Before You Tune Anything: The 12-Minute Mechanical Check That Comes First

Every setup guide ever written — including ours — starts from an assumption it never states: that your car is straight, symmetrical, and mechanically healthy.

Almost nobody checks. So people read "the car understeers, try thinner front shock oil", change the oil, and it barely helps. So they change springs. Then camber. Then toe. Three weeks later the car still pushes, they have spent money on parts, they have lost confidence in their own judgement, and the actual cause was a slightly bent front hub carrier from a crash a month ago.

This is the check that goes first. It takes about twelve minutes, needs almost no tools, and it is the difference between tuning a car and guessing at one.

The rule: if the car is not mechanically right, every setup change you make is being applied on top of a fault. You are not tuning — you are compensating. And compensation always falls apart the moment conditions change.

Do this in order

The order matters. Each step assumes the ones above it passed.

1. Is the chassis flat? (the tweak test)

Tweak is a twist in the chassis or suspension that makes one corner sit lower than its diagonal opposite. A tweaked car pulls to one side, feels different turning left than right, and traction-rolls in one direction only.

Test: put the car on a genuinely flat surface — a glass shelf, a granite worktop, a proper setup board. Not a table you assume is flat. Push down gently on the chassis at the front, let it settle, and watch whether both front wheels return to the ground together. Repeat at the rear. Then lift the car by the middle of the chassis and set it down; if a wheel is off the surface, or if it rocks diagonally like a wobbly restaurant table, it is tweaked.

Fix: adjust spring preload on one corner in small steps until it sits flat. On many cars there is a dedicated tweak screw. Retest after every adjustment.

Why it comes first: every measurement below is meaningless on a twisted car, because you would be measuring the twist.

2. Is the steering mechanically centred?

Test: set the steering trim to dead centre on the transmitter. Do not adjust anything else. Now look at the front wheels. Are they pointing straight?

If they are not, do not fix it with trim. Trim shifts the servo's neutral point, which means you now have more steering lock available in one direction than the other. The car will genuinely turn better one way than the other, and you will spend a season blaming the diff.

Fix: adjust the steering turnbuckles so the wheels sit straight with the trim centred. Trim is for the last tiny amount, not for hiding a misaligned linkage.

3. Bump steer

Bump steer is the car steering itself when the suspension moves, with no input from you. It makes a car dart unpredictably over bumps and on landings, and it appears out of nowhere after a rebuild or a ride-height change.

Test: hold the steering straight. Lift one front wheel through its full travel and back down, slowly, while watching the wheel from directly above. It should stay pointing the same direction throughout. Any visible toe change through the travel is bump steer. Check both sides — they are often different.

Fix: shim the steering link ball studs up or down so the link swings on an arc that matches the suspension arm.

4. Bent parts

After any real crash, something is usually slightly bent. Slightly is the problem — badly bent is obvious and gets replaced, slightly bent gets driven for months.

Test: with the car on a flat surface, sight down the front from a metre away and then the rear. Look for one wheel leaning differently from the other, or one sitting further forward. Compare left and right camber by eye against something vertical. Roll the car slowly and watch each wheel for wobble.

Then take the wheels off and check the parts most likely to be bent: hub carriers, steering blocks, turnbuckles, shock shafts, and — very commonly overlooked — the chassis plate itself.

Fix: replace. A bent turnbuckle can sometimes be straightened; a bent hub carrier should be replaced, because the alignment it sets is the whole point of the part.

5. Ride height — measured, not eyeballed

Test: on a flat surface, battery fitted, measure the chassis height at the front and at the rear with a ride height gauge or a ruler. Compare left to right at each end as well as front to rear. They should match side to side.

Why: unequal side-to-side ride height is another form of tweak. Unequal front-to-rear changes the car's balance and is a legitimate tuning tool — but only once you know what it currently is.

6. Droop, both sides equal

Droop is how far the suspension extends downward before it runs out of travel.

Test: with the wheels off the ground, measure the same reference point on each side. Left and right must match at each end.

Why: unequal droop makes the car behave differently turning left versus right, which is exactly the symptom people misdiagnose as a diff problem or a bad tyre.

7. Bearings

Test: wheels off. Spin each wheel hub by hand and listen. A healthy bearing spins freely, quietly, and coasts. A dying one feels gritty, sounds rough, or stops abruptly. Do the same for every bearing you can reach — hubs, diff outdrives, transmission, and the motor itself.

Why it matters more than people think: a dragging bearing adds load the motor pays for in heat all run, and one dragging front bearing will make a car pull to that side under power.

Fix: clean and re-oil, or replace. Rubber-sealed bearings last longer in dirt and water; metal-shielded ones spin more freely. Both are correct choices for different uses.

8. Differentials

Test: hold one wheel on an axle and turn the other. It should turn smoothly against a consistent resistance. Notchy, gritty, or dramatically different resistance between front and rear that you did not set deliberately means a diff needs attention. Look for oil on the outside of a gear diff case — that is oil that is no longer inside it.

