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R/C Combat Vehicles

Steering Linkage

by
Doug Conn

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I use this device on my 1/6 scale remote controlled paintball vehicle. The heart of the device is a high-torque servo gearbox that controls the steering. On smaller vehicles, a 'servo saver' helps to isolate the steering servo from the bumps and jolts that the front wheels experience. Those commercial servo savers are too small to handle the demands of a larger, heavier, vehicle. This device is basically a beefed-up version of the same thing.
A 1 1/2" PVC end cap is screwed to the output gear of the servo gearbox. This is the servo saver shaft. A plastic flange rotates around the shaft. Two heavy springs transfer the rotation from the servo gearbox (via the servo saver shaft) to the flange. The springs absorb any lateral shock that would otherwise be transferred directly to the servo gearbox. Tie rods connect to the flange and to the arms on the steering knuckles (not shown in the drawing). A standard 3/16" turnbuckle is used to change the length of the tie rods, adjusting the toe-in of the front wheels.

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Start by acquiring the basic parts and tools you'll need. The trickiest part of the building process is to cut the correct size hole in the flange. It needs to be just large enough to allow the flange to rotate on the PVC end cap without binding. Experiment on scrap wood until you adjust the hole saw to the correct diameter. Next, drill a hole through the top of the PVC cap (the servo saver shaft). A bolt through that hole holds one end of both springs. Two holes at the bottom of the flange connect the other end of each spring. Those springs are the only mechanical connection between the servo saver shaft and the flange.

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Build a tie rod by starting with two pieces of 3/16" steel rod of appropriate lengths for your vehicle. For the upper tie rod bend a 90 degree angle about 3/4" from the end. Thread both ends of this rod with the #10-24 die. The lower tie rod is straight. Thread one end at #10-32, it connects to the ball linkage. Thread the other end with the #10-24 left handed die. The turnbuckle joins the upper and lower tie rods and allows toe-in to be adjusted. Repeat for the second tie rod.
Stretching the springs around the servo saver shaft and connecting them to the flange can be a chore. I secured the flange in a vise used pliers to stretch and connect the flange and shaft spring bolts. Wear eye protection! It's easy to lose your grip on the spring and it may go flying !
Tools and Materials List
These are some of the parts and tools I used, where you can get them, and what you can expect to pay.
  • Servo gearbox from www.servocity.com - about $90
  • 1/2" UHMW plastic from www.onlinemetals.com.
  • 1 1/2" and 2" PVC pipe from my local home improvement store
  • PVC cement from my local home improvement store
  • Springs from my local hardware store. You might have to try a few different ones to get the length and strength works for your vehicle.
  • 3/16-32 or #10-32 standard threading die from a hardware store or www.mscdirect.com. About $8
  • 3/16-24 or #10-24 standard threading die from a hardware store or www.mscdirect.com. About $8
  • 3/16-24 or #10-24 left hand threading die from a hardware store or www.mscdirect.com. About $13
  • 3/16 steel rod from a local hardware or home improvement store
  • Die holder from a hardware store ow www.mscdirect.com. About $6
  • Adjustable hole saw from local hardware store or
  • http://www.acehardware.com/sm-skil-and-reg-adjustable-dial-hole-saw-73400--pi- 1333354.html about $17
  • Hex bolts and nuts from a local hardware store.
  • #5-40 cap screws for connecting the servo gearbox output to the servo saver shaft - www.mscdirect.com
I used Google Sketchup to create an assembly drawing and information. I uploaded the model to their 3D Warehouse as Large scale toy vehicle steering linkage and servo saver, so anybody can grab it down and take a look. Search for "rc tank combat steering linkage" if the link no longer works. Google Sketchup is a nice tool. It is free, easy to use, has good 3D drawing features, and it provides a repository.

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Parts Description:
  1. Servo gearbox from www.ServoCity.com
  2. Spacer ring - cut from 2" PVC pipe and split at the bottom. Glue it to the saver shaft (#3) with PVC cement.
  3. Servo saver shaft - 1 1/2" PVC end cap (file the bottom flat)
  4. Shaft spring bolt- 1/4 20 bolt drilled through #3 and secures the springs to it.
  5. Servo saver flange - Cut from 1/2" UHMW. The center hole is just large enough to allow it to rotate on #3 without binding. Cut it carefully with an adjustable hole saw.
  6. Upper tie rod - 3/16" steel rod bent 90 deg. at one end. Threaded with #10-24 on each end.
  7. Servo saver springs: There is really just one spring on each side of part #3, but I didn't know how to draw a curved spring :) These are extension springs and wrap around the servo saver shaft (#3) to help isolate the servo rotation turning the flange (#5).
  8. Upper tie rod retaining nut - #10-24 lock nut secures the upper linkage arm (#6) to the servo saver flange (#5).
  9. Linkage adjuster - 3/16" turnbuckle. Get it from a local hardware store.
  10. Lower tie rod - 3/16" steel rod threaded with #10-24 left handed on the turnbuckle side and #10-32 right handed on the rod end side.
  11. Rod end - 10-32 ball linkage from www.ServoCity.com (http://www.servocity.com/html/10-32_super_duty.html).
  12. Flange spring bolt - 1/4 20 bolts that retain each spring to the servo saver flange (#5)
  13. Flange retaining block - This is a small piece of 2" PVC pipe that is screwed to the servo saver shaft (#3) to keep the flange (#5) vertical on the shaft (#3).