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Larger Rear Tire for 18x9.5, stock on 2029 Slingshot S

2K views 11 replies 6 participants last post by  Cobra 
#1 ·
Having recognized the bad news that my post stroke balance issues were going to keep me from riding any two wheel vehicle, bike or motorcycle, and taking advantage of the factory rebate on the 2019 slingshot models in June I ended 50 years of motorcycles and bought a 2019 Slingshot S at a very good price. The S fit my needs just fine as I did not need a sound system or nav (mini iPad, google maps and Pandora work fine and were already on hand), the engine on all the 2019 models was the same, and the smaller wheel/tire package on the S for real world driving (i.e backroad corner carving not track days) was just fine for me.

I have done several simple mods to make the S more responsive and I am very pleased with the result with one exception. Powering out of a corner especially in second gear has you sliding sideways more than necessary which I believe is an issue as to the ability of the rear tire to get enough traction. The fronts are fine after adding the lowering and new spring kit so I would prefer to keep the existing 18x9.5 rear and not go to a new wheel/tire set.

Does anyone have any experience with the results of going to a wider tire on the stock rim? And what is the largest tire that will fit on the stock S rear wheel?

Thoughts/advice please.
 
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#5 ·
Valid point in that the tire sizes Polaris selected seem designed to spin the rear tire and protect the belt. As you go to a wider tire, the loading on the tire goes down, so you can lose traction at larger widths. I view the challenge of NOT spinning the rear tire while accelerating as quickly as possible as part of the fun of driving a Slingshot.
 
#4 ·
I have a 2015 Slingshot and have run as wide as a 295/30ZR18 tire. Generally speaking, a 285 tire is the widest most tire manufacturers will spec for an 18x9.5 wheel. The safest thing to do is to identify a tire you're interested in running and then check the manufacturer's website to see what widths they recommend for an 18x9.5 wheel to ensure that you're operating the tire within the manufacturer's design limits. www.willtheyfit.com can give you a visualization as to how the tire/wheel combo may affect tire positioning, but the website's speedo calculations are incorrect. One big advantage of WillTheyFit's site is it can also handle different wheel Offsets, but https://www.discounttire.com/learn/tire-size-calculator provides a more accurate speedo calculation. I have been running a 17x10.5" (28mm Offset) 10th Anniversary COBRA Style wheel from American Muscle to mount a Nitto 555 G2 315/35ZR17 tire. The 28mm Offset means the tire sits in the same position with respect to the right side of the wheel/tire, but the center of the tire is shifted around 1.5" to the left, but I have never experienced any handling problems attributable to the off-center tire/wheel. I have also run a Circuit Performance CP32 18x10" wheel (35mm Offset, IIRC) to run an Achilles ATR Sport 2 305/30ZR18 tire. Both of these tires are roughly the same diameter as the stock tire, so there isn't any significant impact on handling, etc. The drawback to the Achilles ATR Sport 2 is that the 30 profile tire is a PITA to mount to the CP32 wheel. Both of these wheels cost me less than $150 each.
Looking at the Nitto 555 G2 specs, Nitto recommends anywhere from a 255/45ZR18 to a 315/40ZR18 for an 18x9.5" wheel. Achilles lists the max width for an 18x9.5" wheel is a 275/35ZR18 or a 275/40ZR18 tire. Nankang lists a 275/35R18 NS-25 tire will fit a 9.0-11.0" wheel. Sumitomo shows a 285/30 or 285/35 18" tire as the largest for an 18x9.5" wheel. Whiole I have tried tires from all of these manufacurers, I haven't necessarily tried all of the recommended sizes. As long as you stick to a tire whose width meets the recommended wheel width range, you should be OK. Try this search - Search results for query: wide tires for posts I have made regarding tires and sizes. Other folks have also made a lot of posts about tires, but I think I probably have the most posts about fitting tires on a Base model Slingshot, especially when using wheels originally meant for older Mustangs.
 
#6 ·
My issue is not losing traction while accelerating but breaking the tire loose and having the rear tire break away when accelerating out of a corner. I am not talking about the Slingshot’s inherent oversteer When paused in a corner. I’m an old (and I do mean OLD) road race driver. My first race car was a beat up 1959 Porsche Speedster. Rear engine, crude rear suspension and 3” rims with mid 60’s race rubber equalled world class oversteer. Corners were normally negotiated with the tail hanging out at a 45 deg angle, so I have no problem dealing with the inherent oversteer. What I am trying to get is a tire back there that will not breakaway so easily and with little feel/warning when it does. I want to carve corners, not drag race. I would also like to maintain the current ‘flat’ attitude of my modestly modified Slingshot by going to a tire with the same sidewall height.
the lowering kit dropped the rear and the front ends equally and I want to keep that intact for the sake of handling.

Hopefully that makes my question clearer. Wider and stickier is what I am looking for and for cornering not burnouts which will strews the belt.
 
#7 ·
A wider tire will in fact increase traction, however the opposite occurs on a wet roadway due to the lower psi of wider tire. One other consideration needs to be body roll, which reduces tire contact in a turn. Because the rear tire and accompanying suspension is rigid to the frame, except for vertical travel, if you can envision going into a turn and the tire has a lateral movement due to body roll, that leaves even less of the tire making contact with the roadway. Consequently, if you have a wider tire, more of the tire should remain in contact with the roadway. One other important consideration is the suspension. If you reduce body roll via suspension and/or heavy duty sway bar, more of the rear tire will remain in contact with the roadway. Have fun in the curves, I know I do. That's what it's all about.
 
#9 · (Edited)
Also, try looking for a stickier tire, say something around a UTQG of 200 or less compared to the 300 rating applied of the stock Slingshot tires. Unfortunately, that type of tire usually is more expensive and has a relatively short tread life compared to tires with a higher UTQG rating.
 
#11 ·
Thanks, that is my understanding as well. Staying as close to the same tire diameter to maintain a level ‘posture’ the 285/30-18is within .6” of the stack tire, a 265/35-18. The 285 per the mfg’s website will fit nicely on the 9.5” wide stock wheel, and that’s what I am planning on doing.
 
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