Tire size and the effect on handling

I've finally got my used IM roadster home. In looking it over to try to determine which options the PO took I see the standard tire size, 165/65HR15 all around.

I know that IM offered as optional 185/65HR15 in front and 205/60HR15 at the rear.

So I'm asking if it'd be worth changing to the larger tires on a car that will likely be driven on the street with enthusiasm, but not rallied or pressed too hard.

Is the appearance changed substantially with those sizes of bigger tires or are they only wider in footprint for more stickum?

Is there a need to change the ride height in order to avoid tire contact in the wheel wells during turns?

Things like that there are popping into my head with the arrival of this car.

Good start is to use the Miata tire size calculator to see the differences (you'll need Java runtime enabled):

http://www.miata.net/garage/tirecalc.html

going from 165 to 185 increases ride height 1" but drops revs per mile by 70. 165 to 20/601 will be 1+". So ride height will need to be adjusted if you like current stance (that could impact clearance). On my CMC (with the 165's)I used a 3/8" spacer on front to move wheels out and clear calipers - also better fills wheel well. (I have no room in back because of wheel offset with Porsche 2L) Not only is ride height affected but your speedo/odo are impacted too. GPS readings would accurately confirm.

Ken,
185/65/15 is a popular size for 356 replicas as it is the largest tire most of us can squeeze in the rear without rubbing issues. However, newer Intermeccanicas can handle wider tires as there is more room in the rear with the tube frame. Obviously, wide-bodied cars don't suffer from lack of tire space. ;-)
The most important thing you'll find in handling is the amount of air pressure in your tires. Most of us are running front pressure in the low 20s and rear in the higher 20s. IMs weigh a bit more so you might raise the psi a bit but I'm sure one of the IM owners will chime in here.

I was with my mother in law last week when she got pulled over for speeding.
I asked the Officer about the differrenc between what my GPS says and the speedo. My speedo say's I'm going 10mph faster than the GPS reads
His advice was that if the tires are standard size Ignore the GPS.
He claimed that GPS's are notoriously inaccurate in regards to the MPH

Jay

Well, whatever I eventually decide to do I'm already in awe of driving this car and all I've done with it was drive about four miles of semi-mountain road to get it home from the nearest place the delivery guy could get his tractor with 72' trailer. We misused a church parking lot for a few minutes.

It was wet, I'm both unregistered and uninsured so I knew I had to cool it, but this little guy goes exactly where pointed like it's a slot car with a slot in the road.

I haven't even looked underneath yet to see if the PO opted for anti-roll bars or the like, but was happy to be able to spot rear disc brakes through the slots in the wheels.

I had known that the original buyer had spent some extra money - the car has seat heaters, courtesy lights, leather, the German style carpeting, Nardi wheel, phone and iPod ready radio with CD and an amp in the trunk, but it hadn't been my impression that they were performance oriented people. More a moneyed couple who wanted a unique drive for wifey to make her salad lunches in and impress her tennis club. (we found a woman's white leather fingerless glove and a hair tieback thingy under the seat). I guessed tennis because a set of clubs would be inconvient in this car so she'd take the SUV to golf.

LOL! The things I can think up. I really have no idea of the previous owners except that they live in Los Gatos, CA and I'm too familiar with that place and places like it. I was born in Sausalito and raised in Tiburon, CA. My schoolmates were all rich kids.

Ken, what year is your IM?

It's a 2001 with 04750 miles on it. Apparently the couple who bought it began having kids even before it arrived and now have four, oldest 11. So they found they had little time to enjoy the car and it spent most of it's time waiting for later maybe, until they realized that maybe wasn't coming and sold it. It's a good find and the price was right too.
I'd have chosen gray, and when I sent a photo to my Marin county sister she said "OH.....MY.....GOD! That is the SWEETEST car I've ever seen!!!My dream car!" She offered to trade her '97 318ti even.

Thing is,it's a chick's car. Maybe Henry will take it in trade, or maybe I should go get my nails done? :D

Ken, my IM Roadster had 165/15 tires as original equipment, and still did when I bought it with 3500 miles on the clock. They were Dunlops in good shape. I put about 15,000 miles on them and they were wearing down to the wear bars. I put on a set of Yokahoma 185/65 15 and am amazed at the improvement in all aspects of handling. Road feel going down the freeway at 75MPH is rock solid, and dry cornering is much better. I would recommend checking the age of your tires. There is a code on the sidewall that will give the month and year they were produced. If they are original from 2001 they are old and should be replaced. The 205/60 15 tires that you see offered by IM on some cars requires a wider rim on the rear. Henry makes these wide rims on a custom basis. If you don't have a big motor and drive hard the wider rear tires/rims are not needed.

Thanks Will, that's good info and I think I'll follow your lead.

Yeah, I'd seen on the option lists that the 205 tires were on 7" rims while the standard on the car is 5 1/2" all around.

