rear track width

John Kelly · Post by **John Kelly** » Thu May 30, 2002 6:53 am

Can anyone merasure the rear track width of a 914 for me? I need the distance from the wheel mounting surface of one rotor to the other with the axles parallel (or close) to the ground. This is for my mid engine Ghia project.

Thanks, John www.ghiaspecialties.com

John Kelly · Post by **John Kelly** » Thu May 30, 2002 12:36 pm

Hi Phil,

Thanks for taking the time. One thing I'm concerned with is that the semi trailing arm suspension does not just go up and down in a simple arc like a VW. The pivot points for the suspension are not parallel to the ground. This is, as I understand it, to make the required camber changes during cornering. This is why i want the measurment with the axles parallel to the ground as a base point. If that's not convenient for you, or easy to measure I understand completely.

Thanks again, John www.ghiaspecialties.com

94teener · Post by **94teener** » Thu May 30, 2002 4:43 pm

John,
I think the 914 trailing arm axes are parallel with one another. When the specified tow, 0 to 15' and camber, -30' +or- 20', are set, then the trailing arm axes are offset from parallel based on this stock alignment. The arc described is then also oblique due to the alignment. Therefore, the track of the rear wheels will vary to that extent. You are correct in that the track with vehicle at rest on the ground will differ from that of the track when the trailing arms are suspended due to the car being on jacks. In order to measure the correct stock track, one would have to firstly insure that the rear alignment is set correctly and that the car is at rest on its wheels. The measurement that I had given you, above, is with alignment unknown and rear suspension hanging down. My only point is that the angle of the axles to oneanother doesn't effect the track.
Phil

Dave_Darling · Post by **Dave_Darling** » Thu May 30, 2002 9:13 pm

Not to be pedantic, but the angle of the axles does have an effect.

The idea is that having the axles parallel to the ground means that the trailing arms are at a specific angle relative to the body. (Slightly upwards toward the aft end, I do believe.) If I'm right about the angle, it will be pretty unusual to find a 914 that low in the back...

Anyway, since the droop of the trailing arms affects the track measurement, and since the droop of the axles is determined by the droop of the trailing arms, you can specify one given track width (for a particular car) by specifying the angle of the axles.

And, as pointed out already, the toe angle will affect the track measurement as well.

--DD

------------------
1974 VW-Porsche 914 2.0 (Type IV powered!)

Pelican Parts' 914 Tech Geek http://www.pelicanparts.com

94teener · Post by **94teener** » Thu May 30, 2002 11:52 pm

I have a 2.0L up on jacks with the rear wheels removed. I measured 56.9375 inches,
rotor wheel mounting surface to rotor wheel mounting surface.
This is with the trailing arms hanging, axles not parallel. I don't think it makes any difference whether the axles are parallel because the CV joints allow the axles to move in and out somewhat as the suspension operates through its range. It is likely that the track will change slighly as the suspension moves up and down based on what the rear wheel camber is set at. So I would take the above measurement as a close estimate.
Phil

94teener · Post by **94teener** » Thu May 30, 2002 11:54 pm

Also, the rear tow setting will change the track.
Phil

94teener · Post by **94teener** » Fri May 31, 2002 5:36 am

Not to be padanic II, but the angle of the axles DOES NOT have an effect. Only the trailing arms alignment and the arc positions of the individual trailing arms effect track. The angle of the axles to one another reflects not effects.
Phil

John Kelly · Post by **John Kelly** » Fri May 31, 2002 6:43 am

I figure if I can get the measurment with the axles parallel to the ground it would be a nice start. I don't know what the trailing arm pivot point deviation from level is...I just remember that there is one. I'm going to use the stock geometry, but with custom trailing arms. Anymore help appreciated.

Thanks, John www.ghiaspecialties.com

Dave_Darling · Post by **Dave_Darling** » Fri May 31, 2002 8:53 am

Pedantic III:

Unless you remove the axles altogether, the angle that the axle makes with the ground is a direct result of the angle of the trailing arm to the ground. The axle itself does not determine the angle of the trailing arm, but for any given axle angle there is only one trailing arm angle that produces it.

So specifying the axle angle also specifies the trailing arm angle.

Anyway, this really is pedantic...

--DD

------------------
1974 VW-Porsche 914 2.0 (Type IV powered!)

Pelican Parts' 914 Tech Geek http://www.pelicanparts.com

94teener · Post by **94teener** » Fri May 31, 2002 9:26 am

Whatever...

ray greenwood · Post by **ray greenwood** » Fri May 31, 2002 5:16 pm

Yes the angle of the axles does affect track....but no one is being clear to each other as to what the angle of the axles is doing to the wheel, where true track width is measured. If there was no toe or camber at all...the track width at the leading and trailing edge of the wheel and at top and bottom would all be the same measurement..12,3,6,9 o'clock respectively. Weight and droop aside for the moment...if the stub axle itself has any angle to it other than parrallel with the ground, then two of those clock positional measurements will be altered. This changes the effective track of the vehicle...by force...if the 3 and 9 measurements vary (toe in), as it changes the angle of attack of the tread to the pavement...increaseing friction and torsional force. If the 12 and 6 angle changes...(camber) the bottom can have an actively wider track than the top. At rest...maybe not too noticable...but the track is changed. But during travel movement, this increases due to droop and arc design. If the stub is parralell to the pavement, and swings in an arc from a single point...nothing changes. But I believe the 914 uses a two point trailing arm mounting(?) correct? no?...whose mounting bushes are off camber (off plane) to each other. They change the arc I believe you will find...and change the wheel angle...because the stub mounting is probably off plane as well. Most cars have this to increase camber without increasing toe in while cornering. So yes, I would believe that the angle of the axles has quite a bit to do with it. The rear trailing arms on my 412 are similar in design, but not execution. The stub mountings are electrowelded in at very specific angles (you can see where the jig was mounted). The trailing arm mounts are off plane...and adjustable in all directions...and have excentrics on them. Ray

94teener · Post by **94teener** » Sat Jun 01, 2002 5:48 am

Ray, You are correct that the 914 trailing arm axes are off plane with each other but the axle half shafts still have nothing to do with affecting or effecting track. Each half shaft elongates or contracts via the CV connections as much as 1.25 inches (2.5 inches rotor to rotor) and this range of movement is to allow the trailing arms to change track when moving through their arc range. The half shaft angles to each other at any time are only a reflection of what the track might be as determined by the trailing arm positions and alignment. For example, one could remove the half shafts from the car and it will have zero affect on rear track.
Phil

Racer Chris · Post by **Racer Chris** » Sat Jun 01, 2002 1:59 pm

To John Kelly,

From the factory 914 manual - the rear track width is 54.4 inches with 5 1/2" wheels. I measured the offset of a 5 1/2" Fuchs wheel at 1 3/8". Therefore, by adding 2 3/4" to 54.4", the number you are requesting is about 57 1/8" between rotor mounting surfaces.
I will have an opportunity to verify this in the next couple of days.

John Kelly · Post by **John Kelly** » Sat Jun 01, 2002 4:04 pm

Hi Chris,

Thanks, that's real close to Phil's measurement. Please do verify and get back to me. Thanks to all who replied.

John www.ghiaspecialties.com

genrex · Post by **genrex** » Mon Jun 03, 2002 11:09 pm

A quick question: could someone please define "track width" for me?.. is this the measurement from outside-of-tire to outside-of-tire? Thanks!

Roger out.