Static Chassis Balance Brushless

[Mark Payne]

Hi All

Right... I already know I am going to take flak for this.... but I write in the hope that someone may have thought of this already or may want to think about it with me and discuss.... Alternatively go ahead and take the p*ss.

The topic is lateral chassis balance. Specifically on a CRC CK with brushless but the principle will apply everywhere me thinks.

Set the scene: If you take your rear pod off your car and keep it complete with rear axle, wheels, average 10 turn modified motor and pinion and then pop it in balancing posts.... it will balance (left to righ lateral balance), pretty much spot on.

So.... join the car up again and put the whole thing on the balancing posts and balance it out by the arrangement of the electrics and you have a tricked out car. I have already documented this all in www.12thrc.com

Now the brushless:

Ooooh ... interesting (come on, if you are down this far you are interested) again my brushless post on 12thrc tell us that my LRP brushless setup is about 30g heavier than the brushed. This is not news but the balance of the motor is different. There is a lot more weight at the (I was about to say commutator end!) ... at the sensor wire/connection end, away from the pinion. This unbalances the pod.

As a result, if you then correct this in the front part of the car you end up shifting weight over to the pinion side of the car. In fact, I can only achieve overall balance by adding a 5 cell RX pack right over on the pinion side.

So.... I am left thinking... is this the right thing to do? When this lot sits down on the deck the front chassis is applying a slight twisting force through the tweak springs to corner weight the rear tyres.

So I have decided to remove the pack which balances the front part the the chassis (take off the tweak brace and you can pivot the chassis around your front end lifting point and the rear pivot ball) but leave the car overall with an inbalance due to the pod.

I think the real fix would be a modified pod with the bulkheads moved over to the pinion side (yes resulting in a shorter diff side axle offset and a longer offset on the other side). However this is really NOT an easy win.

Also in Worlds brushless trim without my pack the car is at 885g (that is with my 3700s in!). The limit is 865g. If I were to run the pack and the 4300's I am going to be well over 900g.

Wanna discuss or am I boring you?

Mark

[jo90]

Not that I have run 12th in ages, or gotten down to the specifics of balancing one, but I get where you are at. IT isn't going to be an easy job, and it would seem it will leave most either setting up to solely use brushed or brushless if this is the case.

Not sure apart from what you have stated (shorted diff side etc) what else can be done, as surely (again stated above) you shouldnt load a chassis to force it to untweek with weights ??

[gaz_d]

Mark,
I believe Viking just threw the brushless in his L4 and went racing. (Mick will no doubt confirm this.)
Wasn't too shabby either looking at his car at the Euros.

More food for thought........

[sidtsloth]

Mark,

Its an interesting question. I would have said that if you were going to worry about it then the rear pod is the place to do the balancing - ie you shouldn't try to even it up at the front. As I see if you have 3 options:

1) ignore it - set the car up as you would normally but with the balances on the rear out. Which is what I assume you ran at the euros.

2) Balance the rear pod by adding weight - I wouldn't have thought you needed much to bring it back into balance, the challenge is where to mount it.

3) As you say, widen the rear pod, so that there is a lesser offset on the diff side - This shouldn't be too hard to do depending on how much you need to move the pod across - its just a new more central lower pod plate, a new upper plate with holes to match the lower. and a new diff side hub. There even appears to be room on the 12th axle that I have to allow for this - failing that I have a selection of 10th axles which are more than long enough. I think the non-diff side would need to be left the same, just to avoid having to shim it, and because of where the T-bar meets on an associated - not a problem on the CRC.

I have already asked for a few diff side hubs to be machined for the 12th car as I have a few sets of 2 hole rear wheels and so am having the hub made to accept either 2 or 3 hole wheels - much like the non diff side. I should have a couple of these at the worlds if anyone is interested - I'm sure I cant be the only one with 2 hole wheels lying about.

Is that enough thinking about it for you?

[Mark Payne]

Hi Gaz

Mate, that is not my style. You know I think about this stuff. I like to understand it so I can write it up and help others.

I also threw in a brusless at the Euros. I am now tring to think about it and do it right. Viking will still beat me but thats not the point ;-)

Cheers

Mark

Mark,
I believe Viking just threw the brushless in his L4 and went racing. (Mick will no doubt confirm this.)
Wasn't too shabby either looking at his car at the Euros.

More food for thought........

[Mark Payne]

Hi Richard,

Yes .. I am going for the option of ignoring the rear pod and not tring to balance the car overall. I have balanced the front section as an item, it seems the right thing to do.

I may need to update my notes on 12thrc.com , I have readers around the world that now regard this as a bit of a "bible" for CRC and I need it all to make sense.

