wheel alignment - need help!
 

Hello, I am an HPI Sprint 2 owner and I have issues with the driving.

My problem is that I still haven't achieved to make the car go straight all the time. After minimizing the steering system's slop putting better ball-cups I still get random course changes. Even if when the car is trimmed properly, while I am going straight , at a random time it starts to turn to a random direction...Sometimes it's from the rear wheels, like a dog walking (stupid example).

After a front wheel alignment (they used to point outwards) the car driving has improved a little and my conclusion is that this happens because of the rear wheels' inward alignment causes this: when the grip is better on the left tire it starts steering to the left (because the tail goes to the right) and opposite.

I need some expert's help please because I don't know how to solve this problem and it's being a pain 2 months now.

***P.s. Is there an article or picture about proper wheel alignment / adjustment for street racing?

Honestly you don't want to mess with asymetric geometry.

You should start to check that there is nothing's bent. Then you should check the bearings: if you have one of them with is crappy, you could have some strange reaction when driving. Finally you should check you rear differential to see if everything is in place and if it is working properly.

Are you using the servo which came with the kit? This is a entry-level servo, it could be worn (electronically) or pinions inside can be out of order.

The car is very new and nothing is bent because I haven't crashed it or something. I've noticed almost all cars have their rear wheels pointed inwards. I'm on the stock servo but I've tried stronger servos too, I don't think it's a servo problem because when I have it lifted up , with some throttle going on, the wheels seem to always keep straight even after I steer left or right they return to their original position, so it must be the grip thing I describe above...

Random RC car with the wheels pointed inwards: http://www.rcdriver.com/wp-content/u...RC-Cars-59.jpg

Rear wheels pointing in is called toe in. This is normal and it makes the rear of the car more stable. It's not causing your wandering issue.

What kind of surface are you running on? If the surface isn't smooth and flat then it can push the car off course. How sharply is it changing course? Is it doing it while at constant speed or only when accelerating or braking? Is it the same direction each time or random? If it's only while accelerating or braking and in a consistent direction then weight distribution and droop settings are the most common causes.

It's independent to the acceleration and braking, let's say you go straight at 10 km/h while it goes straight suddenly starts a course to the left or to the right. It happens even with the steering servo off and the steering system tide up straight. It can happen after 2 or 3 or 7 meters after going completely straight...
I drove it on flat marble and flat asphalt.

1. Front toe out is a common setting for 4 wheel drive RC cars - aprox 1 degree per side
2. Rear toe in is also common - typically 2.5 - 3 degrees per side

I'd suggest you let an experienced driver try your car. It may be normal and you are not used to it. I've raced for years, but when i recently changed to a new racer my car wandered all over like a drunk driver ( OK, maybe not that bad, but noticeable ), reason being the new radio responded quicker and it took me awhile to get used to it. Trying driving in a straight line and take you hand off the steering - does it still dart to the side?

Yes this is how I did the test.
I trimmed until it went dead straight
Started throttling to 20-30% and check
Traveled around 20 feet and then reverse
2/3 times it left the straight line
Usually the rear end was "drifting" not sliding but you know what I mean.
In higher speeds this can cause you to loose control.
I can't imagine how this can be considered as normal.
I need to find someone experienced, in my city there must be a few but don't even know where they hang out...

See if the Servo has come loose. This is very common.

Take the body off and grab the front wheels and start moving them left and right you will see if something is loose.

The servo doesn't even have to be loose. Try a different servo yours could have a problem in the feedback loop.

The servo doesn't even have to be loose. Try a different servo yours could have a problem in the feedback loop. Or a bad gear in it.

I have tried a Turnigy 15kg servo that is better, the problem was the same.

There is a play in the front wheels because of the servo saver (video below)
http://www.mediafire.com/file/ewptp9...1511450086.mp4
but even when I eliminated it by disconnecting the servo cable and blocking it on the chassis with a wire just for the sake of the test I had the same results (maybe 20-30% better).

The most random steering looks like it's coming from the rear side because it has that boat-y way of moving...

Search for the "Hudy setup book" which is an old document but based on a car simulair to the HPI Sprint 2

It is also important to check the tires if all is glued well to the rim.

Here's a couple of really short videos I made to show you what exactly is the problem!

Notice:
Full battery: check
Perfect trim: check
Wheel alignment check
Heavy duty RPM ball cups: check
Tested a new turnigy 15kg servo: check
Car in perfect condition / no crashes: check

Download link: https://we.tl/UkFqjnDPu4
(Details about videos in each title. In general I'm using only throttle and reverse, no steering except when to prevent from crash)

This is no car setup issue, I would check the play on the whole steering system and if it is not that much I must be the servo or the transmitter.

This is the servo-saver slop and it's the whole play there is in the steering system, is it much? (If I hold the servo saver blocked there's no play)
BUT , even with the servo locked down with a hard wire on the chassis so it can't move and the servo off the power, I still had the random steerings!!!
https://www.mediafire.com/file/pq23b...ver%20slop.mp4