Unsprung Weight & Rotational Mass

[thezoneR32]

I came accross these articles which Im sure may be common knowledge to some but I thought very interesting reading.

Unsprung Weight

Rotational Inertia

Now Im starting to worry about replacing my 17"x7.5" Denvers with 18" x 8.5 GTC? They will have less unsprung weight but will have more rotational inertia.

Although this calculator http://www.csgnetwork.com/tiresizescalc.html

tells me that my 225/45/17 Pirellis have a tire diameter of 24.972inches and my 235/40/18 Re002's have a tire diameter of 25.402inches which makes an increase in rotational mass of only .428 of an inch or 1cm! I just went out and measured them and this is correct! So...........what does this mean? Is 1 cm of extra rotaional mass a problem considering Im losing about 1.5kg per wheel?

[Stiggy_21]

interesting articles.
stick with what wheels looks best, i wouldn't bother getting all the equations right unless your doing track/hillclimbs.

[thezoneR32]

interesting articles.
stick with what wheels looks best, i wouldn't bother getting all the equations right unless your doing track/hillclimbs.

Cheers mate, was an intersting read nonetheless. Might look at brakes to reduce unsprung weight, ****, I didnt even know what that meant until today...........now where was that bbk group buy thread........

[dylan8]

why not go to 225/40/18 if thats the case about the diameter change

[dainese]

Its still a healthy weight saving if I'm not mistaken

Sent from my Transformer TF101 using Tapatalk

[kaanage]

You will only gain rotational inertia if your are running a tyre that is heavier than the original or if the rim portion of the wheel is noticably heavier than the Denvers. It is VERY hard to predict if that will be the case due to the possible variations in tyre and wheel construction. The only way to be sure is to test the rotational inertia with both wheels with tyres mounted.

I seriously doubt you should be worrying about this - I'd be more worried about the harsher ride and the greater likelyhood of the rims being damaged by potholes.

[fred27]

Losing 1.5kg and reducing unsprung weight is good.
Would not worry about the rotational inertia, you would be hard pressed to tell the difference, probably only with a stopwatch at the track.
With a lower profile, you need to be careful with big potholes and the kerbs when parking.

[thezoneR32]

All good advice thanks fellas!