Steering setup

[J@mes 2326]

Hi folks! 

I could use some guidance on how to setup the steering because I struggle to get my RC vehicles to a place where I am happy with straight line driving. Even when I use turn buckles and spend ages tinkering it doesn’t always resolve all the issues.

is the a technique or some gadget that can help? All my vehicles are vintage monster trucks, no buggies or road cars, maybe the slop is all of the issue and I just need to try harder!

TIA

[Falcon#5 594]

Are they wandering, or too twitchy?

[J@mes 2326]

Hi @Falcon#5 they’re wandering. Also seems the turning circle varies in size left to right, is that my setup or is it  character due to servo location / steering arm length or similar?

[KEV THE REV 2310]

Centring the servo position can help and have the tie rods the same length  . Is your servo set to centre , or do you have to adjust the trim a lot to get the wheels straight ? , this can have an effect on the turning circle . . Check slack on the wheel bearings ( plastic ?) and any other related steering parts

[MockTurtle 239]

check if you've that plastic washer in the servo saver installed. if its not installed the horn will kind of 'bind' and not center properly. then again it may be just slop which is inherent in these monster trucks... i've started to replace mine with the high torque servo saver and that helps a lot.

[Juggular 4964]

Are they toed in?  or toed out?  

When the 2 front wheels are parallel, you would have the most trouble going straight.  

For straight line, toe-in is better. But it's tipsier at the corners.  Toe-out will also help straight running, but it's better at corners.  

During 80's and 90's, all Tamiya cars were toed-in.  But nowadays, most are toed out.  

 

Most will have some difference in turning circles.  

 

[Big Jon 684]

A quality digital servo will center better and hold center better, too, while a stiffer servo saver will prevent the vehicle from being knocked off course as easily. I've also seen cheap or worn out transmitters refuse to center.

If you have a radio with a bunch of adjustments, you can add some expo, which will make the steering less sensitive near center. This really helps me to go straight, especially with twitchy on road cars.

A pinch of toe out improves straight line stability.

 

[nbTMM 433]

I actually find front caster has a more profound effect on stability than toe. Positive caster will make the front wheels want to steer in the direction the car is travelling in so the car tracks straight, as long as the servo is not forcing them to steer off course. Some slop in the steering, combined with positive caster therefore makes a car track straighter. Unfortunately, caster is often non-adjustable on many models because the angle is built in to the plastic parts. Most 4WD Tamiyas have very little built in, and require modification to get more.

Theoretically, toe in at both axles should improve stability and this is mostly true for 1:1 cars. Imagine the car is travelling down the road and the car starts sliding slightly on the pavement and rotates a little to the left (it's now facing a little towards the left gutter instead of directly down the road). Now the left wheels which were previously scrubbing slightly due to toe in are pointed perfectly down the road and no longer scrub, and the right wheels are pointed even further towards the gutter causing them to scrub even more than normal. The right wheels scrubbing gives a slight braking effect and skid-steers the car to the right, counteracting the slide.

However, there is a second effect. If the car starts steering towards the left due to steering slop, a bump/pothole in the road or tyres ballooning and becoming different diameters, the car will experience a weight shift to the right wheels (which in turn causes bodyroll to the right). Now the right wheels experience higher grip than the left wheels and produce a stronger steering force to the left, than the left wheels produce to the right. This results in the respective axles independently steering to the left. If the rear steers to the left and the front axle does nothing, the car overall turns to the right, so toe in at the rear definitely helps stability. If the front steers to the left and the rear does nothing, this makes the problem worse as it causes the car to steer even further to the left, therefore in the case of weight shift front toe in can decrease stability if there is a significant weight shift across the wheels on an axle. That is probably why some people report that front toe out improves stability, instead of toe in.

Personally my go-to setup for stability is toe in at the rear, zero toe at the front, and positive caster. Some toe out at the front seems to improve the aggressiveness of the steering, and might decrease straight-line stability slightly although to be honest I don't notice a huge change compared to adjusting rear toe or caster. If there is a lot of slop in the wheel hubs, you may need to slightly toe out the wheel from the alignment that you want, because any driven wheels will toe in under power.

As above, servo and servo saver make a big difference too. If you can move the mounting point that the steering arm attaches to the servo saver/horn closer to the center of rotation (effectively reduce the length of the horn), the servo requires more rotation to give a certain amount of steering throw. This may reduce the amount of steering, but reduces the effect of a deadband/centering error from the servo. Some servos just have a large deadband, which is how much the input signal has to change before they actually command the motor to move. This stops them constantly hunting around and burning themselves out. You can test this by turning the steering to full lock and back to centre in either direction, and checking if the centre position is the same from both sides. A servo with a large deadband will stop significantly right of centre when returning to centre from the right, and left of centre when returning from the left. Making sure the steering is mechanically adjusted to center (remove the horn/saver and reinstall centered) with no electronic trim will ensure you have close to even throw to the left and right before adjusting end points.

Finally, consider adding a steering gyro.



 

[J@mes 2326]

On 9/13/2019 at 8:10 PM, KEV THE REV said:

Centring the servo position can help and have the tie rods the same length  . Is your servo set to centre , or do you have to adjust the trim a lot to get the wheels straight ? , this can have an effect on the turning circle . . Check slack on the wheel bearings ( plastic ?) and any other related steering parts

What I find is with a 25 T servo I center it then the saver splines won’t line up to slot on dead straight and the compromise is a spline to the left or right then adjusting the trim.

i don’t bave any 23T servos, I expect I’d have the same problem 

 

yeah there’s a lot of slack on all to front wheels on the 2WD ones (regardless of bearing or not) guess I need spacers.

[J@mes 2326]

On 9/13/2019 at 8:17 PM, MockTurtle said:

check if you've that plastic washer in the servo saver installed. if its not installed the horn will kind of 'bind' and not center properly. then again it may be just slop which is inherent in these monster trucks... i've started to replace mine with the high torque servo saver and that helps a lot.

I too use a high torque saver 

the Trucks with the separate kit supplied fixed central pivot might not, I need to check!

[Mad Ax 9310]

I find Tamiya servo savers (which are not the best to begin with) aren't so good on modern servos.  Especially servos with metal gears, as you can get plenty of torque in the thread (and you need to to stop it unthreading).  Too much torque and (IMO) even with the plastic washer installed, the servo saver binds up and won't return to centre after a knock.  A good servo saver is a help.

Plenty of slop in vintage Tamiya truck design doesn't help either.  My KBF (despite being my favourite monster truck) is terrible for that.  Under acceleration, the wheels naturally want to go toe-out - so adding a bit more toe-in helps.  Also the geometry with that centrally-mounted steering post is very bad indeed, when the front wheels go over a bump the steering angle changes.  This is (a form of) bump-steer and is especially pronounced on these trucks.  Check the steering angle throughout all the suspension movement and adjust according to where you think the compression 'normally' is.

Retro-fitting a steering rack to my KBF is one of the first big projects for my 3D printer