How does a larger rim effect the car

[Jack960]

Hey guys, I know there are many posts like this but I have some more specific questions.

Over the years I have heard much debate about what larger rims do to a car. No one has really given a real answer with a reason and I am curious because I like the look of 19's and 20's but not if it takes away the performance.

From my understanding, as long as you keep the same rolling diameter (larger rim smaller tyre wall) and have a rim that weighs roughly the same as stock, there should be an improvement in handling. Due to the fact that a larger rim is generally wider giving the wheel more surface contact with the road. Is the correct?

I realize the biggest compromise is the comfort as the tyres will be low profile and will absorb less shock meaning you feel more. Is this damaging to the suspension of the car?

I have heard things about moments on the rim, a larger rim has a larger diameter meaning the bulk weight of the rim is distributed further away from the center. Thus it would take more effort to rotate it. does that make sense?

Thanks for your time

[Nikked]

Performance...are you going to be regularly taking the car to track days?

If not...then you need not worry.


As long as they arn't chrome 20's....

[mik]

Just my opinion, but, I reckon larger wheels can have a detrimental effect on steering, with extra weight , and with changing the geometry of the steering,, I guess it's about weighing the plusses against the minuses, myself I think if a car was primarily designed with wheel size x, I would'nt go much beyond that, again just my opinion.

[tweeked]

Depends on your suspension set up. A very firm suspension and very low profile tyres will most probably have less overall roadholding, especially with a few bumps thrown in.

You need to look at the overall package, and the intended purpose you are trying to build the car for. Straight line grip and cornering power often have conflicting requirements.

[dimka100]

If all you want is bling then by all means go 20inch and live with the consequences that come with it.

If you want more performance then look into proper performance tyres and lightweight sports wheels (e.g. Enkei).

[Ben73]

I have 20s and they feel the exact same as stock 18s.
There is a little more road noise, but 99% of the time you can't hear it.
My 20s are not even much of a low profile due to the large size of my car.

[Whitey-AMG]

Just a thought here.......unproven - but I have a suspicion

Larger rims are invariably heavier....so they will contribute to more unsprung mass - meaning more pressure on suspension components and handling.

I'm also thinking that larger / heavier rims will also have a higher rotating mass - so peak power to the wheels "might" be affected. There might not be too much in it - and I'm not sure this has ever been replicated on the dyno - but the physics dictates that you'll need more power to spin the assembly. Of course - this argument goes out the window if the larger rims are better quality and of a lighter material - magnesium wheels etc......

As an example - the 19"rims on a BFGT ( my personal ride ) weigh a tonne. They're built like the proverbial and I've had no issues with runout or bent wheels over potholes - but I reckon with a substantially lighter magnesium wheel - there may be better suspension control and small acceleration difference - particularly in roll on.

[ebxr8240]

Larger rim less tyre so it may counter itself ??
Breaking and bent rims over our potholed roads don't mix..
BTW when have I ever seen race cars on 20" rims ??

[FPV GTHO]

There's a lot of different sides to this argument. For a Falcon running a 26" height tyre it's generally a given that an 18" is best. That will give a normal width rim a 245/40 tyre as the closest match.

17s and 18's give better grip, but 18's and 19's have a wider range of tyres available. 20's don't have anything going for them performance wise though.

That doesn't mean 20's are naturally bad. A GT500 runs 28 inch tall rubber, that gives it a 255/40/20 front tyre. Almost the same as the 245/40/18 Falcon.

[Shonky.]

You need some sidewall in your tyres. It forms part of the cars suspension.

In my opinion 18's are probably best for a large car.

Mine has 19s from the factory, so if I get aftermarket rims I will probably stick with this diameter and go 19x8 or 19 x 8.5 and 19x9.5 or 19 x 10

I reckon 20s are a mistake if you care about anything other than looks

[Jack960]

Thanks for all the information. Think ill be looking for a set of 19's as close to the stock weight as possible.

