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Stock tire psi recommendation vs +1 wheel change .

2131 Views 14 Replies 4 Participants Last post by  1stiski
Going from the stock 16” wheels to 17’s and having a increase in width (205/40-17) as recommended by Lorinser , is having negative effects with my TRACTION control kicking in more often, and as the temps drop , I’m noticing it even more (all season tires no less) I have them aired to stock 29 psi front and 36psi rears. My thinking is that having more volume in these tires is making my tires less sticky (harder than they need to be) to the pavement on such a light car . I’m planning on starting to air down a lb or two on each tire to see how the system handles the change . I think that allowing the tire to grip the road better will cut down on the traction control kicking in so much on turns that I’m familiar with and at the speed I’m going , it shouldn’t get funky . Thoughts ?
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I do drive more aggressively with the current set up vs my original stock 453 , but at no time does the car feel at limits and the system kicks in at surprisingly moderate turns .
What's the all season tire (brand/model) you are running?
Some random thoughts.

The vast majority of owners who track and race their first gen Miatas on 205 width tires find maximum grip with cold pressures of 26-28 PSI at each corner. These Miatas are roughly 2150 pounds with nearly 50/50 weight distribution, so pretty close to a 453 in weight, moderately close in axle loads but probably very similar wheel loads as a Miata is capable of cornering and braking so much harder, and pretty similar power levels to manage.

However, I've also found there are times when I don't want to run at those optimal pressures. On bumpy tracks or on the street, a lower pressure can help the tire more closely those imperfections. For instance, cornering on a washboard surface, a highly pressurized tire will tend to skip and chatter, whereas a lower pressure may wallow a bit but the tire will remain in more consistent contact.

Similarly, there may be times when you want more than the optimal pressure. Slightly higher pressures can give you a wider plateau of slightly lower grip, along with more warning of impending traction loss as you near the limit, and a bit easier recoverability if traction limits are exceeded.

Last random thought - tire pressures have to work within "the system", that being the car, its attributes, and its nannies.

Examples of each. The fortwo is a short, narrow vehicle with a high polar moment of inertia - once the back end loses traction in a corner, things get out of hand very quickly. So maybe it's way more important to try to retain grip than to create easier recoverability from loss of grip, even at the expense of lower pressures and a more wallowing response.

The fortwo is a short, narrow vehicle with a high center of gravity. This means it more heavily loads up the shoulder of a tire when cornering. Most low profile tires are speed rated, and accomplish that by using cap plies under the carcass to restrict circumferential growth and reduce heat at high speeds. These cap plies also stiffen the tire carcass under the tread, so if pressures are not high enough, tire sidewall can roll underneath and cause the inner portion of the tread of the outside tire to lift off the pavement. This wears the shoulder quickly and it may be better to accept a bit of skipping and chattering in bumpy corners by using higher pressures, and then also keep more of the tread in contact at other times.

The fortwo is a vehicle that, due to it narrow width and short length, can get out of shape so quickly that even experienced drivers find it hard to catch, and therefore it has an undefeatable stability control system that activates early and in an intrusive manner. Personally, this has been where I've put most of my energies - trying to determine what triggers the system to react, how to get the system to not react, and how to handle what the system does when it intervenes. So far, I can pretty confidently state that I can reduce the number of interventions by running very high pressures, as it seems to be lower frequency vertical oscillations in conjunction with cornering and/or braking that really make the system think a crash is imminent, so much that it will then do something which could instead trigger a crash. For instance, the freeway offramp nearest to my home is heavily used by transit, and crosses a busy overpass. The braking zone is rutted and grooved, and as I take a left turn I cross over the wheel ruts of three lanes of traffic. If I an braking hard before entering the intersection, then turning hard as I go over those tire ruts, the vertical oscillations in conjunction with the cornering always triggers the stability system to aggressively intervene. If I run higher pressures, my tires don't follow the contour of the groove - instead they kind of crash into the rising far lip of the rut, but somehow the stability system is less responsive to this.

All of that just to say there are a lot of factors and you may have to pick and choose the evils you want to live with, but at least give it a try with some more reasonable pressures, like 28 front/30 rear to see what that gets you.
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^^^ I agree .. I’m realized after I looked at my preferred tech sheet I keep in psi’s in my different vehicles , my stock 36 was way to high .. I had moved down to 31 in the tears and kept 29 in the front , it’s behavior was much more acceptable. The higher psi’s were causing early activation .
^^^ I agree .. I’m realized after I looked at my preferred tech sheet I keep in psi’s in my different vehicles , my stock 36 was way to high .. I had moved down to 31 in the tears and kept 29 in the front , it’s behavior was much more acceptable. The higher psi’s were causing early activation .
You might be able to go a bit lower still, and see if that further improves the skipping and chattering you're still experiencing. 26-27 PSI cold would seem to be a reasonable lower limit that still will provide enough rim safety, yet make enough of a difference that it might solve your problem.

