Sunday, April 17, 2011

Rally computer magnet mounting

The Jetta is equipped with a US made and supported Alfa Pro rally ( computer to measure time, speed, and distance.

The Alfa uses a switch to ground to trigger the pulse counter to measure distance- for cars with a VSS, it will interface directly, but on the Jetta, we used the included Hall effect probe and magnets.

The magnets were glued inside the rear left rotor, with the probe mounted off an ear from the rear stub axle fixing bolt.  This worked great until the rotor finally succombed 5 seasons of use and cracked.  In the rush to replace it, something went wrong with the magnet mounting leaving the unit unable to measure distance.

A Garmin Zumo GPS measuring speed and distance has been the extended temporary workaround for the past couple of seasons, but definitely wasn't optimized for rally use.

With the Jetta starting duty as a rental the rally computer made the list of "must fixes".

An email to Mike Friedman later, and I had a new probe and magnet kit sent out.  We are using the same basic layout for the probe, but with a removable ring mounted to the inside of the disc.

This only requires that the rotor be drilled and tapped to mount the ring.  The holes are easily layed out with a caliper, then drilled and tapped by hand from the front side.  The magnet mounting disk is reusable when the time to replace the rotors comes around again, which should prevent a repeat of the issues encountered last time.

Serviceable, durable, and done.

Friday, April 15, 2011

Small big lathe project

Over the course of the last 5 seasons, the bearing housings on the strut mounts have gotten pretty beat up.  With one cracked, it's time to make new ones.

This part starts off life as 4" diameter stock, and is stepped down to 2".  A 4" chunk of aluminum is reasonably hefty. This piece, 16" long, cost a cool $100.  With the relatively large diameter, the linear speed across the cutting tool is somewhat high, and will load up the machine if the depth is too great.

It sure does look pretty impressive sitting in the lathe, though.

For something of this size, the use of a live center while turning is a must.  The chuck jaws have been reversed, and the part is centered with a dial indicator prior to center drilling.

Then it's just a matter of whittling away- it's going to take about 65 passes. Yikes!


No pictures of the boring steps, but eventually the housing made it's way to the mill for holes and getting squared up.

The DRO speeds up punching holes in the part, as well as the milling operations. Rather than spin in the part center with a indicator, an onsize plug for the bore was turned center drilled.  A center drill in the mill to pick up the center gets within .004" or so of center.  For this part, that's accurate enough for the remaining operations, and significantly faster.

The balance between time and cost comes down to knowing required tolerances for a piece part, as well as the assembled part.

Monday, April 11, 2011

Bending stiffness- door bars

Multiple tubes in the same plane are required to prevent side intrusion. The builder is left with several options as to tubing diameter and wall thickness. This spreadsheet compares idealized properties of the tubes to give an idea of how adding extra elements would affect weight of the assembly, or overall bending stiffness. (Stiffness largely drives energy absorption unless the tubes buckle or otherwise plastically deform)

Number of Tubes

constant OD (inches) thickness I 1 2 3
Relative mass per unit length
Stiffness to mass ratio

0.04900 1.50000 0.095 0.10376 0.1038 0.2075 0.3113

0.04900 1.50000 0.120 0.12456 0.1246 0.2491 0.3737

0.04900 1.75000 0.095 0.16937 0.1694 0.3387 0.5081

0.04900 1.75000 0.120 0.20482 0.2048 0.4096 0.6145

0.04900 2.00000 0.083 0.22964 0.2296 0.4593 0.6889

Friday, April 8, 2011

Steering stops

It's difficult for privateer competitors to fully develop their vehicles in a vacuum. Even after years of active use, I find myself poking through some of the parts manuals that Volkswagen had provided for both their 2nd and 3rd generation Golf platforms.

Both prep manuals show spacers on the steering rack, and after incorrectly implementing them, I've realized they're primarily there to limit steering rack travel.

Evidence of tire contact on the transmission and/or brake lines in the wheel well sometimes appears, but I hadn't connected the two until this weekend while replacing the steering rack.

If you have bare spots where your tires have been rubbing inside your wheel wells, some spacers will prevent that, as well as take a bit of strain off your CVs.

Contact me for more information on the where/what/how of installation...