rolling mass vs diameter which is the greater evil

Hi

My quaife issue installation resolved, moving onto tyres....

I have 2 sets of wheels.
1. 195/80/15 Gen4 steels with Dunlop grandtrek AT20. Tyre and wheel weight 17kg
2. 215/75/15 Gen3 SZ4 Alloys with Falken wildpeak AT3wa. Tyre and wheel weight 20kg.

At just £30 per wheel, (Rim plus tyre), I purchased the 195/80/15's as a short term solution having been let down with the OME lift kit and facing a six month supply delay having already paid, but wanting to wait. 215/75/15 were rubbing (with spacer) and rather than start trimming I bought 6 of the £30 wheels, thinking it would be a stop gap solution.

Turns out as a day to day driver , on road, the  lighter 195/80/15's are more nimble and feel better (in summer). So to preserve the tread depth on the 215/75/15's i've left them on until winter.

there is only .4 of an inch difference in diameter between these 2 tyres.
There is 3kg per corner difference in weight between the 2 wheel options.

There is a noticeable difference in acceleration between the 2. the 215/75/15 blunts the performance.

So to my point and question. Which is the greater evil to blunting performance, increased diameter or rolling mass ?

with this in mind, is it logical to look at a 205/80/16 tyre size if wanting to gain ground clearance without too much extra mass ?
With a 2 inch lift is this size doable without trimming ? without reduction gears ? without upgrading further ?

Anyone got 205/80/16's ?

As always help greatly appreciated.

Thanks
Andrew











 

I keep looking at 205.50r16 as my next set in a medium compound gravel spec. Obviously not for off road use as the ground clearance is significantly reduced. But being significantly smaller and lighter they are going to have a rather noticeable impact on acceleration. Back to your question some of the jimny I have driven you can definitely feel the increase in weight as you say but the additional weight is a function of them being a much more robust construction which is relevant if you are using it off road, I would be more interested in that than a few kg rotating mass on an off roader.

hi

Agreed but the crux of my question is all things being equal a narrower tyre should weigh less than a wider tyre of the same type. 205 width appears to be the sweet spot to get the increased diameter (205/80/16) without excessive width and corresponding weight. more diameter for your weight of rubber.

It's not clear which vehicle we're discussing, so my comments on rubbing probably don't apply to a Gen 3, the remainder, on rotating mass are applicable to any & all vehicles, and they are in fact not based on a Jimny.

That said, I'm quite surprised that 215/75R15 tyres rub, primarily because that's what I've been running, for the past four years on my wife's three door Gen 4, and the last six months on my five door, and they don't rub.

That said, we are running them on the OE Gen 5 alloys with no spacers, I don't know what the offset on the Gen 3 alloys is, or whether spacers are needed with them, but, I can't help but wonder if that's the cause of the rubbing.

"Rolling mass" is a bit of a "simplification - theoretically the steel rims will be heavier than the alloy rims and the 195/80R15 tyres will be lighter than the 215/75R15 ones. The complication here is the distance of the rotating mass from center of rotation.

A heavier tyre on a lighter rim will put more of the rotating mass further from the center of rotation than a lighter tyre on a heavier rim.

This, in my experience has a bigger impact on braking than it does acceleration. The rolling diameter has a bigger impact on acceleration.

Jimny specific observations...

Switching from the OE 195/80s on the Gen4 to 215/75s does have an impact on rolling resistance, to my mind it's negligible, it certainly did not show up in fuel consumption, but my daughters certainly noticed it - unknown to me, they were in the habit of rolling the car into the corner of the garage when putting it away, and one day it was noticeably harder to roll, that was the day the tyres were fitted.

The impact on acceleration has barely noticeable on the three door (manual transmission), it didn't quite have the same "zip", the only noticeable difference on the five door (automatic transmission) was a slight increase in tyre noise.

Both cars are noticeably more "planted" on the road with the wider tyres, my wife's comment was "it dances less".

I did/do have certain reservations about upsizing the tyres, especially when it comes to braking as previous experience with larger Suzukis have taught me just how much of an impact the increased rotating mass can have, and I made a list of parts to upgrade the brakes on the three door should it be necessary, turns out that for a 215/75R15 the OE brakes are fine.

I have half my OME suspension (for both Jimnys) on hand, the other half is in Florida waiting to be shipped out to me, I should have everything on hand late in December, so I'm probably looking at a January install, I will run the 215/75s for at least another year before I consider a change in size, gearing, etc.

Not sure if you have a Gen 3 or 4.

Personally I really wouldn't worry about rolling mass unless you are going for lap times. With such small differences you are talking about, I'd be surprised if you could really tell from the drivers seat. Or even if you could, you'd very quickly get used to the new normal.

I have a stock power Gen 4 on 235/75R15 AT's in place of the 195/80R15 originals and a stop at 215/75R15 on the way. The gearing is a bit different, but tbh it feels just as lively as it ever has. I cannot feel or notice a detriment caused by bigger heavier tyres.

I wouldn't rush to the assumption that the arbitrary size on the side of the tyre is a useful indicator of what that tyre will weigh. The construction type of the tyre will have a significant impact on it's weight, you won't find a 205,80r16 in a light highway construction, it will be either a "c" an "at" or "mt" all of which will have increased sidewall and tread plies over the much smaller 195,80r15 road tyres that are standard on the 4. Therfore they will be heavier. But less prone to damage and puncture.