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Old 05-24-2002, 02:21 AM   #121
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mcrisp, thanks for the reply on the hinge pin toe-in/out angles- I can live with that answer. Now what's the benefit of a car with most of it's weight located down the middle of it's chassis(Kyosho KX1, HPI Pro3) gonna do for handling vs. a car where most of it's weight is on the sides(Yokomo MR4's, XXXS's, TC3's)of its chassis? It seems like there won't be much to "balance" from side to side with the chassis configuration since all the weight is in the middle- is this an advantage when balancing side to side? The more I think about it the more I see these cars have trouble transfering weight from side to side in turns which inturn leads to less traction.

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Old 05-24-2002, 02:32 AM   #122
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proudwinner:

Putting the batteries down the middle does a couple of things. First it could help achieve a better 50/50 left/right weight distribution. Secondly there is less weight being transferred when the car leans into a corner, which should allow for quicker direction changes.

There are likely some other benefits....
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Old 05-24-2002, 05:02 AM   #123
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mcrisp, hope you don't mind if I give a more complete answer to proudwinner's question.

If you look at a car with it's mass on the centre line from the rear, all it's mass is concentrated nearer to the roll centre than if the weight was off the centre line. This means the car has lower inertia with respect to the roll centre. The car will roll more quickly.

This means when you initially turn from the straight ahead position that response will be improved.


Now look at the car fom above and picture it going in a large circle. You now need to consider the centrifugal force with relation to the centre of that circle. Your gut reaction may be that as you have moved an equal amount of mass towards the centre of the circle and an equal amount away from the centre of the circle then the centrifugal force is unchanged. However removing mass from the outside of the corner is more significant than removing mass from the inside of the corner. The centrifugal force has in fact dropped. There is less sideways loading on the tyres.

So round large fast corners you have more grip with mass on the centre line.

Now we consider a really tight corner. To simplify the theory assume the car can turn with a radius of zero. Look at the car from above. The car will rotate about a point somewhere near the centrepoint of the chassis (if it is well balanced). By moving the mass to the centreline you are moving the mass nearer in the left and right direction but you are moving the mass even further away in the front to rear direction. The "polar moment of inertia" is increased. This makes it harder to start the car rotating about this point and easier to stop it.

Round very tight corners the car is initially reluctant to turn but may then spin.

Now look at the side of the car. The final consideration is front to rear pitch or weight transfer. Assume that the car rotates roughly about the centre of gravity / middle of chassis. As mentioned already you have moved more weight to the front and back of the car. Therefore you have also increased inertia in this plain.

Front to rear pitch / weight transfer is going to be slower. This could cause scrubbing of speed and understeer coming off throttle and loss of traction under acceleration. Stability of offroad cars is also improved with the front or rear less likely to shoot up in the air over bumps.
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Old 05-24-2002, 08:42 AM   #124
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Hey dw,

Good explanations and thanks for adding more to the answer.

I was a little confused about your explanation on how the increased polar moment of inertia causes the car to be harder to start the car rotating and easier to stop it. I agree with it being harder to start the car rotating, but I would think it would also be harder to stop the car from rotating given the increased mass at both ends of the car.

Perhaps I am missing somthing here?

cya,
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Old 05-24-2002, 10:20 AM   #125
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Yes I typed that wrong .
Increased inertia makes it harder to stop something turning as wells as harder to start it turning. The conclusion for that section is correct though.

I also notice I have made another error in the section about large radius corners. Firstly I meant inertia relative to the centre of the circle and not centrifugal force, although this makes no difference to the final assumption.
In order to reach that conclusion I assumed the mass all fell on a line passing through the centre of the circle. This is normally a reasonable approximation to make if the radius of the circle is much bigger than the length of the car. For what we are trying to calculate though it invalidates the parrallel axis theorem for inertia. This basically says that the inertia of the car driving round a circle is the sum of the inertia of the car rotating about it's centrepoint and a constant which depends on the radius of the corner squared and the mass of the vehicle.

Guess I should think more before I type.

Amending my theory to take this into account then the effects on the car round large corners will be the same as the effects round tight corners but less pronounced.

So in summary, weight moved to the centre line of the car gives:

Quicker roll so better initial steering response, especially for small steering inputs.

