Home alignment Question re Hubs/track measurements

[jjeffries]

Hello All,

I'm preparing to do a home corner balance and string alignment....I've read every thread (thanks to all who've created and added to them).

One item I didn't complete yet is to snap a simple front to rear center chalk line, because the car is on a mid-rise scissor lift which obscures access, and I don't have/haven't borrowed a laser thingamajig (yet).

Measuring from the hubs (wheels removed) as a precise way to set the side string distances makes sense, so my question is about the Front/Rear Track specs for my 82 SC: the owner's manual lists them as 1369mm front, and 1379 rear. One might deduce that means the front hubs are each 5mm closer to the car's center line than the rears.

True? OR, is that number misleading? It could be if the front and rear wheels have different offsets, since (I believe) that factory measurement is to the outside of the wheels/tires (?), not to the hub faces

thanks, John/CT



You can see from that RR wheel that I've had the rear spring plates off.

[Knockdown]

Equal distance from fronts to string and rears to string should get you square if the string boards have equal spaced holes and are close to perpendicular to car centerline.

[ClickClickBoom]

FSM shows the factory centerpoints, they are not terribly accurate, in the sense of a formed sheetmetal pan. Pulling measurements from anything other the pan is potentially introducing error.
The pan is your foundation, everything is built off of that. I used concrete pier blocks, some nails and string. What ever method you use, consistency is paramount.

[WP0ZZZ]

Originally Posted by jjeffries

Measuring from the hubs (wheels removed) as a precise way to set the side string distances makes sense, so my question is about the Front/Rear Track specs for my 82 SC: the owner's manual lists them as 1369mm front, and 1379 rear. One might deduce that means the front hubs are each 5mm closer to the car's center line than the rears.

Track is defined from wheel center to wheel center. Changing wheel width or offset will change the track of a car.

You can see in Porsche's own specs how moving to wider wheels/tires sometimes results in a narrower track. This happens when the wheel centers move inboard due to the wider wheels having larger offset.

[jjeffries]

Thanks gentlemen.

[Oneredspyder]

I used a carpenter's laser pointer aiming from the back to the front. It was amazingly accurate, one wheel was off less than one degree when it went into the shop.

[Tom '74 911]

John,

You are on the right track. I am ahead of you by 1 weekend - I checked my car last weekend! I've got some SmartStrings and a SmartCamber tool to help - your home-brew setup will do the exact same thing for much cheaper... : )



A few things you probably already know, but others may not... make sure your car is set up on a level surface. I do this often enough that I've marked on my floor in sharpie where the wheels need to be located and exactly how many spacers are required under each wheel to get them on a level surface as my shop floor is not level. I used a laser level, a tape measure & 1/8" tiles to get it pretty darn close. This step is pretty important!

Also, after EVERY adjustment, you'll want to roll the car around or better yet, drive it around the block to settle the suspension. If you jack it up w/your scissor lift or otherwise, when you set it back down on the sweet risers you built, the suspension will bind as you lower it and will not give you correct readings when you measure/check your adjustments. One way to help avoid having to do this (as it requires disassembling the strings and everything which are a PITA to set up) might be to put some casters on your risers... (not my pic... saved from somewhere...).



This will help, but I still think rolling the car back & forth or driving around the block are the best to get it to settle. Putting some weight in the driver's seat is also an advisable trick too...

There is an old home alignment guide written by Ray Scruggs that you could likely find by searching here if you don't already have a copy. It is aircooled Porsche specific and does a GREAT job explaining EVERYTHING - the what, why and how. It's worth tracking down.

Best,
Tom

[stownsen914]

Not sure if this was mentioned earlier in the thread, but you can also use slip plates (at each corner (a pair of flat steel plates with oil between them placed on top of your scales or measuring surface) and some light bouncing to level the car after each adjustment. They work well in my experience.

Scott

[jjeffries]

Thanks much Tom and Scott. I spent a bunch of time leveling those risers and making the home-brew kit; will mark them and floor so I have easy repeatability before/after road-tests.

I (somewhat guiltily) copied the Scruggs book from an older thread here.

Will use gallon baggies lubed with something slippery for slip plates. Have made a knock off camber gauge with a digital angle finder. I also modified my spring plates as per the thread started by Craig Garrett (who did my cylinder heads). Cheers, John

[jjeffries]

My homemade camber gauge. Little white buggers are PVC plumbing caps; bevels make good feet against rims. Yes, I like zip ties.

[jjeffries]

My modded spring plates. I cannot claim much credit: I looked at others' examples, then made a sketch and showed it to my son's friend, who works someplace with a machine shop. He outdid himself. I sent them and some hardware out for plating, then installed Rebel parts thanks Clint and Jim Tweet). You'll see that concurrent with my suspension rehab, I also installed thin-flange SSI's (thanks R Liberty) and pipes (thanks 914-6 in Vancouver and especially Len Cummings).