TIRE DOWNSIZING EXPERIENCE! aka 15" to 14"

[Guest jjohart]

Hi there, from a 280ZXT owner who may or not have the best of both worlds (ride/handling) with one definite shortcoming..literally!

I found myself scratching my head last spring, being that I got my 83 in fall, and bought snows for it immediately....not wanting to keep changing wheels for winter/summer duty, and also against buying a 2nd pair of 15"s...seemed redundant..I scoured for a staggered, 16" wheel set, but couldn't find anything exciting for less than 2 grand at least (i.e Compomotive).

Anyway, a local fellow sold me a set of 14" ET wheels for 50 dollars. I had them painted body color when I had the whole car repainted. I went through this forum for opinions on using low profile, ultra high performance/summer tires, and was warned that my speedo would "go nuts", that the ride would be harsher than the 15's. Well, yes, the speedo is optimistic by 5 mph, but the ride is supple and performance is downright amazing. I don't mean to brag, but the pundits were dead wrong..you can have it all with a 15 to 14 downsize, provided you don't need all season traction.

My only point here is in the question or two I still have. It seems in going from stock 205 60 15 to 195 50 14, I have inadvertently "lowered the car". Would anyone be able to tell me how much, just out of converting the numbers...it seems like a small amount when I look at the car, but boy can I brush the undercarraige/exhaust if I go over a poorly maintained steep driveway lip!!! I am also wondering if, in effect, I have gained some, if not all of the suspension "improvement" (and liability) from this lowered set up that some do by buying the Eibach kits, etc (which incidentally weren't made available for my 2 + 2! Either way, the car feels supple, less herky jerky up/down on take off, and perhaps less stress is placed on the suspension bits by the reduction of rolling mass resistance (if I understand that correctly!)..not a bad deal for older Z's posing as daily drivers!

Thanks

John

[blueovalz]

I'm seeing about a 38mm drop in ride height (25.5mm due to tires, and 12mm due to wheel size).

 

The 60 series tire will have a sidewall of 123mm vs the 97.5mm sidewall of the 50 series tire (thus the 25.5mm difference). This, in addition to the 12mm drop due to the smaller diameter wheel, adds up to 1.5".

 

Lowing the CG of any car will improve the handling. 'Nuf said.

[A. G. Olphart]

Almost 'Nuf said .

 

The lowering kits run stiffer springs to avoid the bottom dragging that you are experiencing.

Suspension setup always involves priorites and compromises; tight handling normally comes at the expense of some ride quality.

[Guest jjohart]

Hi, and to last respondent, yes, agreed, some compromises are necessary, yet, as somewhat of an anti SUV response, am I not technically in the right that slightly smaller wheels and lower profile tires provide some, if not all of the benefits of much more tricked out lowered suspension bits and big fat tires, with greater mileage, less rolling resistance, and better mileage and accelleration, with just a nominal increase in ultimate handling?!!? Seems to me slightly smaller is better, but then didn't Howard Stern get in trouble with the FCC for pontification of that sicko attitude?!!?!

Thanks

John

[A. G. Olphart]

There are others on the board who race (and win) who may chip in here, but I'll give you my .02 worth.

 

Smaller tires and wheels tend to weigh less, and less unsprung weight helps in rough (bumpy) corners. The shocks have less to control, so the tires are in more continual contact with the road surface. There is another penalty in running bigger/heavier wheels and tires beyond unsprung weight: rotational inertia. The wheels/tires act as flywheels, making the vehicle marginally slower to accelerate and decelerate. Bigger wheels, bigger flywheel effect.

Your lower profile tires have less sidewall flex, which helps lessen body lean and promotes a more direct/responsive steering feel. I'd guess that and the lower C.G. is why you like them.

You are correct in that they also give you a slightly higher (numerically) effective overall gear ratio for quicker acceleration (but likely poorer gas mileage- sorry).

 

Tire construction and compounds can play a huge part in both rolling resistance (for mileage) and roadholding. That is one of many subjects of which I have no knowledge.

[Brad-ManQ45]

The reason that the ride is suppler than the 15/60 setup inspite of the reduction in sidewall height is the higher loading of the tire contact patch along with the lower unsprung weight.

 

One has to be careful when changing tire sizes to make sure that the load capacity stays near the same or higher than stock, or else at high speeds or heavy cornering the tires won't hold up to the demands put upon them.

 

A great example of this is the '90-'96 Q45 (4000 lbs + passengers). Stock tire was a special Michelin 215/65/15 tire on 6" rims. The current crop of tires this size's load ratings are about 25% below the original tire specs.

 

To get a current tire with the load capacity that that special tire had nowadays you have to get a Michelin 235/60/15 Pilot HGT4 (actually a smidge higher in load capacity). This is a bit big for 6" rims, but the other choice is a 225/60/15 which is smaller in diameter and no extra load capacity.

 

Fortunately I have aftermarket 7" rims, and love the Pilots, although the Q can chew tires UP!.

[260DET]

True, big is not always best, 15" wheels seem to be around the biggest desirable, for round the corners work anyway. 14" work well too, if you can get the right tyres which seems to be the biggest problem when using smaller wheels. But don't forget the limitation on brake size when using 14" wheels..