, I think when measured the improvement will be in 0.00?.

The only real reason for picking one over the other is personal preferrence.
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      07-07-2008, 03:34 PM   #27
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Despite weighing almost the same, the 19" rims will have slightly more rotational inertia than the 18" rims as the majority of the rim mass is distributed further away from the rotation axis (by 0.5" in this case). But that effect would be so small that one cannot feel it.
the 19" rims MAY have slightly more rotational inertia - key word MAY. the forged 19 rim may wigh less at the outer rim. the spokes may wight less. the 18 inch tire could wiegh more. we dont have specs on the moment of inertia of the two do we?
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      11-04-2008, 05:03 PM   #28
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Quote:
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Despite weighing almost the same, the 19" rims will have slightly more rotational inertia than the 18" rims as the majority of the rim mass is distributed further away from the rotation axis (by 0.5" in this case). But that effect would be so small that one cannot feel it.
that's exactly right, theoretically (not taking into account distribution of mass throughout the wheel, which can vary) a 15" rim (notwithstanding the brake clearance issues!) with a tire size to have the overal wheel size of these 19" and 18" setups and of identical weight, would have a more noticeable difference in rotational inertia, as the mass is concentrated around the hub....most drag strip cars have pretty small rim sizes, im thinking this plays a factor in that decision!
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      11-08-2008, 07:16 AM   #29
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The 19" tires are slightly lighter than the 18s and that mass is even further away from the center than .5" for the rims. I doubt the difference between the two stock sizes is significant or even noticeable.
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