[svo supporter]

Aftermarket rim weights vary considerably...I have struck 18" factory rims that are heavier than aftermarket equivilant....I have also struck the complete reverse.

So if it's to do with a weight saving, you'll need to spend a little more coin to get a lighter weight rim.

The problem with the bending of the inner edge is due to the profile of the tyre, not absorbing the impact of the holes in the road...This combined with no support on the inside of the rim, all adds up the problem.

So having a 245/45X18 won't allow the rim to bend as easy as a 235/40X18, as the tyre profile is larger, therefore allowing the tyre to absorb more of the impact.

Food for thought

[F6E]

[QUOTE=shonkymofo;4508633]
I reckon 20s are a mistake if you care about anything other than looks[/

Should we let HSV in on this ?

HSV consistently smashes FPV in the ride/handling stakes in every comparo ( even in the latest FPV/HSV/SRT face off, the Clubbie had a win based on being a better steer, even when slower in a straight line ).

Ugly, Clown Face cars, I know, but they handle ( and on 20's )...go figure...

[FPV GTHO]

HSV run the same profile tyres on their 20's as FPV do on their 19's though.

[Mont5.0]

I dont know if it was coinsedence but my power steering hose gave way when i went to 19's.

[F6E]

So , in response to the OP and following that theory, assuming a similar total weight, there is no performance disadvantage running 20's over 19's with the same profile tyres ie 35's ?

[Angeldust]

put it this way, my HRT clubbie came out with 18's and semi'slicks..(not 19's like normal ones did) specifically for amateur track work.

[noflac52]

Top Gear tested this using a HSV commodore and found that a road car performed best in all conditions with 50 series tyres. Any less than this and the performance dropped off due to variations in the usage that a road car has to contend with. The original rolling diameter will dictate what size rims you will need to use to be able to run 50 series tyres.

[LG17]

I know this isn't scientific but remember when Top Gear did a run on their track with a Clubsport (I think). They did runs with 18s and 20s (?). The 18s were far superior with lap times.

[Sprint]

Originally Posted by Jack960

Hey guys, I know there are many posts like this but I have some more specific questions.



I have heard things about moments on the rim, a larger rim has a larger diameter meaning the bulk weight of the rim is distributed further away from the center. Thus it would take more effort to rotate it. does that make sense?

Thanks for your time

Perfectly true with Physics, Momentum and Inertia, Imagine a small spiniing top on the end of a stick, change of direction might not seem hard.

Now increase the size of the spinning top and see the required effort to control the change in direction.Even try this by increasing the diameter of the spinning top but keep it the same weight, it will still have a effect.

Also yes there are accelleration and braking issues also with all that weight near the outer edges as you put it.

cool science forum im on and it answers all lifes physics questions lol

http://www.thescienceforum.com/

[lilmattie]

Not sure how it affects the newer models, but I've found that with the older falcons, the larger diameter the rim = less profile on the tyres = harder ride.

I've also heard that the profile of the tyres also spreads when accelerating, and the lower profile tyres don't spread as much because they have less tyre to work with? Can someone clarify?

[2011G6E]

Mate, even on a plushly-suspended G6E, 19's give you a harsher noisier ride...sitting in our shed is a set of 18's for when these tyres wear out on the 19" turbo wheels we have now...
Not to mention you become paranoid about big potholes, bitumen edges, and general lump and bumps.

Unsprung weight...,I remember reading years ago when 16's and 17's were starting to appear an article that said people might be interested to go to a tyre place and physically lift up a big wheel and tyre combo and a "standard" steel wheel and tyre combo and you would be surprised at the weight difference...this is why there was a minor craze in some areas back around 2000 for done up Falcons and Commodores (older ones) to have 16" "Interceptor" steel wheels painted black...light weight, and still able to fit what was, at the time, performance tyres in a "larger" size (when 16" was seen as "big").

