Hi Bob, I had a similar problem. Turned out to be a transmitter problem. Probably got damaged in the mail. Don't get me wrong, I'm no expert it took me a long time to find the problem. I cleaned the gravel from underneath the servo saver , replaced the servo , and finally got a new transmitter. Problem solved.

Sorry Bob, I could not see the videos. I'm having problems with my computer. Hopefully my advise will be helpfull.

If it's happening when the servo isn't powered then it can't be a transmitter issue.

So after some testing , walking next to the car while going straight I noticed the car goes a lot like a boat almost all the time while sometimes it kept following a turning course while none of the wheels were moving left or right, mainly the back side of it...
So I think the 80% of my problem is because of this:

*let me wear my glasses*

The back tires have the inward toe so each wheel pushes to the center, so the back side in order to stay centered, drifts all the time like a boat. Now, when a wheel grabs better for some reason, forces the car to take that direction leaving the other back tire drifting because on such a light weight car is easy to drift. See the picture below. Kg = weight = better grip.



Now what confuses me is that almost all RC cars I've seen have the inward toe on the back wheels, so why isn't everybody having this problem? Is it because of the flat tires I am on? Should I 3D print the back suspension arms so the wheels don't have inward toe....?

Yep , I just proved my theory right!!! After a very long time I finally did a test half-removing the chassis shafts of the rear suspension arms, straighten up the rear toe by putting some spacers between the arms and chassis , I did some tests and YES it's fixed!

(this is just temporary you can't run it like this for a long, the suspensions hardly work and did it just for the shake of the test)

Now the problem is that I don't know where to find adjustable rear suspension arms, or the STL file to 3D print a pair that has less inward toe! Can you help???

Before and after


Close up of the temporary mod

That's just a band aid. The car won't handle properly at speed without some rear toe and any car with a couple of degrees rear toe should track straight. There's something else going on here.

Also, most common 3D printing materials aren't suitable for suspension parts. Too fragile or flexible.

http://users.telenet.be/elvo/1/1_content.html There's a little go tab to the right. kinda hard to see.

For HPI Sprint 2 - Yeahracing - Yeah Racing Aluminum Rear Hub/Knuckle Arm Toe-In 3 Degree For HPI Sprint 2 #SPT2

You keep saying the car runs like a boat. Basically you r saying one of the rear tires is not turning but sliding on the ground right? That is not because of toe. Something is binding.3 degree angle never makes tires slide instead of turning unless u have drift tires or running on very low traction surface.

That is fine, but move them all the way back and forth and look for something else moving. The play in the linkage isn’t what’s causing it to wander left and right down the straight. Either something is loose or the Servo is bad. That’s the only two things it can be.

3 degrees is the stock too...

Three degrees of rear toe-in is standard and helps keep your car going straight. Reducing it or eliminating it altogether will make your car harder to drive. I recommend you go through the rear suspension as follows;

1) Remove the rear tires and rear shocks.
2) Move the rear suspension arms up and let them drop on their own. They should drop all the way down without you touching them. If not then there is binding somewhere. Fix as necessary.
3) Ensure the rear inner hinge pins are perfectly flat and not bent. If bent, replace. You will need to disassemble to check this.
4) Ensure the outer hinge pins are also flat and not bent. Replace as necessary
5) Make sure the rear droop is the same left to right. Adjust until they are and use the setting from your manual.
6) Check the rear shocks and look for a bent shaft. If straight, ensure they compress and expand without sticking. Fix as necessary.

If all checks out, then do the same to the front suspension. Fix or adjust as necessary.

The tiny bit of slop shown in your video is normal and should not affect the way your car tracks much if at all. Shimming to the point of zero movement might make things too tight and make it worse.

Thank you for the detailed message, but the car is pretty much new, the suspension arms move perfectly and when you lift the car a little the wheels have the same distance from the ground. It seems really weird to me that the angle helps the car go straight because
a) IMAGINE the toe was 20 degrees in, then the wheels would be drifting towards each other even more making the car's tail drift left and right.
b) because I did a test with almost no inward toe and my flat tires were cooperating nice with the ground making it drive straight with no weird and unpredictable movement.

I am really confused :-s

You have to understand that EVERYBODY is running toe in the rear. That's a way to compensate the fact the front is too efficient compared to the rear.

Now, the toe must be symmetric. There is no way that it will make the car drift the way you are describing it. Please read the definition of toe and how it works (mainly by getting rear tyres hot).

On drift car, people are reducing the rear toe to obtain a more stable drift. In this case it is preferable to have outboad toe instead of inboard toe.

If you're able to change toe like this, there's something wrong with suspension arm mount. It shouldn't let you change toe like that. Maybe it's too soft to hold constant angle, that's why is your car unpredictable.Look forcaluminium mount, which could solve it.

As everybody here already said, reducing toe won't help. 0° toe-in makes rear end unpredictable at high speed, especially while braking.