XRAY - Model racing cars

Since building my ‘012 a month or so ago I have failed miserably to get it balanced (using my Hudy gear). I did wonder whether something came bent in the box or maybe my tweak station has had it. But I set up the 009 for my son at the weekend and immediately got it balanced by going through my standard setup process.

So something is definitely wrong with the car.

Would front/ rear weight distribution affect this? It balances left/right but I have not tried to check the front/ rear balance. I must say that lazily I pay little attention to front/rear balance and have never failed before to get my XRAYs balance on the tweak station.

When I say I cannot get it balanced, it is out by one mark on the station and I never muck around with the spring adjusters as a cure, as in my view that is hiding an error and I always go back to ground zero and redo all the relevant settings, which without fail gets the car balanced (but not in this case).

The car is running fine, great in fact but I do not like seeing it sitting there on the station with tweak.

What is irritating me is that after setting droop and raising the axles, separately, the setup wheels on the axle are leaving the floor at the same time. Then ditto after I haved attached the springs and set the ride height.
And ditto when I then attach the roll bars

You say you don't use the spring adjusters when on the tweak station, but that's exactly what you need to do.

If your car is anything like usual you will find that the front left and rear right appear to be heavier when put on scales.

I would loosen all the bulkhead screws etc and retighten them, then do the same with the top deck.
How are you measuring the ride height, and if doing it near the droop screws are the ride heights EXACTLY the same.
Are the shocks all exactly the same length, do they have exactly the same rebound.

Front/rear should not affect tweak, just weight balance.

Skiddins

Hi Skiddens, thanks for your help, much appreciated. In particular you have confirmed, as I was thinking, that the weight distribution did not come into this.

To answer your questions/observations…

I have, twice – once after build and then again after the 1st race meeting, loosened all the top deck and bulkhead screws to try and remove any chassis tweak. Placing the chassis on the setup board suggests there is no chassis tweak, as does placing a metal ruler along the bottom of the deck.


I think we have to accept that we are just different on this one . I know that using the spring adjusters is what the manual says and that is what I used to do; but over time, rightly or wrongly, I came to feel that that this is treating the symptom rather than the cause, which is why I no longer do this.
I find that going back to the start and redoing/checking the settings has always resulted in my car passing the tweak station test on the 2nd or at worst, 3rd time.


The front shocks are the same, as are the rear, I check…
• Rebound with a vernier guage
• Using the vernier outside measurement I measure from the top of the top cap to the bottom of the bottom cap. Accepting manufacturing tolerances they should be the same unless one of the caps has been screwed down more than another.

• Using the vernier inside measurement I measure from the bottom of the bottom cap to the top (as you look at it) of the ball joint. I assume that the manufacturing tolerances on the ball joints will have no significant impact on shock length – centre point to centre point of top and bottom connections

I do this every time I top up with oil

The only point I wonder, where I could be failing, is the ride height– although I have used the same process for the past 5 years without issue.

Very very rarely do I measure the ride height on both sides of the chassis; I measure under the centre line. I believe the process below negates the need to (but I will double check the individual side measurements).

With shocks and roll bar disengaged...
• I fit the Hudy setup wheels to remove any wheel/tyre variance

• I set the downstop on both sides of the axle and then remove the droop bar for that axle

• I slide the ride height gauge under the chassis along the centre line and check that both wheels leave the surface at exactly the same time
- note that when I say” exactly the same time”, this is a visual assessment but I have never had a problem; a difference of a 20th of a millimetre does not affect the tweak station. I know from experience that a very noticeable difference (say .25 to .5mm)will normally show up on the tweak station.

• I fit the pre-measured shocks and place a mark on the top cap and an aligned mark on the adjuster to help me ensure the adjusters are moved the same distance on both shocks.

• I use the adjusters to get the required height

• I place a droop bar under the opposite axle and then slide the ride height gauge along the centre line to again ensure that the two wheels leave the surface at exactly the same time .


I accept that chassis tweak will negate the above process; but if the tweak station is displaying tweak the first port of call for me is the top deck, then followed by the above. If I am still failing I then hit the bottom deck and unscrew everything. (If I have had a heavy crash I start with the bottom deck and unscrew everything as well as the top deck)


I will try and check the side ride heights tonight. If that throws up nothing I suspect I may be into swapping in components from the 009 (where possible) to try and identify what is causing this.

