Chassis flex

Old 02-28-2013, 08:42 AM
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Originally Posted by YZFAndy View Post
The only instance I know where chassis flex is desirable is in motorbikes......
Racing karts.
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Old 02-28-2013, 01:30 PM
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Originally Posted by Johnny Wishbone View Post
Racing karts.
Sprint cars too.
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Old 02-28-2013, 02:40 PM
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Originally Posted by Johnny Wishbone View Post
Racing karts.
Because they have no suspension
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Old 02-28-2013, 02:44 PM
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Originally Posted by TryHard View Post
Sprint cars too.
Because they have no suspension.
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Old 02-28-2013, 03:20 PM
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Why have chassis flex at all, isnt that just trying to do what the suspension is there for ??
Dont know, just asking.

Xray T3R :FRP chassis, very very flexy. Yet no-one swapped out their Carbon T3 chassis for the T3R frp chassis. Why is that if a flexy chassis is better on low grip tracks ?
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Old 02-28-2013, 03:29 PM
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Originally Posted by YZFAndy View Post
Because they have no suspension.
Sprint cars have suspension, I guarantee it, shifter kart not so much.
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Old 02-28-2013, 03:58 PM
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Originally Posted by 1101 View Post
Why have chassis flex at all, isnt that just trying to do what the suspension is there for ??
Dont know, just asking.

Xray T3R :FRP chassis, very very flexy. Yet no-one swapped out their Carbon T3 chassis for the T3R frp chassis. Why is that if a flexy chassis is better on low grip tracks ?
A chassis is basically a spring without a shock absorber which you have to trust to attach the relative roll tendencies of your front and rear suspension. It is basically a torsion blade, twisting with every weight transfer. To make it even more complex, the chassis twist is not linear like a coil spring, it's an exponentially increasing spring rate.

I understand it as when you're on low traction (rubber tires, bad grip) a soft chassis will allow the front and rear relative amount of roll to increase without needing a corresponding increase in cornering forces. This will allow the front end to roll and dive (the front wheel spring rate of most touring cars I see is much softer than the rear due to where the shocks are mounted) without torquing the back end over with it. This slows the response, and as a result is less likely to overpower what little traction you have.

This is an educated guess based on what I have seen and learned. I may be completely wrong
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Old 02-28-2013, 04:04 PM
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Originally Posted by DesertRat View Post
Sprint cars have suspension, I guarantee it, shifter kart not so much.
I thought they had a solid front axle and a floating beam suspension on the rear(might as well not have any)! Don't really have them here in the uk so not 100% sure on them. An oval racer is not the best example of needing dynamic suspension. Oval recers tend to have a different setup on either side of the car as well.

But out of the 100's of forms of motor racing and explanations of why flex is traditionally thought of as undesirable i had already bet money that someone was going to spend their time trying to find rare examples of racing vehicles that use do use flex just to prove what i said was wrong.

What i was saying is flex tends to be used to compensate for suspension or geometry deficiencies but ultimately if you can control the forces accurately it is better. If not 90%+ of the motor racing world has got it wrong and a toy manufacture has got it right.
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Old 02-28-2013, 04:22 PM
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Originally Posted by YZFAndy View Post
I thought they had a solid front axle and a floating beam suspension on the rear(might as well not have any)! Don't really have them here in the uk so not 100% sure on them. An oval racer is not the best example of needing dynamic suspension. Oval recers tend to have a different setup on either side of the car as well.

But out of the 100's of forms of motor racing and explanations of why flex is traditionally thought of as undesirable i had already bet money that someone was going to spend their time trying to find rare examples of racing vehicles that use do use flex just to prove what i said was wrong.

What i was saying is flex tends to be used to compensate for suspension or geometry deficiencies but ultimately if you can control the forces accurately it is better. If not 90%+ of the motor racing world has got it wrong and a toy manufacture has got it right.
Ah, i get it, sprint cars do run solid axles but they are sprung and full floating, and they can have a LOT of suspension travel and are absolutely necessary to even completing laps, let alone going fast. Dirt oval racing is not easy in suspension setup, and is probably the best example of needing dynamic suspension and chassis tuning, as the asymmetric car setup demands it. They may go faster with four wheel independent suspension, but rules are rules.

