Unsprung Weight / Rotational Mass / Wheel + Tyre Weight

[ATM]

Is this the correct place to ask about Unsprung Weight / Rotational Mass / Wheel + Tyre Weight.

Would a change of wheels to reduce weight by 1kg be noticeable?

[Matt Seabrook]

That's very much dependant on home much the car weighs. Any reduction is good but cost needs to be factored in as to the viability of it.

[ATM]

Standard Road car weighing 1,730kg.

Currently wearing 19 inch Run Flat tyres.

Cant decide if 19 inch Go Flat tyres will make much difference or if I should go all out and switch to 18 inch wheels and Go Flat tyres.

I know Run Flat tyres are heavier than Go Flat.

When comparing the standard 19 inch and 18 inch wheels for my car there is a 1.5kg weight difference per wheel.

[Matt Seabrook]

1.5KG per wheel on an almost 2T car is not going to make a big difference at normal road speed depending on how much the wheel and tyre combo already comes in at. If driving at ten tenths you should feel a difference. Then again if you have runflats fitted its unlikely to be a track only toy anyway. Changing from ROF tyres is going to make a bigger difference anyway as the tyre construction is different. Just something to keep in mind is the insurance company needs to be informed of going from runflat to a non runflat tyre as some of them see this as a modification and may want to charge a premium for what they would say is a down grade on the tyres. Suspension, ABS and ESP have been optimized for ROF tyres and BMW have put much effort into checking all eventualities so something else to think about before changing. I know lots of people have done it and are happy with the results but always a good idea to go into a modification with eyes open and have the facts before you change a safety related item. BMW say you can do this as long as its all four tyres of the same construction but advise against it. I hope this is of some help.

Matt

[dblack1]

Matt Seabrook said:

1.5KG per wheel on an almost 2T car is not going to make a big difference at normal road speed depending on how much the wheel and tyre combo already comes in at. If driving at ten tenths you should feel a difference. Then again if you have runflats fitted its unlikely to be a track only toy anyway. Changing from ROF tyres is going to make a bigger difference anyway as the tyre construction is different. Just something to keep in mind is the insurance company needs to be informed of going from runflat to a non runflat tyre as some of them see this as a modification and may want to charge a premium for what they would say is a down grade on the tyres. Suspension, ABS and ESP have been optimized for ROF tyres and BMW have put much effort into checking all eventualities so something else to think about before changing. I know lots of people have done it and are happy with the results but always a good idea to go into a modification with eyes open and have the facts before you change a safety related item. BMW say you can do this as long as its all four tyres of the same construction but advise against it. I hope this is of some help.

Matt

Matt is right on. You will notice very little difference because the car is on the heavy side. Your biggest difference will likely be in braking and acceleration due to reduced momentum.
Most auto manufactures spend a lot of time engineering on every vehicle the produce, and it is unlikely that you have considered everything that the manufacturer did when they selected runflat tyres.
If you select a smaller rim (such as the 18 inch you mentioned) you should (assuming everything else is the same except the diameter of the tyre) have reduced the momentum your wheels carry. The decreased momentum should give you more of a go cart feel; however, it will also make it ride worse at high speed. Reducing the diameter of the tyre will also lower the vehicle without modifying the suspension geometry (which also reduces ground clearance)
If this is for a street car, you should have a contingency plan in place in case you have a flat. If your tyres are runflats, you can just drive to a shop, but without the runflat tyre, you need to at least consider a spare.
If this is for a track car, ditching the runflats should show a gain in performance (don't expect anything amazing though) and regular tyres are generally cheaper than runflats.
If it is a daily driver that sees track use, you could get a second set of wheels and tyres for track use.

[ATM]

19 inch wheel weight 13kg.
18 inch wheel weight 11.5kg.

BMW also sell some forged 20 inch wheels which only weigh 10kg - tempting right.

The car wears exactly same size tyres as the e46 m3. So I popped along to the m power section and did some digging. Seems the Goodyear asymmetric 2 is recommended and available for 660 for a set of 4 in 19 inch. 18 inch is pretty much the same price for tyres only. So in terms of price 19 inch or 18 inch are pretty cheap I'd say. 150ish per corner isn't awful. 20 inch are more like 250 - ouch.

I will try to find some info on tyre weights. Then I can compare the overall weight of wheel and tyre.

[Locknut]

ATM said:

I will try to find some info on tyre weights...

If you go to the Tirerack site and do a search on your tyre size you will be presented with results in the form of a panel for each tyre. At the bottom of each panel there is a grey area with the words "Additional Tire Information", click on this and it will bring you to a new page with seven panels near the bottom, click on "specs" and this will open up information including the tyre weight that you are looking for. Here's a link to my size:

http://www.tirerack.com/tires/TireSearchResults.js...

My size is 225/45/17 and according to Tirerack the difference between run-flat and conventional tyre in my size is 4 pounds (2.3kg)

[ATM]

Camskill quote weights for some tyres and are better than tyrerack as they include decimal places. I've found looking at different tyres and their weights that they get heavier as the load rating increases. Interestingly the load on the Run Flats supplied by BMW are quite low and therefore not too heavy.

Current Bridgestone Run Flats supplied by BMW

Front
225/40RF19 89Y

Tyre Weight (kg) 11.5
Tyre Weight (lbs) 25.31

http://www.camskill.co.uk/m55b0s5062p110650


Rear
255/35RF19 92Y

Tyre Weight (kg) 13.1
Tyre Weight (lbs) 28.86

http://www.camskill.co.uk/m55b0s5068p110648


Goodyear Eagle F1 Asymmetric 2 Go Flats

225/40 R19 89Y

No weight on Camskill

http://www.camskill.co.uk/m55b0s467p105524


255/35 R19 96Y XL

Tyre Weight (kg) 11.6
Tyre Weight (lbs) 25.52

http://www.camskill.co.uk/m55b0s337p109698

[ATM]

I've been toying with the idea of getting some light weight forged wheels. 9kg compared to 13kg for my OE fitment wheels. Will I notice a difference?

[dblack1]

Don't go with the goodyear (most of there tyres are low quality and they don't hold any top track times that I know of) tyre. Pick michelin, they have a lot of good stuff in there pilot line of tires. Just a thought, your driving a heavy BMW, don't worry about the heavy tyre so much as the sidewall strength of your tire.

[PaulKemp]

Locknut said:

ATM said:

My size is 225/45/17 and according to Tirerack the difference between run-flat and conventional tyre in my size is 4 pounds (2.3kg)

4lb = 1kg 814.37g

[one eyed mick]

ATM said:

I've been toying with the idea of getting some light weight forged wheels. 9kg compared to 13kg for my OE fitment wheels. Will I notice a difference?

If you curb them they will bend easier !!!!!

[ATM]

one eyed mick said:

ATM said:

I've been toying with the idea of getting some light weight forged wheels. 9kg compared to 13kg for my OE fitment wheels. Will I notice a difference?

If you curb them they will bend easier !!!!!

I thought forged meant strong. Do they bend?

[dblack1]

Forged can bend, but they are more likely to crack (than cast). Usually lightweight forged wheels are fragile.

[ATM]

dblack1 said:

Forged can bend, but they are more likely to crack (than cast). Usually lightweight forged wheels are fragile.

So where does one go for lighter wheels. I see a lot of people using Racing Dynamics. Some of the OZ wheels specs sound light too.

[cptsideways]

dblack1 said:

Forged can bend, but they are more likely to crack (than cast). Usually lightweight forged wheels are fragile.

Its the other way round Forged wheels are always stronger, due to their density & the nature of the material. Hence why almost all OE wheels are forged.