Frank Calandra will be at the worlds and I will see if he thinks this kind of thing warrants a modified pod. I have already posed the question to him also.

Cheers

Mark

Mark,

Its an interesting question. I would have said that if you were going to worry about it then the rear pod is the place to do the balancing - ie you shouldn't try to even it up at the front. As I see if you have 3 options:

1) ignore it - set the car up as you would normally but with the balances on the rear out. Which is what I assume you ran at the euros.

2) Balance the rear pod by adding weight - I wouldn't have thought you needed much to bring it back into balance, the challenge is where to mount it.

3) As you say, widen the rear pod, so that there is a lesser offset on the diff side - This shouldn't be too hard to do depending on how much you need to move the pod across - its just a new more central lower pod plate, a new upper plate with holes to match the lower. and a new diff side hub. There even appears to be room on the 12th axle that I have to allow for this - failing that I have a selection of 10th axles which are more than long enough. I think the non-diff side would need to be left the same, just to avoid having to shim it, and because of where the T-bar meets on an associated - not a problem on the CRC.

I have already asked for a few diff side hubs to be machined for the 12th car as I have a few sets of 2 hole rear wheels and so am having the hub made to accept either 2 or 3 hole wheels - much like the non diff side. I should have a couple of these at the worlds if anyone is interested - I'm sure I cant be the only one with 2 hole wheels lying about.

Is that enough thinking about it for you?

[Viking]

As I got a mention - here are my thoughts........

Sling it in, check tweak and go racing.

At the Euro's during practice, I fitted the brushless and went straight out. My car was good already, so I didnt fiddle. The result was a car that was still good, but was faster and durated - those that know me, a car that durates its pretty awesome to me!!!!

In a nut shell, it worked for me - the theoretial amongst us can theorise, thats what they are good at. For me.......just go race it!

[PDW]

Just in case this glass of wine (like to copy Mark P's good points!) is affecting my thinking, feel free to comment...

If the weight in the pod is a bit lop-sided, how is altering the tweak on the springs/t-bar going to bring that back? The main chassis is disconnected from the pod by the T-bar pivots, or single pivot. It wouldn't matter if you stood on the left-side wheel, when you picked up the chassis at the front, it will simply equalise itself based on the position of the springs (CRC) or the t-bar screws (12L). How will altering the springs/tweak screws alter the weight distirbution in the motor pod?

I cite the fact that we change motors ad nauseum during racing, but I have never had the tweak alter if I take out a (plastic endbell) Yokomo design, and put in (aluminium endbell) V2 design. Come to think of it, I have never had the tweak alter whatever motor I put in, including one of Mr Ward's lightweight rare-earth magnet jobbies. The weight in the pod is distributed across the rear axle, and has no impact on the position of the main chassis once you lift the front wheels off the ground - the chassis is articulated in the middle, not fixed like a Touring Car or Buggy.

I agree that the weight distribution across the rear axle might not be as it should be, and moving the motor pod side plates would place the weight centrally, but I don't see how you can alter that weight distribution any other way. I can also see that this skewed weight distribution might affect traction, as there is more weight on one wheel than the other.

I cite lastly as evidence (M'Lud) the heroic Viking who says he had a good car, slung in a (heavier) brushless motor, and got the same handling he had before. With the amount of front end Mick runs, any tweak from the weight distribution change would have shown up immediately, especially at the speed Mick drives!!

I don't doubt the desire to get even weight distribution is right and proper, but balancing it out with weight on the main chassis won't help IMHO. And at 900g Mark, you're going to need a brushless to make the race time - wouldn't it be cheaper to buy a brick, instead of building one??!!

[KRob]

Stick it in the car and drive

[Mark Payne]

Errr Drinking a beer...

Pete, if the pod in isolation is not balanced then you agree that if you put it down one wheel may be carrying lets say 20g more than the other. Now attach the rest of the chassis with perfect balance, perfect geometry and do the coin thing and now you sill have 20g more on one rear wheel compared to the other (more weight on both obviously). I agree that this cannot transfer to the front wheels.

Put the car on the balance pivots and it will be "wrong" ... move things around in the front of the car to make it balance (as people do) and then put it dowm. Question.
What has happened to the 20g difference in the rear wheels?

Answer this and I will move on to the next question.....

Just in case this glass of wine (like to copy Mark P's good points!) is affecting my thinking, feel free to comment...

The weight in the pod is distributed across the rear axle, and has no impact on the position of the main chassis once you lift the front wheels off the ground - the chassis is articulated in the middle, not fixed like a Touring Car or Buggy.