The first time I changed a tyre on our G6E I was honestly shocked how much the damn things weighed...no wonder new cars are getting so porky...many years back it was about reducing weight where ever possible to make a car more efficient...not it's not a concern anymore apparently...

[mik]

i struck a bloke in an xr8 stranded in the bp at campbellfield about a month ago, he did a tyre in the morning and buggered another that night, both his tyres where 19 or 20 inch wheels with low profile tyres, the bloke asked for some help , i lived 5 minutes away so i wizzed home and got one of my old xr wheels to get him home.
the moral of this story is low profiles suck on crappy roads, the other thing is if you do a good Samaritan thing, make sure you get a deposit for good Samaritan items lent, because me bloody wheel never came back ....lesson lear`nt.

[F6E]

Originally Posted by Sprint

Perfectly true with Physics, Momentum and Inertia, Imagine a small spiniing top on the end of a stick, change of direction might not seem hard.

Now increase the size of the spinning top and see the required effort to control the change in direction.Even try this by increasing the diameter of the spinning top but keep it the same weight, it will still have a effect.

Also yes there are accelleration and braking issues also with all that weight near the outer edges as you put it.

cool science forum im on and it answers all lifes physics questions lol

http://www.thescienceforum.com/

People ! We are NOT increasing the size of the "spinning top" here !!

It remains the same size/diameter, just the ratio of tyre to rim fluctuates between 18, 19 & 20 " rims ..the overall diameter remains the same....

Not rocket science. OD remains constant, tyre wall reduces, rim size increases

[FPV GTHO]

There is more weight in the wheel than the tyre though, which I think is the intention of the comparison. I know my car is faster on the quarter mile with my chasers than my normal 275/35/18s, despite the chasers being taller.

If you could have the same profile tyre on a small rim and a large rim, all things equal including weight, centre of gravity, polar moment etc, then they should perform the same and the larger diameter should even last longer as there's more tread face. Problem is for a given wheel design, the larger diameter will generally always be heavier.

[F6E]

Originally Posted by FPV GTHO

There is more weight in the wheel than the tyre

Says who ? A speculative assumption . The comparison was based on the size of the spinning top not it's composition...

Not being argumentative but the popular misconception is that 20's are sooooo incredulously/ridiculously bigger than, say 18's, where in fact , with the correct +2 figment they are the exact same size/diameter, just the tyres have 1 inch less sidewall....therein lays the contention, does that loss of sidewall impeded or improve performance ?

[Jack960]

When talking about spinning tops, I think he was using it as an example for "moments" on a wheel. Because, with a 20" rim, there will be more mass located further away from the center of the wheel (as opposed to a 17" or 18") thus it will have more momentum on the outside. Meaning you would need more force to get it rotating, but once its rotating it has more momentum. Is this not where people get the increased top speed but less acceleration theory from?

[Nikked]

Physics aside...


Nothing wrong with 20's for general road use, aslong as you have some side wall, and don't mind dodging the odd (or many) potholes. All depends on the car, i wouldn't have 20's on my car, but on a BA, i can see the point...

[F6E]

Originally Posted by Jack960

When talking about spinning tops, I think he was using it as an example for "moments" on a wheel. Because, with a 20" rim, there will be more mass located further away from the center of the wheel (as opposed to a 17" or 18") thus it will have more momentum on the outside. Meaning you would need more force to get it rotating, but once its rotating it has more momentum. Is this not where people get the increased top speed but less acceleration theory from?

Point taken, But we are still taking the tyre out of the equation, Rim and tyre rotate as one, their combined mass acts as one, they do not differentiate where the bead meets the rim, their respective masses are not that dissimilar ....
Surely a 25kg 20" rim/ tyre combo has the same inertia as a 25kg 18" combo ?

[BaraFGXR6]

Dont people chasing big Dyno numbers go with a smaller and lighter rim? Whether this is just to fudge the dyno numbers or actually provies benefit on road im not sure.