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Old April 29th 2010, 21:03
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Post Look At My Skinny Arms!

by Johan Kanters
Feb 09, 2003


You have just bought those killer wheels that you've always wanted. When you got home you took a rear wheel out of the box and test fit it. "Looks pretty good" you say to your self, but they could be tucked in some more. "Hey !" you proclaim, "What I need to do is to narrow my trailing arms!"

Tool List
  • Angle grinder
  • Measuring tape
  • Mig welder
  • Metric wrench set
  • Die grinder
  • Compressor
  • Procedure

Start by placing the vehicle on axle stands and removing the arm from the car or alternately obtain some used ones for narrowing to be installed in the car as an exchange. Completely dismantle the arm, removing the axle- backingplate and wheelbearings. Thoroughly clean the bearing hub by removing all the grease and dirt etc. Remember grease is VERY flammable when welding later on! Its probably a good idea to get the arm sandblasted prior to narrowing as it makes welding a whole lot easier too.

The jig I made allows me to narrow the arms a maximum of 29mm, turn the arm over so that you are looking at the inside of the arm. You'll see that the round bearing carrier is welded to the arm. This weld must be cut through as we are going to remove the whole bearing carrier from the arm. You can cut through the weld with an ordinary angle grinder, I find that a 2mm thick cut off disc works best, try to cut through the middle of the weld by moving the grinder in a circular motion. Its important not to cut into the arm or bearing carrier too much as this means more work later.

Now turn the arm over and you'll see the square mounting block which holds the backing plate, the arm must be cut through just above the block, just below the block and in front of the block.

The bearing carrier should now be able to be removed from the arm providing you cut through the weld enough, it shouldn't take much persuasion with a hammer to come out.

As I said before I made a jig to be able to line everything up with when narrowing the arm, if you don't have this you will have to make one or carefully take some measurements before removing the bearing carrier from the arm. With a die grinder, the hole in the back of the arm must be made larger for the bearing carrier to be able to slide through, this is because the bearing carrier is tapered -meaning the diameter gets larger towards the backing plate mounting face.

Once you have decided how much to narrow your arms you can slide the bearing carrier back through the arm, set at the desired distance and weld in place. A powerful welder should be used or you could just tack in place and get an experienced welder to complete the weld for you.

Turn the arm over and weld the bearing carrier to the arm where the backing plate mounts on making sure you have left enough clearance for the backing plate to mount on again. You will see from the pic below that I have cut a large section out of the arm just in front of the backing plate mounting block, this also needs narrowing and to be welded back onto the arm.

Turn the arm over again and the next step is to reinforce the bearing carrier to the arm by making a brace from some 50x5mm flat plate. At one end it should be ground out to follow the contour of the bearing carrier hub, weld this to the arm as shown in the pic.

There you go a narrowed IRS arm! Job well done. Make sure that all welds are approved by an engineer before you use the arms in a vehicle, this article is a guide only and it merely shows how I have narrowed some arms for my own use. Make sure you have a good understanding of engineering before attempting to narrow your arms.
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Old February 16th 2012, 20:45
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Join Date: Jul 2002
Location: Forest Grove, Oregon
Posts: 697

In reviving this old thread because Humble and I were talking about narrowed semi-trailing arms and the effect they'd have on handling.

How much of an impact - negative - will going to a narrow arm so I can fit larger wheels / tires in the rear of the Ghia be for handling? This is a fun car which'll see some driving events, but not all-out racing.

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irs, narrowing, trailing arms

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