Does anybody know what's the OPTIMUM wheel size for the car? The original (SLK320) had smaller ones. The 18" and 19" appear to be too large, and sized upwards for cometic reasons.

For explanation: I have a Miata roadster and thru trial and error of the tens of thousands of owners, the optimum wheel size is about 15", but no bigger than 16" for this car. (Motocross racers use 14" or even 13" wheels). Beyond that 15 - maybe 16" size there are all kinds of obvious performance issues, especialy acceleration reduction, and even handling suffers, the car is too twitchy.

With these huge wheels on the Xfire my concern is unsprung weight, and rotational inertia, adversely affecting acceleration, braking and handling. Per test reports, the Xfire already has a 25 to 30' longer braking distance from 70-0 than the Miata ('99) 130+ feet vs 161' (Car&Dr. 07/2003).

This posting is not for comparison purposes, both cars serve a completely different function for me, but I wonder if I should be looking for new (smaller) wheels. (Would help the tire replacement costs too when I think of it). No, I wasn't thinking of replacing the Miata with an Xfire roadster, unfortunately, the Xfire (roadster) just doesn't have THAT amazing feeling the Miata has.

Any ideas or actual experience?

 

The XF weighs about 500 lbs more than a Miata, and braking distances really can't be compared from one to another. The XF does have great brakes though, owing some to the Michelin Pilots and the wide tires.

Some guys are runnning 18" all around; i'm not sure 17" wheels would clear the brakes. Hopefully someone else who has done it will chime in.

Off-topic, i had an '01 Miata, FWIW (as my username suggests). awesome car, took it down to R Speed twice for their open house.

 

I have 18"s all the way around but kept overall tire heights the same so I still have the larger in the rear look. I would have preferred 17's in front and 18's in the rear but could find the wheels to do this. I moticed and improvement in the rear by reducing the unsprung weight. The car doesn't get that unsettled feel on bumps any more. 17" wheels will clear the stock brakes just fine, not sure about SRT front brakes though.

My 18's are staggered. 8 inch wide in front and 9 inch wide in back. With 18's in the rear I think the perfect tire is a 265 40 which is 10mm wider than stock and only slightly taller than stock.

 

Banjoman, I don't have the magazine in front of me, but as formerNXM5 pointed out, the Crossfire is one of the best stopping cars around.
You're probably correct about the choice of using wider tires as being mostly cosmetic on the Crossfire, but in a head to head comparison test between a 2003 SLK320 and '04 Crossfire automatic, the Crossfire came out ahead of the SLK in the handling arena. Only the SLK's 1/4 mile times were better (by close to a 1/2 sec.) they did say the smaller tires gave it a more supple ride.
Since the Limited Crossfire uses the same brakes as the SLK, the 17" wheels will go right on. My biggest concern would be the offset. I've never had the greatest luck using wheel spacers, but others on the forum have used them with no problems.
I know what you're saying about the Miata, nothing I've driven is more tossable, but since I've purchased our Crossfire, I haven't driven our Miata even once.

 

The optimum wheel size should be what the car came with, because that's how it was designed. You can't directly compare the wheel sizing on the Crossfire to those of the SLK since the suspension is quite different in the two cars. The Crossifre makes use of modified CLK front and S-Class rear suspension to accomodate the larger wheels.

I moved from a '99 Miata to the Crossfire back in July and I've never looked back. They are very different cars for sure, with the Miata being a true lightweight (I had 12lb wheels on mine), but the Crossfire stops beautifully, corners with much more overall grip than the Miata and will outrun it in a straight line by a healthy margin. Plus it's quiter, stiffer and more comfortable all around. Yes, it lacks some of the Miata's overall tossability (you can't defy the laws of physics and something as light as a Miata will change direction like a housefly) but I've had the Crossfire on a road course and it was just a dream. Absolutely wonderful.

