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Old 30 Nov 2023, 11:17 (Ref:4187847)   #1
Johnno3
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Benefit of Lower Unsprung Weight?

Hi,
I’ve been using 15” mags with semi slicks in club motorsport, and was wondering about the benefits of lowering the unsprung weight by switching to 14” mags. (The consensus seems to be that lowering the unsprung weight is beneficial.)
Some details:
195/50 x 15 tyres. Combined wheel and tyre weight 16.75 kg (37 lbs).
185/60 x 14 tyres. Combined wheel and tyre weight 14.5 kg (32 lbs).
That’s a weight saving of 2.25 kg per corner, BUT with the 14s the sidewall is taller and the tread width slightly narrower. Any thoughts on how they might compare assuming an identical make of semi slick?
I’ve run both 14s and 15s in the past with semi slicks but no back to back comparison available.
(The car is a sedan weighing about 900kg, runs about 1.5 degrees negative camber, stiffer than standard suspension and has OK power.)

Thanks,
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Old 30 Nov 2023, 19:43 (Ref:4187893)   #2
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A lower unsprung weight means better grip and worse comfort.
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Old 1 Dec 2023, 07:52 (Ref:4187915)   #3
GreenMachine
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GreenMachine should be qualifying in the top 10 on the gridGreenMachine should be qualifying in the top 10 on the grid
Looking at the two combos, you are trying to keep a similar rolling diameter so as not to alter the gearing? But the tread width is lower, so LESS grip and less precise handling (sidewall slop)?

We don't know important details - what sort of suspension, what rule set you have to comply with and what does it say about this. What is your tyre of choice, and how much freedom does that give you?

First question is, can you run wider tyres on those 14s, maybe go down to /50s too and wear the gearing change?

Can you buy lighter rims - I have ~140rwkw running 15x8s that are about 6kg, 225/45 semis and I would not be within cooee of 16kg.

Tyre choice/rim size can get too big and the drag from the big sticky tyres can make the car slower on the straights (if you run on longish straights), what is your output?

Too summarise, you are looking at a binary solution to a complex problem. Wheel diameter, rim width, power, rubber availability are the main variables, they all presume you have the lightest wheel you can afford.

If you have a tyre supplier that knows their stuff, they can offer good advice. What do the leading cars in your class run (or maybe the fast competitors in same cars as you), that is a good starting point if no other available.
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Old 1 Dec 2023, 22:16 (Ref:4187992)   #4
Johnno3
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Thanks for the replies.
RE: "...you are trying to keep a similar rolling diameter so as not to alter the gearing?" Yes. But now that I think about it, the car has relatively tall gearing after changing the diff (3.9 changed to 3.3) so 14s with a 50 aspect ratio tyre could help lower the gearing.
RE: "Can you buy lighter rims?" Yes, that is an option. (Comes at a cost of course.)
Something else I just thought of is that there are still a few choices in 15" semi slicks but not much choice in 14s. Nankang make the AR-1 in a 14, but only as 185/60 x 14.
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Old 1 Dec 2023, 22:46 (Ref:4187996)   #5
GreenMachine
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GreenMachine should be qualifying in the top 10 on the gridGreenMachine should be qualifying in the top 10 on the grid
Yes, I run 15s because we have (still) good tyre choice in that size. This is what I alluded to above - if the gun tyre is brand X, you are locked into those sizes available, otherwise you have to take the laptime/tyre life penalty. You then build your suspension around the size you select. I'd go with 15s over 14s any day, because of the larger contact patch, and your power will dictate the width - big hp turbo/V8 as wide as you can, at least 8s, 1.5l atmo, 6-7.

Take this with a grain of salt, I am not racing where you are racing and I am not racing your car. Again, look around, talk to the tyre people.
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