Increased polar moment of inertia so car may be reluctant to turn initially and then want to spin. Especially round tight corners.

Slower weight transfer from front to rear. Causing possible understeer and scrubbing coming off power, less traction under acceleration. Also gives improved stability over bumps.
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Old 05-24-2002, 11:29 AM   #126
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Hey dw,

I wondered if you meant to say that, given your conclusion of oversteer on exit. .

Good to chat with you,

have a good one!

cya,
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Old 05-24-2002, 06:28 PM   #127
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dw, mcrisp- you guys are awesome. Now I see more to what I was picturing- thanks for the answers.

mcrisp, I really hate to ask you this question but why didn't you talk about overdrive and underdrive in your setup guide. I know not all touring cars offer the option(TC3, XRAY) but it's still a parameter with some(XXXS, Pro2, V1R).
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Old 05-24-2002, 09:41 PM   #128
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PW- I would guess becuase he runs a 414 he hasn't had much experience with under or over drives, but that's just my guess.

One thing I've been wondering myself is camber link length and relative connecting points in relation to camber change during chassis roll and the effects to be expected, assuming roll center stays the same. So far I haven't seen anything about it in the book.

Martin or DW- would you care to give your thoughts on this subject?

Thanks guys- THIS is the kind of information and discussion that stimulates my mind. I can't think of another discussion that I look more forward to reading.
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Old 05-24-2002, 09:52 PM   #129
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Thanks for so much insight into car handling guys.

What is overdrive and underdrive?

I race TC3.

Thank you in advance.
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Old 05-24-2002, 10:10 PM   #130
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rcruv, over/underdrive is where one end of the car is turning faster than the other. Excuse me, tc3's can be over/underdriving but only with tires. Smaller tires on the rear will make your car underdriven where smaller tires on the front will overdrive it. Underdrive will give your car more steering when entering a turn with less exiting. If overdriven, your car will have less off-power steering when entering a turn but more steering when exiting a turn.
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Old 05-25-2002, 12:23 AM   #131
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Proudwinner: (Over/Under drive)

Glad you like what dw and I have to say. Good point about the over/under drive. I have had the opportunity to try it when I had a XXXs, but after trying it I went back to no under/over driver. I personally found it did not make for a smooth driving car. But to be fair, I did not give it a lot of time. I remember the first time I tried a front one-way, I did not like it either. Perhaps I will add it in a future version of the book.

BigDogRacing: (Length of upper link)

As discussed in my book the more the upper link angles down the higher the roll center (RC) and thus the less the car rolls over in the corner. One thing I did not discuss is how the length of the upper link affects things. A lot of drivers I talk to understand that the shorter the upper link, the more the camber changes, but there is more to it than that.

There really are a lot of possible scenarios here, but as a general rule, the longer the upper link the more deeply the cars rolls or leans. This is because the RC, which tends to get lower (depending on the upper link angle and length) as the car starts to lean (or roll) into the corner, will go through a bigger change with the longer upper link than with a shorter upper link. In other words the longer upper link will have a lower RC than the shorter upper link at a given degrees of roll. This lower RC will produce even more roll and thus the longer link will actually roll further, or have a "deeper roll".

For example, if you have short upper link that is parallel to the lower A arm, the car starts with an already low RC and thus the car initially will roll easily, however with the short upper link the car will not lean (or roll) as much as with a long upper link.

As I said above there are a lot of possible scenarios. For example as the angle of the upper link gets steeper then the RC tends to not go lower during cornering. If you make the upper link angle even more steep, then the RC will actually get higher reduce the roll.

So how do we apply this....? If you want a little more steering at the middle of the corner, you can increase the length of the front upper link so the front of the car leans a little more mid corner.

Of course like any chassis adjustment there are exceptions to this. For example if you had too much camber change as a result of the shorter link you might actually loose traction at mid corner. There is a lot more to this, but hopefully this gives you some things to try out.

Whew!!

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Old 05-25-2002, 07:35 AM   #132
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I can see your already very busy in here, but I have a quick query.

My pro3 gradually acquires more and more understeer as the grip of the track comes up, mainly midcorner and on exit.

I'll end up stiffening the rear by at least one spring grade as the night progresses.

Basically it's a hassle, and means I can only get a run with the car "dialled" a couple of times a night, when the grip matches my suspension settings.