I agree that the weight distribution across the rear axle might not be as it should be, and moving the motor pod side plates would place the weight centrally, but I don't see how you can alter that weight distribution any other way. I can also see that this skewed weight distribution might affect traction, as there is more weight on one wheel than the other.

I don't doubt the desire to get even weight distribution is right and proper, but balancing it out with weight on the main chassis won't help IMHO. And at 900g Mark, you're going to need a brushless to make the race time - wouldn't it be cheaper to buy a brick, instead of building one??!!

[PDW]

Mark, you're not allowed to change chemical fuel without telling me!! How on earth can we converse if you're on unleaded and I'm on four-star?

The 20g is still on that rear wheel, and that wheel is still carrying more weight than any of the other wheels. My point is that with an articulated chassis, how does the rear imbalance transfer to the opposite corner?

If it can't transfer to the opposite corner, as you are saying, then the impact of the rear imbalance can only be on traction at the rear wheels. However, that additional 20g is not going solely to one wheel. Since the whole rear end is solidly connected, some must be going to the pinion side too. If that is the case, you may end up with as little as 10g extra on the left rear wheel.

A 10g imbalance is no worse than some paint, stickers and manufacturing tolerances in the parts on the left-side of the car, so the effect is marginal at worst, and may already be there anyway. On a 900g car (God knows how you're going to get that whipped through a chicane in a hurry!! ) you are talking about 1% of the weight being in the 'wrong' place; absolute max 2% if you were on the 800g limit.

The fast boys (Viking and KRob) are telling us to get out and drive, and neither suffered any problems switching from one motor to the other - nor, from memory, did Jim Spencer or Griff. This supports the hypothosis above - it is too small a change to worry about.

[Mark Payne]

We can continue this at Chesterfield if you like.
I will put my car in your hands and prove it to you.

Meanwhile. The lateral balance of the pod expresses any balance errors by compressing the rear tyres more or less on one side cf. the other. The tyres are relatively stiff compared to the imbalances we are speaking of and can be considered as solid. As a result the physical position of the centre pivot ball and (more importantly) the spring balls do not change even with imbalance. In this sense errors in the rear pod balance cannot be passed to the front wheels through the chassis. I said this already and you agree BUT the reverse is NOT true, this is where we differ I think:

If you load up the pinion side of the car (in the front section) with the extra 20g you need to balance the car out then this weight load DOES pass to the rear pod via the the compression of the "tweak spring" on the same side as the extra weight you have engineerd in the front to counteract lack of it at the back. Of course on a CRC link car the springs are not just tweak adjustments thet are a fundemental part of the suspension.

I actually now think is is counter productive to balance the car out in this way. I think it is better to leave it and not try to achieve the balance on the pins overall with the motor pod carrying a balance error. Rob and Mick will agree, they are better drivers than I and are instinctively doing a thing right by not doing a thing... get it?

Meanwhile I have a 1/12th website with over 16,000 pages viewed since christmass where I explain to people how I think stuff works. I like to be right. I owe it to people. Thats why I wanted to discuss.

Posts like " oh mr_faster_tha_you just slings it in and drives faster than you so who cares" are empty air to me. I like model engineering I like to play with stuff. I would rather be slower and understand. My sponsors seem to understand that and place value on my efforts....

Not a dig at Pete.... thanks for your part of the discussion. You see I think I have been wrong to persue this balance thing.

As I said, I knew I would take flak. Maybe this is the wrong forum for 1/12thscale tech stuff?

Cheers

M

Mark, you're not allowed to change chemical fuel without telling me!! How on earth can we converse if you're on unleaded and I'm on four-star?

The 20g is still on that rear wheel, and that wheel is still carrying more weight than any of the other wheels. My point is that with an articulated chassis, how does the rear imbalance transfer to the opposite corner?

If it can't transfer to the opposite corner, as you are saying, then the impact of the rear imbalance can only be on traction at the rear wheels. However, that additional 20g is not going solely to one wheel. Since the whole rear end is solidly connected, some must be going to the pinion side too. If that is the case, you may end up with as little as 10g extra on the left rear wheel.

A 10g imbalance is no worse than some paint, stickers and manufacturing tolerances in the parts on the left-side of the car, so the effect is marginal at worst, and may already be there anyway. On a 900g car (God knows how you're going to get that whipped through a chicane in a hurry!! ) you are talking about 1% of the weight being in the 'wrong' place; absolute max 2% if you were on the 800g limit.

The fast boys (Viking and KRob) are telling us to get out and drive, and neither suffered any problems switching from one motor to the other - nor, from memory, did Jim Spencer or Griff. This supports the hypothosis above - it is too small a change to worry about.