Why: a diff quietly leaking oil changes the car's balance a little more every run. Chasing that with setup changes is maddening, because the thing you are tuning against keeps moving.

9. Slipper clutch

Test: hold the spur gear and try to turn a wheel. It should take real force to slip. If it slips easily, it is too loose; if it will not slip at all under a hard launch, it is not protecting anything.

Why: a slipper set too loose costs acceleration, generates heat and glazes its own pads. It feels exactly like a car that has lost power, and it gets misdiagnosed as a tired motor constantly.

10. Shocks

The most common source of left-right asymmetry, and the hardest to see.

Test: take all four off. Compress each one slowly by hand. They should feel identical side to side, move smoothly with no dead spot at the top of the stroke, and rebound at the same rate. A shock with air trapped in it feels spongy and has a soft spot near full extension. Look for oil on the shock bodies — a leaking shock has less oil in it than its partner, which means the two ends of one axle are damped differently.

Fix: rebuild in pairs, bleed properly, and use the same oil and the same technique on both. If you rebuild one, rebuild its partner.

11. Tyres and inserts

Test: spin each wheel and watch for the tyre running out of round or wobbling. Squeeze each tyre — the inserts should feel the same on all four, or at least matched front and rear. Check the glue line all the way round; a tyre unglued in one spot balloons at speed and behaves like a completely different compound.

Why: inserts collapse and take a set over time, especially after hot runs. A car whose handling has slowly drifted with no setup change is very often just running tired inserts on one end.

12. The boring hardware

Wheel nuts tight. Wheel hexes not worn round or spinning on the axle. Suspension pins not walking out. Ball studs tight. Servo horn screw tight — a loose servo horn feels exactly like a failing servo. Nothing rattling that should not be.

Symptoms that are almost always mechanical, not setup

If you have any of these, stop tuning and go back to the list.

• The car behaves differently turning left than turning right. Tweak, unequal droop, unequal ride height, a bent part, or one dead shock. A setup change cannot be asymmetric, so a symmetric setup cannot be the cause.
• It pulls to one side under power but tracks straight when coasting. Drivetrain — a dragging bearing, a diff problem, or a bent driveshaft.
• It darts unpredictably over bumps but is fine on smooth ground. Bump steer.
• Handling changed suddenly rather than gradually. Something broke or moved. Cars do not change their character on their own.
• Handling drifted slowly over many runs with no changes made. Wear: inserts, bearings, shock oil, diff oil, a slipper glazing.
• It was fine, you crashed, now it is not. Do the whole list. All of it.

After a crash, specifically

A crash is the single most common cause of a car that "suddenly needs setup work." Work through this before you touch a setup screw:

1. Tweak test — a hard landing twists chassis plates.
2. Sight down both ends for a bent hub, block or shaft.
3. Check every turnbuckle for a bend by rolling it on a flat surface.
4. Check the shocks for a bent shaft — compress each one and feel for a catch.
5. Re-check bump steer, because a bent steering component reintroduces it.
6. Re-check gear mesh and that the drivetrain still spins freely.
7. Range check the radio. Aerial damage is invisible.

Why trim is a liar

This deserves its own note because it causes so much wasted effort.

Trim moves the servo's neutral point. It does not move the mechanical centre of your steering. So a car trimmed to drive straight, with a linkage that is 5 degrees off, now has 5 degrees more lock available one way than the other.

The driver feels a car that turns better one direction than the other. They check the diff. They check tyres. They swap shocks side to side. The cause is on the transmitter, and it was introduced by them, deliberately, to fix a symptom whose real cause was in the linkage.

Set the mechanics right, then trim. If you ever find yourself using a lot of trim, treat it as a fault report rather than a fix.

Then, and only then, tune

Once the car passes this list, you have a baseline: a straight, symmetrical, healthy car. Now setup changes mean something, because a change produces a difference you can attribute to the change.

From there:

• Change one thing at a time. One end, one step.
• Run the same section of track between changes. Comparing a change against a different corner tells you nothing.
• Write it down. Setup sheets exist because human memory of how a car felt three packs ago is unreliable.
• Know how to get back. Note the setting before you change it, so a change that made things worse costs one minute, not an evening.

Our tuning guide covers what to change once you are at this point, symptom by symptom.

What we could not confirm

There is no universal specification for how much tweak, ride-height difference or droop difference is "acceptable" — it varies by chassis, class and how precisely the car is being run. We have deliberately not published tolerance figures, because a number that is meaningless for a bashing monster truck and far too loose for a carpet touring car would be worse than no number. The principle holds for everything: left and right should match, and any difference you did not put there deliberately is worth removing.

Manufacturers who publish setup sheets for their own cars give figures for that car. Those beat anything general, including this page.

The short version

Before you change one setup screw, in this order: flat, straight, centred, unbent, equal, free, sealed, matched, tight.

Twelve minutes. It will save you three weeks.