Looking at Road & Track Magazine test data panels for the current NC Miata and the Crossfire we see things like this:

0-60: Miata 7.0 seconds, Crossfire 6.4
1/4 mile: Miata 15.4 @ 88.7 mph (the '99 Miata was a little slower than this), Crossfire 14.7 @ 95.9 mph
Skidpad: Miata 0.86g, Crossfire 0.89g (this is actually a very noticeable difference)
700 ft Slalom: Miata 67.7 mph, Crossfire 67.5 mph
Braking 60-0: Miata 112 feet, Crossfire 113 feet
Braking 80-0: Miata 200 feet, Crossfire 203 feet

I used these numbers because they were done by the same magazine using the same test methods and even the ambient weather conditions were similar for both (+/- 1 degree F and 1% humidity). But these are just raw numbers and say nothing about the actual driving experience. I loved my '99 Miata but the Crossfire just offers so much more of what I want (I know lots of guys at miata.net will argue this!!) and I bought a "new" '05 Limited Roadster for less money than a new '07 Miata would have cost me.

But back to the orginal question about wheel size/weight and performance. Sure, putting lighter wheels on a Crossfire would have some benefit just as reduced unsprung weight on any vehicle would. But the more a vehicle weighs and the more power it produces, the less important a few pounds of weight makes. The situation with the Miata, especially the first two generations, is that they so light and so low on HP and torque, that larger wheels have a huge negative impact on all performance parameters. It's no wonder then that Miata owners get slammed at miata.net when they want to put 25 lb, 19" wheels on their little cars. Those wheels would transform a delightful little sports car into a ponderous, sluggish economy ride.

 

I would have to disagree. I have read some of the misinformation that has been put out there to distance the Crossfire from the SLK320. CLK front suspension is BS as the CLK is McPherson strut. Our front suspensions are Identical to the SLK as well as the rear suspension. As long as you have comittees designing cars which also include the bean counters, very few cars are optimized but rather jsut a bunch of comprimises. Getting the stock boat ancors of my car made a lot of difference.

 

I love the stock wheels and the staggered look.

 

My God does this mean if I slam on my brakes my Crossfire may flip end over end? I thought they screwed up the design on this car, in fact the smaller tires should be on the back so that doesn't happen! Gonna go change mine right now.

Pat

 

My info is based on an article from SAE International's Automotive Engineering Online website, specifically the article found at: http://www.sae.org/automag/globalveh.../1-111-7-8.pdf (scroll down to page 14).

 

Sorry, but that article is complete crap and proves you can't believe everything you read. I have seen the same information printed almost word for word in other articles that came out in 2003. Most of the articles written around that time try to down play the SLK320 connection. Mercedes has never tried to correct any of the misinformation that is out there. I suspect they originated most of this BS so that their MB buyers wouldn't think the same basic cars can be pruchased with a Chrysler badge on them for half the cost.

for example, CLK front suspension is complete Bull. Our cars don't even have the same type of front suspension as a CLK/C class (unequal A arms/coils vs. McPherson strut). 39% Mercedes? What is the other 61%? Our cars are just re-skinned SLK320's A Crossfire is to the SLK320 what a Taurus is to a Sable. IMO, this is is the major reason the Crossfire is currently the best bargain on the market.

 

what's a miata??? never heard of it.

 

It's true that our cars are "C" parts so is the
SLK !! So they added some "S" parts too!

 

You would know it as the MX-5.

 

Probably something like a 17x8.5 front and 17x10 rear...if you want to fit as much tire as possible while keeping weight down.

 

But very few (if any) cars are designed with only performance in mind...I'd say the OEM wheel size is almost never the *optimum* wheel size.

 

Absolutely right! I wish I had the 17" inch wheels you mention above. Just going to 18's made a big diference in the rear of my car. It seems much more planted on bumpier roads. I believe it is the reduced unsprung weight that helped the most.

 

20x9's in the front 22x10's in the rear that's optimum. hehehe.