******** DISCLAIMER ********
THE FOLLOWING INFORMATION IS AN ACCOUNT OF MY OWN EXPERIENCES, AND ANYONE WHO USES THIS INFORMATION DOES SO AT THEIR OWN RISK.
The following is detailed information on everything that you will need know to run 345/30/19 tires on 19x12 rims, on a '97-'04 C5. If you have any questions feel free to ask.
There are three phases to this project:
- Tubbing your C5
- Limiting the Shock Travel
- Determining the Rims Offset
Depending on how fast you want to complete this project will all depend on what order you complete the phases. To minimize down-time or wait-time, I personally would order the rims first as they can take up to 4 weeks for a manufacturer to make and ship. Second I'd send off the shocks to get modified because this takes about two weeks to get back. And third I'd tub the C5, this phase can be done over the span of one weekend (Sat & Sun). So with this information you be the judge as to when and what order to do each phase.
Tubbing your C5:
The following information is a compilation of my own experience and tips received from fellow forum members, and was born out of the desire to run wider wheels WITHOUT spending $2000+ for someone to tub my car. Here's a "How To" write up I did on Tubbing a C5 for less then $50. All the info you need is in a PDF...shoot me an IM and I'll send you the PDF.
Limiting the Shock Travel:
The following specifications were created by myself, and was born out of the desire to run wider wheels WITHOUT spending $600+ for the LPE Shock Kit (Shock-Travel-Limiter & Shock-Mount-Relocation-Bracket), and still keep the wheels tucked under the rear fenders. Quick background info on this shock-limiting issue: First of all, in order to run any rim wider then 19x11.5" and still keep the wheels under the fender you will need to limit the downward travel of the rear wheels (in other words limit the total expanded length of the shock). From my understanding LPE achieves this with a new upper Shock-Mount-Relocation-Bracket that basically just has a deeper "hat", thus allowing the shock to sit deeper into the shock tower (effectively reducing the total downward travel of the rear wheel). However this gives rise to another problem, when the shock bottoms out (total shock compression) the wheels will now hit the top of the inside fender well. This is fixed with a Shock-Travel-Limiter (which is a thick disk type spacer that slips over the shocks center rod), thus effectively limiting the shocks upward travel. Both pieces work together to achieve the result needed to push the wheels further under the car. I however came up with an alternative to the LPE Shock Kit. My idea was to modify the shock directly. Since there are many different brands of shocks being used on the C5 you will have to approach this phase with caution. The main thing here is to measure the total EXPANDED length of the shock (with the shock standing on the floor measure from the floor to top of shock rod). Take the shocks total length and subtract 18", this difference DIFF1 will be the amount that the shock needs to be shortened. This can be done by adding a spacer in the size of DIFF1 to the shock internally(DANGER: THIS CAN ONLY BE DONE BY THE SHOCK MANUFACTURER!). So be sure to check with the shock manufacturer to see if they will modify the shock for you. The rest of this phase is Bilstein specific. Since I already had Bilstein Sport Shocks, and I knew that Bilstein will modify your shocks for $65 each, so I had Bilstein do the modifications. If you have Bilstein Sport Shocks, then all you have to do is send Bilstein your rear shocks and have them add a 1.125" spacer inside the shock body to shorten the total expanded length of the shock. This modification achieves the same desired result as the LPE Shock kit, but for only $130 (the cost of modifying two shocks). Another advantage to this is that Bilstein will also re-valve your shocks (make them stiffer or softer) if you want them too, and it's all covered under the original $65 (per shock) charge. Just give them a call at Ph# 800-537-1085 to discuss any question you may have.
When you mail out your shocks be sure to include a note that states what work you want them to perform, and how you want to handle billing (they do take credit cards). Note that if you send in more then just the rear shocks be clear that you only want the 1.125" spacers added to the REAR SHOCKS. Their mailing address is:
Bilstein of America
Attn: Jack French
14102 Stowe Dr.
Poway, CA 92064
Determining the Rims Offset:
The following specifications were created by myself, and was born out of the desire to run wider wheels and still keep the wheels tucked under the rear fenders. First a little background info on the offset issue: Originally I was running 19x12 rims with a 70mm offset. This is perfect for those who don't have any type of a Shock Travel Limiter, however your wheels will stick out ~7/8" past the fender lip. The max offset that you can have without using any type of shock-travel-limiter and without causing any interference is 74mm. However you will be right on the edge of interference, so be cautious when using this offset....and yes your wheels will still stick out about ~3/4". OK, now on to the good stuff. I took numerous measurements to figure out exactly what the maximum offset I could use without causing any kind of interference. I came up with an 84mm offset as the prime choice. I could have gone with an 86mm offset, but I would have been right on the edge of interference. Anyway, when I went to have my wheels worked on by FIKSE they told me that the closest they could get to 84mm without going over was 82.5mm, so I agreed. And that's what I got. Now, when compared to a LPE Mini-Tubbed Z06 with the LPE Shock Kit, my wheels stick out only 1/16" further, not much considering the tire's section width is 13.6". Plus, I believe this small difference can be corrected by running the little more aggressive Z06 alignment (more negative camber).
All that is really needed to run 19x12 rims and still keep the wheels under the fender is a 1-1/4" mini-tub (if you don't plan on tubbing the liners) or a 1-1/2" mini-tub(with tubbed liners), modified shocks with a maximum of an 18" total expanded length, and an 82.5mm wheel offset on a 19x12 rim.
An offset larger then 86mm can be used to push the wheel further under the car, just be sure to take numerous measurements from all angles to ensure proper clearance for the offset you wish to use. Of course a bigger mini-tub will be needed (up to 2" can be done), followed by a shorter shock travel which can be done by using bigger internal spacers. However, keep in mind that by shortening the shocks travel (regardless if you're using the LPE Shock Kit or not) you are reducing the shocks intended operating range. Shortening the shock too much can translate into poor shock performance or even shock breakage. So keep the size of the internal spacers to the bare minimum needed to achieve your goals.
FYI: The desired wheel offset was determined first, that in-turn lead to determining how much the shock needed to be modified to achieve this offset.
**** NOTE ****
The rear wheels in my sig pic is how my wheels looked before I corrected the offset (they stuck out ~ 7/8").