Racing Power Wheels Part 11: Fixing the Caster for Stable Steering

So the next thing to work on was fixing the instability of the steering at high speeds. After doing some online research on steering geometry, I found that the caster plays a very important role in the stability of a vehicle. The current caster of the Power Wheels had negative caster. Negative caster is when the kingpin is leaning forward at the top. Below is a picture of the negative caster on the Power Wheels.

Below is a diagram of positive and negative caster that I found online.

Negative caster causes a vehicle to become unstable at higher speeds while positive caster causes a vehicle to be more stable at higher speeds. But having too much positive caster can cause the Power Wheels car difficult to steer at higher speeds. The current caster on the Power Wheels has a caster of -7 degrees. I measured it by using a level as my vertical axis and measuring the angle of the spindle bracket from the level. From doing some online searching, I found that a good amount of caster is +10 to +15 degrees. These numbers were for gas powered go karts. I found most of this information from diygokarts.com. There I found caster can be generally set to 10 to 15 degrees of positive caster.

I will first need to correct the caster to give me neutral caster (no caster angle) on the steering axis. From there, I can then add positive caster. The way the first brackets were welded onto the front bar were to have no caster at all. But when the front bar is mounted onto the frame, it becomes negative caster. The larger wheels in the back causes the frame to have a tilt and this causes the front brackets to have negative caster. If both the front and rear wheels were the same size, then the Power Wheels would have no caster.

So to work on this I needed more square tubing to weld new brackets onto it. Luckily I still had another square tubing that we had used for the rear extension of the frame. Below is a picture of the square tubing when I was beginning to prepare to cut it to the same length of the original front bar. The bar had some rust but I think it'll be ok. The bar still feels sturdy.

I first had to cut off the end because the end had some indentation and a hole as well. So I clamped the square tubing down and got out an angle grinder from Harbor Freight to do the cutting. I also got a face shield from Harbor Freight. I'm no professional machinist so I wanted to be as safe as possible when doing the cutting. Below are pictures of the square tubing clamped down and the tools used.

Below is a picture of the part that I wanted to cut off and how the final square tubing looked like afterwards.

 

The cut was pretty straight to me and that's a good thing since on this new end one of the spindle brackets will be welded to. I went back and measured the original front bar which was close to 20.75". So I marked the approximate location on the new square tubing and began the cut. I didn't have another table nearby to support the bar that is being cut off so I improvised and used a chair with some boxes to support the bar that is being cut off. Below is a picture of the bar after the cut was done.

This was probably not the most ideal way to support that part that is being cut off but it got the job done. Also, the cutting wheel of the angle grinder wasn't big enough to cut all the way through the square tubing. I had to stop and cut from the other side of the square tubing to complete the cut. The size of the cutting wheel is 4.5" and that's the maximum recommended cutting wheel size for the angle grinder I was using. But the angle grinder cut through pretty easily.

I then compared the newly cut square tubing to the original front bar on the Power Wheels as shown below.

The newly cut bar is pretty close in length to the original front bar. This was good so I can still reuse the same tie rods that were cut to accommodate the length of the original front bar. The next thing to do was to weld on the spindle brackets on the ends of the bar with some positive caster to it. The welding will be done again by John from Small Engine Rescue. Below is the bar with brackets and a drawing.

Below is the drawing from the picture above.

So from the drawing above, the brackets will be welded to the square tubing with a 15 degree angle from the vertical axis to give it some positive caster. When it will be installed on the frame, the actual effective positive caster will be 8 degrees because 7 degrees of it will actually make the caster neutral to cancel the negative caster created from the lean of the frame.

If the frame is to be ever leveled out by using same size tires for the front and the back, then the effective caster will then be 15 degrees. 15 degrees of positive caster is the max recommended caster for go karts while 10 is the recommended minimum. But I think 8 degrees might suffice, or at least be a much better improvement over the negative 7 degrees of caster.

One thing I forgot to account for is the flat bar that is welded to the front bar. The flat bar is used to hold the steering column in place. So I had to cut off the welds when it comes to removing the front bar. Below are pictures of the flat bar I am referring to.

John did a good job on welding the brackets onto the new bar with the designed positive caster. Below is a picture of the new front bar with positive caster.

So I went back to the frame and began to start cutting of the welds of the flat bar with an angle grinder. Below is a picture of the flat bar finally removed from the front bar.

I took some pictures to compare the new front bar to the original one. They're pretty close in length.

The next thing to do was now to drill holes into the new front bar to be able to bolt to the frame of the Power Wheels.I first did this by laying the frame onto the new front bar to be able to drill new holes straight through the frame's bolt holes. Below is a picture of how I had positioned the front bar initially to drill the holes.

I first made sure that the distances on each side of the bar from the frame were approximately the same lengths. Then I proceeded to drill the holes into the front bar. Below is a picture of the front bar with the newly drilled holes.

Then I bolted it the frame.

From the side view of the frame, you can now see the positive caster of the spindle brackets. This can be seen below. The brackets are slightly leaning towards the back of the frame at the top. Now the axis of the spindle will be in front of the contact patch of the tire. This is good compared to the negative caster that caused the tire contact patch to be in front of the spindle axis. With negative caster, when driving, the contact patch will always want to go behind the spindle axis. This is what causes the steering to be unstable at high speeds.

Now the next thing to do was to figure how to reattach the flat bar onto the new front bar. I decided to just drill a hole into the flat bar and a third hole on the front bar to bolt the flat bar and the front bar together. This way I can always swap out the front bar with another if I want to change the caster or for any other reason. I began to drill the holes but I had a difficult time drilling through the flat bar. I barely made it to produce the following small hole.

So I borrowed some cobalt based HSS drill bits from Small Engine Rescue to complete the drilling. I then bolted the flat bar to the front bar and I was done with the steering after adjusting the toe to accommodate the new front bar.

I had also decided to replace the chain since the old one was binding up at some parts and was probably on too tight. So I installed a new one with some slack to it. The first chain was on too tight that there was always a point where the motor had to work much harder to be able to rotate. This caused the motor to heat up during runs. I was worried that this may damage the motor in the long run so I decided to install a new chain and let the chain have some slack. I may need to go back and adjust the tension because the chain may be on too loose right now.

So I went to test the new steering around the Boca Bearing building and found that the Power Wheels was now stable at higher speeds. This time I can push the Power Wheels to full throttle without having to worry that I may lose control. So the steering instability has been fixed. More improvements will be done to car in the near future and the project will now be taken over by one of our recent interns, Michael. He will improve the frame and make it much sturdier. He may redesign and remake the whole frame to occupy space more efficiently. Thanks for reading!

-Gerardo Ramos
 7/29/2016