I do not usually post on the technical thread but I thought this might be of some value... I have a 550 that I wanted to change wheels on (never really liked the stock rims). I first bought a set of 360 Challenge rims (18"), I installed them with no issues (255 40 18 / 295 35 18). Then I decided I wanted 19"s so I bought a set of Challenge Stradale rims, I put slicks on these with no issues (do not have the sizes with me). Well, the slicks went away pretty fast so in June I bought a set of Pilot Sports, 225/35 19 and 285 35 19, the stock tire setup for a Stradale, and on that day I began having ABS issues. At nearly every stop the ABS kicked in, speed and force was not much of an influence. I sent the car off to an authorized dealer and they replaced the front pads and rotors and they said it was fixed. I went to pick it up and as I was leaving the dealer the ABS once again kicked in. It was a Saturday, no service department, so I drove it home, a bit frustrated and a few dollars out of my account. Today I retraced my steps and did some research on Tirerack. The short story is that the 550 ABS system was designed for tires of nearly equal RPM (revolutions per mile) the factory setup is 799 front to 797 rear. The Challenge wheels are 824 front to 774 rear. The number of revolution difference is too great on the Challenge wheels for the 550, so the system activates the ABS on virtually every appilcation. A Challenge Stradale is designed for 824/774. I put my 18"s back on today and it cured the problem, but I would like to run my 19's, so I ordered two new 245/40ZR19 for the front today that have a RPM of 778, a number I hope is close enough to the rear tire RPM of 774. RPM, not tire size (18, 19, 20 etc) and the relationship front to rear is more critical than I believed. Ferrari wheels and tires are not cheap, a lesson I am willing to share. I hope this might help someone else, I will let you know if the new tires cure my ABS problem with the 19's.
Don't know if you want to make this into a bigger "science project", but I think that you might be able to use the smaller diameter tires if you modified item 5 in this jpeg: http://www.ricambiamerica.com/parts_catalogs.php?M=FE&P=&V=diag&I=344 to have fewer teeth (so that you get the "correct" number of pulses per mile from the sensor when using the smaller diameter front tires) -- meaning, most likely, removing the existing toothed section from item 5 and somehow bolting on a custom made piece with maybe 1 less tooth while reusing the bulk axle+flange portion (of course, you'd have to figure the numbers more precisely to see if it would really work as you can only have an integer number of teeth). Just a thought if you'd like to use the smaller tire size...
So we have, yet again, another example of the "experts" at a Dealership not having a clue about what they are servicing. It always makes me sad when a customer dumps $$$ at a Dealership and not only doesn't get anything fixed, but has to fix it themselves as the Dealership is not competent to identify the problem. Of course, they will tell you that "your pads and rotors needed to be replaced anyway" so, "they did nothing wrong" . If it were me, I would make them take back the pads and rotors, re-install the old parts, and refund the entire cost of the service. But, then again, I'm rather a hard-a** about such things ---- which is why I don't take any of my cars to Dealerships --- I don't like them, and they don't like me
Why not put the 550 wheels back on to see if the problem goes away? A large mismatch from factory relationship front to rear can cause a problem. Does the ASR go wacky too? Oh and the slicks you ran if you know the brand then you can guess the size. Slick sizing is nothing like streetcar sizing and there could be only one tire that will fit those rims and then you can calculate the mismatch if any.
"I put my 18"s back on today and it cured the problem, but I would like to run my 19's, so I ordered two new 245/40ZR19 for the front today that have a RPM of 778, a number I hope is close enough to the rear tire RPM of 774." I did put the 18"s back on, it cured it. I decided to order new 18"s yesterday, so I have a set of Stradale rims with new tires for sale.
Here is the best summary of ABS systems written by a former colleague. The 19" are seriously messing with the vehicle speed and/or wheel slip calculations. See page 3, "Vehicle Modifications: Tires". http://www.e30m3project.com/e30m3performance/tech_articles/chassis-tech/abs/index.htm
Chris- There is a 4-5% limit on how different from OEM the tire differential can be before you start running into ABS problems. We could have helped prevent this problem if you had asked on the 550/575 forum. Ferrari fitted 19" OEM wheels to both the 575M and Superamerica. To match the OEM 255/40 18 F, 295/35 18 R tire diameters fitted to your 550 and to the 575M, Ferrari went to 255/35 19 F and 305/30 19 R tires, which is the size you should be running to match your OEM 18" tire diameters. There is a lot of difference between a mid-engine, rear biased weight distribution 360 CS and a front-biased (probably about 52/48) 550 or 575, so the CS tire sizes were inappropriately small up front. The front springs on your 550 are at least 33% stiffer than the rears for this reason. Also the CS wheels are too narrow for a 550 for really vigourous driving. The front wheels are only 7.5" wide vs the 8.5" on the OEM wheels, and the rears are a half inch narrower at 10.0" vs 10.5". Not much difference in normal street driving, but there is a lot of difference between the mass of a 3000 lb CS and a 3700 lb 550. You need more wheel and more tire if you are really going to push it. Thanks for reinforcing the necessity of being careful on which wheels and tires you choose. I have a wheel size chart for most of the late model Ferraris if you would like to see it. Sounds like I need to add tire sizes, too. Taz Terry Phillips
255/40ZR18 has a rolling diameter of (18+(2*255*0.40/25.4)) = 26.03 295/35ZR18 has a rolling diameter of (18+(2*295*0.35/25.4)) = 26.12 225/35ZR19 has a rolling diameter of (19+(2*225*0.35/25.4)) = 25.20 285/35ZR19 has a rolling diameter of (19+(2*285*0.35/25.4)) = 26.85 So, you took a full inch off the front and added 3/4 of an inch at the back. It is NO WONDER that the ABS dislikes this. Now you see what the number are attempting to tell you and how easy the math is to compute what you need to know.
ABS issues aside, moving that far from the original factory specs is asking for handling issues as well. Engineers spend a lot of time matching everything up. Dave
This is the 3rd instance of the same problem on 550 showing up on FChat. To bad there isn't some way to alert 550 owners to the potential problem.
To bad there isn't some way to alert ALL owners as to how to avoid the problems in the first place. Its called math. Too bad we don't teach it in schools.
Paul, Mitch- The problem and the math have been thoroughly covered, ad nauseum, in the 456/550/575 forum. Nobody ever bothers to look. Taz Terry Phillips
It's not that nobody understands the math, it's that not many (including the dealership service techs!) understand how the system works. I'm sure a 13 year old from a good school district could calculate how tall a tire is, but very few car owners, of Ferraris or Fords, knows what impact that has on their slip control system.
Don- That is fairly simple, too. Somewhere around 5% differential, + or-, ABS gets unhappy, and ASR uses the ABS sensors. First clue is usually an ABS warning light. In the preceding case, the 550 ABS system is set up for essentially identical diameter tires front and rear. The change to CS tires left an ~6.3% differential from the ~26" normal tire diameter, way above the ABS limit. Taz Terry phillips