Is this a weight distribution issue, or geometry (I'm thinking it may be too much weight at the rear, because team drivers seem to move things forward in the car)

Any help appreciated.
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Old 05-25-2002, 07:50 AM   #133
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Just one thing I would like to add to what mcrisp has said on roll centres / camber change. Most touring cars have very limited suspension droop. When the inside suspension hits the droop limiter mid corner it doesn't matter where you have the linkage set. The roll centre will always be the centre of the contact patch of the inside tyre. At this point you are trying purely to optimise camber angle.


Never played with over or underdrive but I think it's primary purpose is to alter the amount of time a 1 way engages. As with anything you need to try it.

Below is what I have deduced myself so it is possible I have missed something or there may be slight errors but.....

Unless you have the rear of the car well out of shape (not a quick way round the track) the front wheels always travel further than the rear wheels. (Ride a bike through a puddle then look at the tracks after turning slowly if you don't believe me.)
With a fixed transmission one end of the car must break traction as the gearing means both sets of wheels must travel at the same relative speeds. This could result in over or under steer depending on the rest of the car set up.
With no one way fitted you can use larger front tyres or overdrive so the front wheels rotate more relative to the rear. This will compensate for the difference in distance travelled by the two ends of the car so efficiency is improved as is corner speed. It will only work perfectly for one radius of corner. On the straight one set of wheels has to break traction.

A 1 way allows the front wheels to rotate faster than the rears during a corner which is what the car wants to do as mentioned above.
There is a myth that by using different front and rear ratios with a 1way so the car is 2wd you will get improved efficiency down the straight. If both ends of the car are geared the same then everything rotates at the same rate. Even with the 1 way in action all the drive components are still rotating. If the 1 way is coming into effect it means that you have had to make the components at the front end of the car rotate faster than the rear ones (by driving them through the tyres). As the rear of the transmission is determining the vehicles speed then that can't slow down so you require more energy.

The reason over / underdrive can be beneficial with a 1 way is that when you accelerate the rear wheels have to start to slip before the front drive kicks in. If they slip too much they won't be producing optimum traction. Using bigger front tyres or overdrive so the front wheels are driven faster reduces the amount of time the 1 way is engaged. This makes the car pull harder out of the corner. It may have to break traction on corner entry though as the 1 way is not disengaged quickly. There will also be a drag penalty on the straight as one end of the car must break traction.

I can't see any reason why you would want to use small front tyres or underdrive at the front of the car. This would just make the car into a 2wd with a severe weight penalty for most of the time in my view as the 1 way would rarely engage.
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Old 05-25-2002, 08:12 AM   #134
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sosidge

Moving the weight forward gives reduced turn in and understeer in slow corners. It increases on power steering though but may result in a loss of drive out of the corner.

Moving the weight back makes the car more responsive. It may spin on corner entry or step out mid corner. It will steer less on power.

Didn't you say in a previous post that you couldn't move the cells forward?

What did you try out of my previous suggestions?
Is the car improved?

Difficult to say what the problem is without far more info.
Are you towards the harder end of the spring range?

I remember when I had my sst98 that the chassis was so flexible that with hard springs changing harder still would sometimes have the opposite effect to that expected. The pro 3 is quite a flexible car from what I've heard.
Maybe softer springs all round will give more expected results.
It shouldn't hurt on a polished floor anyway.

What I do remember about running on polished floors is that getting a setup was an absolute nightmare.

Less rear toe in may help.
Changing caster may help too.

These may take too long to change between rounds at a club meeting though if your car is working at the start of the day.
As I said before throttle control is critical on polished floors. You may find that you need to blip the throttle so that you can get the car sliding as you want it in order to counter the understeer, expecially if it is only a problem for a coupl of corners.
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Old 05-25-2002, 09:49 AM   #135
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It's not really the setup that's the problem, it's just consistency through the night.

Last time out for example, I started out with the car running well, but by the end had to go softer on the front springs to keep the balance.

Basically, as the grip comes up during the meeting, the car starts to understeer.

I'm just wondering if there's anything I could do with the setup, be it geometry or weight distribution, that would make the cars balance more consistent regardless of the level of grip.

I run pretty hard settings for that responsive feel.
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