Ken, this weekend is the annual Intermeccanica Owners gathering in Vancouver B.C. If at all possible you should get in your car and make it! You will have a ball, meet some fine people,and learn all about your car! I bought my car in Spokane, and headed for Vancouver the next day. I think that I drove about 500 miles that weekend and had a blast. After the weekend I drove down the East side of the Cascades to the Columbia River, right past Appleton. It is a great way to kick off the fun you will have as an IM owner. Don't miss it!!!!!

Ken, with those low miles I'd check to se if the tires are original equipment. If so, replace soon, whatever size you decide.

Good luck with that great ride.

Will,
Yeah, I knew about that wing ding, but it's impoosible for me as I've got Dr. appts twice in the week and my wife has another. 'Sides, the car's not registered to me yet snd the mounties would keep us out for that if nothing else.

Ron, I just got an email from Henry telling me that it's the 1600 "Super 90" that they were doing in those days. Maybe 85hp on a good day. Hmmm, the dealer lied? - that'd have to be a first.

The higher the tire, the higher the CG and roll center. If I go up 1/2" on my race car, I usually need 1/2* more camber to compensate for that, as we aren't allowed to change the spring rate or roll bar size

Ken, I had the same "Super 90" 1600CC engine in my IM. I really did not feel that it was underpowered. I drove the car 15,000 miles on freeways, twisty mountain roads, and running with folks on this forum. I was never left behind, and had a ball driving every mile. Putting on 185/65 15 tires will slightly reduce the tire diameter, giving a little better acceleration on hill climbs. I could feel the difference when I changed from 165R15. Enjoy your car with the motor it has, and if you want to upgrade to a large engine later like a lot of longtime owners do we will be glad to advise you and help you spend your money!!!

The old 165x15 tires were most likely aspect ratio 80. So going to 165 aspect ratio would result in less sidewall flex (and lower ground clearance due to smaller tire diameter). Better handling but less bump absorption.

oops 80 to 65 aspect ratio. Plus old tires dry out and the rubber isn't "sticky" any more!

Will, that's exactly my plan about the engine. It sounds like it's in good shape and has good OP, so it's enough for now. I've got a little "road test route" here that's all hilly country roads with lots of good curves and when I took the car out by myself to see if I could get the rear end to let go a little I only had to downshift to second on one particularly steep climb that has to be turned into. If I coulld attack that hill from a straightaway I'm sure the car would go right up in third. It feels like a good balance of power and handling, though I think the KWB GS2 shocks are either shot or junk in the first place. The car rides soft but boy did it dislike a pair of speedbumps that some guy put across in front of his house. I think I'd have lost a filling if I had any left.

Wolfgang, they're Pirelli 165sr15. An oldtime sportscar standard and Probably the tires that came on the car new ten years ago. I haven't got the car to feel twitchy or to slide yet, but the rubber is probably as hard as my arteries.

Am I mistaken to think that the suspension looks like it came out of a T3 VW? Is that what parts to buy for it? I like Bilsteins better than Konis but I'll need to know how to crossreference the part numbers. Doubt it would do a lot of good to tell a partsguy it's an Intermeccanica 356. :)

KW,

I'm really liking your speedster, especially the dash with all those gauges set in black.

You said 'racecar'? Do you race in sportscar classes?

Yes, I did a lot of pro racing, and am currently the southern chief instructor for Skip Barber racing school...

Ken - Most likely a T1 bug donor. Does it still carry the VW VIN? That would give you good idea of year and whether a bug or a ghia chassis. Bug would start with 11, T3 31 and Ghia 14.

Wolfgang,

Do you mean a VW chassis or pan? If so I don't have one of those to refer to. The IM cars use their in-house designed and built tubular (square and rectangular) steel frame. The car has a VW front and VW IRS rear torsion bar suspensions. I'm just not sure which model the components were sourced from. The front end looks pretty generic VW but I've never worked with or owned a VW IRS rear suspension to know enough to identify.

My VW experience is from forty years ago and even though I worked on them for pay through college the vagaries of memory make it nearly as if I'd never heard of them at all.

KW, I'm impressed. For years I wanted to road race, but the closest I came was to nearly buy a ragged out Triumph Spitfire for G modified (best I recall). The car with a trailer, four sets of tires, two transmissions, and a spare engine all for $1500. while in college (early '70's folks - that wasn't so great a deal).

It would have been purely amateur and likely to make me broker than I already was by far. I lived not far from Sears Point was never missed a race day there for about four years. Usually made Laguna Seca too, sleeping in my car in Seaside.

edit: I don't know what brain quirk made me think the tires on my car are Pirellis - they're Michelins, every one of them. I'm still trying to figure out where I got the visual remembrance of seeing Pirelli. Must be the drugs. :)

Yeah, I have fun with my job... For sure. I've raced and helped setup a few vintage racing (as in real vintage) Speedsters. An absolute ball .

Ken, missed your reply, thanks for the kind words on our car. I wasn't wild about the gauges, but I think I'll leave them for now.

Ken, we might have crossed paths while 'stealth' camping in Seaside during the 70's (at Del Monte Lake park). We always looked forward to our annual trek to Laguna Seca for the Camel IMSA series. You will most definitely have fun in your Roadster!