I had to go the other way - lower pressures were causing more intervention, and I'm at 32.5/34.5 on OEM sized tires on my 451. There's a lot of impact harshness when running this high, but that tradeoff has been worth it so far.
On our 451’s we are running what the door sticker says we should, and the manners are great, we have plenty of grip without any intervention yet , so the limits haven’t been met yet .. the 453 is a whole different experience, and yeah , Lower psi is working on that better . I’ll have to report back as I go .
205/40R17 tires have very little sidewall. Expect the wheels to be prone to rim damage when going over potholes.

Running 205/40R17 fronts instead of 185/50R16 fronts will increase drag resulting in higher fuel consumption.

Running 205/40R17 all round instead of 185/50R16 fronts and 205/45R16 rears will give you a harder more uncomfortable ride.

It's safer to run front tyres that are 20mm narrower than the rear tyres like Smart specified as standard. ie. 165/65R15 front 185/60R15 rear and 185/50R16 front 205/45R16 rear. It's important to carefully match the front and rear tyres with the same make and model of tire, or at least similarly performing tires in all conditions. For example fitting mismatched front and rear tires such as front tires that work well in the rain or winter conditions and rear tires that don't work well in rain or winter conditions would be a recipe for disaster unless the ESP can save the day as the rears will tend to lose grip first resulting in a dangerous 360 degree spin.

Vredestein Quatrac 5 come in 165/65R15 81T and 185/60R15 84T sizes and 185/50R16 81H and 205/45R16 83H sizes. ie. Smart 453 sizes. They work well all year round where you don't need full winter tires.

Aftermarket 17" wheels might be a lot heavier than genuine Smart 16" wheels as aftermarket 17" wheels are likely to be made for much heavier cars. The aftermarket wheel manufacturer simply using a generic rim and drilling the bolt holes and center bore suitable for the Smart car. This will result in your Smart car not handling so well due to the big increase in unsprung weight.
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^^^ all good points .. but I’m just running what Lorinser recommended for the car . They designed the whole package , so I’m comfortable that the research and development that was put into the design . I’ve hit some pretty good sized holes and although I thought I’d find some damage , they show no such effect , so yes they could be more susceptible to damage , but so far luck is hanging out in my corner . I’ve found my problem with too high of a psi on a more voluminous tire .
The Lorinser wheels are sweet pieces of art , light and well made,not anything cheesy or cheap about them . I agree some inexpensive wheels are questionable in quality and safety .
While it is undoubtedly true that heavier wheels/unsprung weight can cause problems with skipping or chattering on washboard surfaces, my experience is that heavier wheels make much more of a difference in rotational inertia before the extra unsprung weight becomes an issue for slower speed handling. Five years of owning a first generation Miata, with springs that have lower rate than even on the smart, and having experimented with several different sets of wheels of different weights, showed me that I was very sensitive to as little as two static pounds when feeling how it made low speed acceleration feel soft (like the difference between having the A/C on and off) but it required about twice that amount of weight before there was a handling difference, and then only on high speed (70-80+ MPH) bumpy surfaces. Below that and differences in tire behavior overshadowed unsprung weight changes, for me anyway.
What surprises me most about the traction control kicking in so early (IMO) is that with the full Lorinser conversion , the center of gravity has been lowered and it’s track widened , you would think that the traction control would be less active than it was with the stock set up . But I’m sure my psi adjustments will bring about better manners . It’s unfortunate that winter weather approaches and road travel in the 453will be limited till spring, plus the roads will become less than favorable for spirited driving due to due to ample use of salt and grit on our roads , especially in the turns .
Just to be sure, you got the full Lorinser kit, with the lowering springs, correct? I ask because if you use wheels or spacers that increases the track width, as has happened in your case, you effectively soften the spring rate of the suspension. That magnifies the effect of an unsprung weight increase, but would be more then compensated for by the uprated and shorter springs.

Just out of curiosity, did you weigh one of your OEM wheels and one of the Lorinser wheels?
^^^full kit , including the lowering springs . No wheel spacers . And I didn’t weigh the wheels , but I will , I still have the stock wheels and OEM tires in the garage .
Went to 28 psi fronts and 32 psi rears , made a huge improvement .. too bad the roads have cool asphalt, I can’t wait for summer conditions to try more options in the psi settings .
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