If that fails I will get you to check it at the Demon GP

Thanks Skiddens. I see your point. Maybe the weight distribution on the ‘012 is a lot different from the 009/008/007 etc. which is why it doing this now when never before.

It does though raise another question in my mind. Which handles better?

A car where the wheels leave the surface at the same time but the tweak station alleges tweak?
Or a car where the tweak station alleges no tweak but the wheels leave the surface at different times?

My brain is starting to hurt as I cannot visualise whether different weights on opposite sides of the axle would have an impact when raising the car and checking the balance.

Planning on spending an hour in the workshop tonight looking at this; will update with results

What has already been said above, you have to adjust the preload collars, and also your downstop as a result of this, and these two are directly influenced by each other.

Hi Greenwing, thanks for your thoughts, thorough as usual. Broadly, below maybe provides some answers

These are the results of my test, which demonstrated there is always the opportunity to learn!

On both axles…
I set the downstops the same both sides. Raising the axle both wheels left the ground exactly the same time.

Shocks were the same length on both side and the spring tension adjuster was exactly in the same position on both sides.
Fitting the springs; I didn’t adjust the ride height to a desired position, I just measured the ride height on both sides of the axle as Skiddens asked and the heights were the same.

Raising the axles, the wheels on the front axle left the ground at exactly the same time but there was a very very very small difference on the rear wheels. The difference was so small that I would not worry about it for a club meet but I would probably pursue it for the Nationals.

On to the Tweak station…

Surprisingly, but as before as I noted in my first post, the front axle (ie the rear axle placed on the station) perfectly balanced so far, showed 1 mark of tweak on the gauge.

The rear axle showed just a very small amount of extr a tweak, about half the width of the white mark on the station, almost invisible to the naked eye. This I put down to the very small difference in the rear wheels leaving the ground, which was not present when I did the first two setups

Adjusting…

OK this time I used the springs on the rear axle to remove its “tweak”
I then turned the car around to adjust the front springs but saw that the front axle was no longer showing any tweak; ie the adjustment to the rear axle had removed it. I do not for one moment understand why that was the case.

Back on the ground…

Front axle - as expected the front axle showed the ride height unchanged and the same on both sides of the axle. Raising the axle, as expected, the wheels left the surface at exactly the same time.

Rear axle – the ride height down the centre line was unchanged. But there was now a difference of0.5mm between the left and right ride heights. Which I suppose is not surprising.

What did surprise ME (but maybe if I had thought it through it would have been obvious), was that on raising the axle, this time the wheels left the surface at exactly the same time.

So, before going to the tweak station I had a car that was balanced except for a very minor difference on the rear wheels.

After the tweak station I have a car that is “balanced” but with a0.5mm difference in rear ride heights.
So which is the better car to race? I have no idea.

Greenwing is this where your comment on the downstops comes in? I.e. the final step is to adjust the downstops to make both sides equal to the centre line ride height measurement? I guess that just means that the ride heights will be the same but the droops will now be different?

Of course none of this really solves my problem as described in my first post; what should be a perfectly balanced car, new out of the box and set up with all the checks noted above, is showing tweak on the station and I need to cure the cause not the symptoms. This test does not replicate exactly what I was seeing before because originally there was NO difference in the rear wheels leaving the ground. But since then I have raced again and it is possible that a small amout of tweak was put into the chassis.

So tomorrow I am going to repeat the above but firstly pull all the screws and ensure again that there is no chassis tweak.

Hi Alex, sorry I did not see your post before posting. If one then adjusts the downstops to get the ride heights balanced, I assume the downstop measurements are then different?

If so what does this do to the uplift? If I am setting downstops to say 4 to get the uplift I want for handling, if I have one side say 3.5 and the other side 4.5, is the uplift the same?

..... and will it be the same when transitioning a corner, left or right?

The lift could also be influenced by the weight balance of the car. Since this may not be spot on, the wheels will lift at different times. Yes you can compensate this either by adjusting your downstops or shock preload. I prefer to adjust the downstop, this would always give me the most accurate result.