It’s pretty common for one of these clutch poppers, particularly well worn in ones, to curve off to one side when going at full speed. Some may do that extremely, some only a little, and some by some miracle go pretty much dead straight. Why they curve at speed is complicated and probably a combination of several things. But the first thing is whether you really need to care much about it at all, if it curves just a bit then just pointing it on a angle from the start can help, but if it’s a bit extreme there are some things you can do to straighten them up. If you are doing this with your kids it can be quite a good little project as there is a bit of physics involved here.
So as to why this curving happens, I think it is a combination of a few things, but almost always is relative to the speed the flywheel is turning, which as you may have noticed is very fast. There is a lot of kinetic energy stored in that flywheel and I think that is part of picture. Another part is surface it is running on, and how flat that is and how much grip the car can get on it (for this you may need an anti-slip grip mat for the first couple of meters where there is maximum torque applied). And another is how much side to side movement there is in the rear wheels which can cause the body to shift slightly on the axis, which at speed magnifies this in a small (or large) direction change.
With all of these, adding weight to the right place more or less addresses the issue. This is because it can:
- counter the force of flywheel if it is shifting and causing movement
- push the car more firmly into the ground to counter surface variation and give more grip
- center its weight more and make it more difficult to move much on it’s axles if there is some decent movement there
Generally you’ll want to add about 20grams of weight (maybe more maybe less, depends) to the opposing side that the car is tending to turn/curve. You could use coins or a lead fishing sinker hammered into the right shape. Add the weight to either the far side tray (between the front and rear wheel), or right up the front by the bumper, I’ve found for some right up by the bumper works best, not entirely sure why! Use blu-tack or similar to hold in place so can easily rejig if needed.
Here’s an example, do the opposite if curving to the right.
You may still need to use some PVC anti-slip matting for the first meter if running on a very smooth surface – like this stuff:
So one thing I have found, I think, is that the more horizontal movement in that rear axle is a pretty good indicator of how much it is going to turn. I think this movement is caused by many repeated big hits on the axles after crashes, which causes the internal brass gear and ring on either side of the axle to push the steel side wall out just a tiny bit each time, resulting in a larger gap and therefore more movement. Thing is I’ve got one that is extremely beat up, and not a huge amount of movement, and one that is not too bad that had quite a lot – and that one spun out like crazy, so its not always about how beat up they are. Pushing the side walls back in a bit to close that gap more or less fixed the bad one – but that was very difficult and I very nearly stuffed it for good (and required drilling the motor out and putting it in a vice).
So I wouldn’t really recommend trying to fix like that, and actually the right amount of weight should still more or less sort out even those bad ones, and I wouldn’t worry too much about small veering off – with kids they can even make a game out it as it can make a challenge to go through a certain gap/ posts/ finish line / bowling pin targeting:)
You could also try putting in an e-clip/circlip in this gap, with some grease (white lithium), if you don’t want to push the frame. I haven’t tried that yet but could be a good solution – just need to make sure it’s not too tight as getting one of those clips out once in could be difficult/near impossible:)
They actually addressed/solved this issue in the single clutch Dune Crawler, by adding brass sleeves to the exterior part of the axle between the wheel and the motor which stops/absorbs any horizontal movement before it is translated into the motor gears. Unfortunately for whatever reason they decided not to add that enhancement to any of the subsequent models which is a shame as is quite difficult to add something like that post assembly.
All in all I think real kudos needs to go to the designers of these things, the fact that they continue to operate at all with the force and speed they crash at, repeatedly over years, is a credit to the design engineers, end to end.