Speaking of rules, the only time chassis flex is used as compensation is when rules dictate what the rest of your car can do, like NASCAR years ago when they raced on the bump-stops due to a minimum stationary rideheight requirement. Chassis flex is not undesireable and you will never rid of it all, you just need to make it work with you instead of against you. A good starting point I learned for full-size racing is to make the chassis 10x stiffer in torsion than your suspension, as for how this translates to toy cars i have no idea.
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Old 02-28-2013, 04:45 PM
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Both sprint cars and shifter karts use torsion bars. My shifter kart has 3 removable and adjustable torsion bars, as part of its setup options.
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Old 02-28-2013, 06:20 PM
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isn't that (flex chassis) just for the in-perfect setup problem? more tolerance when something wrong happens.
*even world level drivers still have max turning angle with massive understeer in the apex, most of the time. In real racing car, not even fernando alonso do that extreme.
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Old 02-28-2013, 06:57 PM
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Originally Posted by slotracer577 View Post
Softer means less weight transfer,thus more grip. Traction is normal force x coefficient of friction. Problem is tires are only linear within a certain range do normal forces. Beyond a certain point they make no more grip. So once you get to that point more weight transfer removes grip. A lot of people confuse roll with weight transfer. Then are not the same an are almost opposite.
Yes, agreed.
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Old 02-28-2013, 07:10 PM
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I cannot find a graph of normal load vs. lateral force, but generally speaking with increased load on the outside tires you will find the gains are less than what you loose with the inside tires. Therefore your net lateral force will decrease with weight transfer (which is tied very closely to roll stiffness, increased roll stiffness generally increases weight transfer.)

Now, there is a compromise to be made between having lateral acceleration and having yaw acceleration. If the car can't yaw, the car will be slow in transient conditions. In RC racing you are almost ALWAYS in a transient condition, hence the chassis are generally stiff. I personally find it odd when manufacturers talk about how they have "chassis flex". IT is not necessarily a good thing or bad thing. Everything is all one compromise between lateral acceleration and yaw acceleration.


Here is a yaw moment diagram that can illustrate the relationship. The name of the game is extending the boundary lines. A good racing driver can be very close to the boundary lines all the time.
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Old 02-28-2013, 07:40 PM
  #29  
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I guess if you wanted to have a scale, perhaps you could think of all the things that take load transfer in your car (from soft to stiff):

tires
dampers/springs
suspension arm
chassis

Of course different setups can change this order (use some really stiff springs and the chassis will twist before the shocks will react), but I think this is the natural order of things and if you change it you can confuse yourself easy and start trying to chase your tail with setup. Like for instance using the wrong tire - get a hard one and you'll be in a lot of trouble. Sure, there will be pros who know what they're doing, but I wouldn't necessarily try to emulate their setups given that I don't have the same skill set to make the most of that setup. I think this is a trap for a lot of quick fix seekers, people who want to copy someone else's setup without necessarily having the same skill level.

Personally I prefer to keep the order of softness (or stiffness if you will) as above.

I guess you need to use all of these given that they all react one way or another to weight transfer.

American language differs somewhat from other english speaking countries, hence the confusion for us when we read things like "a flexible chassis generates more grip". I don't think they mean to say it creates more grip, because you can not create grip. I think what they mean is it makes better use of the grip available. Whether or not that is true, is perhaps up for debate.

I try to regard my TC cars as rigid and rely on suspension to fine tune handling. Testing would probably reveal the cars are not that rigid, but if you use a soft enough suspension you won't need to use chassis flex (as long as your suspension can take the loads). I am not sure however where the crossover point is and loads start being transferred tot he chassis, regardless of how soft the suspension is (one point you can be sure of is when your suspension bottoms out - at that point, you can bet something else has to put up with load and perhaps flex, twist or do something else - not always a desirable situation).

But apart from that extreme, I would guess there is a more progressive transfer of strain between various components and no matter what they'll all be under some strain. Like I said, I try to keep it away form the chassis, because I do not know how the chassis reacts, so prefer not to rely on it. the result is that I have a softer suspension than almost everybody else at the track, but the car works and by the looks of it it is more progressive than the rest of them. Maybe not as fast, but I like it this way. Then again, my driving style is not "point and shoot".

Experimentally (feel by hand) you could try this. Place your car with two wheels on a table (front or rear) and grab the other end in your hand. Now try to compress the suspension on the wheels on the table by holding the car from the opposite end. Looking from the side, does the chassis bow before the suspension on the table reaches end of travel? If yes, there you go, your chassis takes some load to flex and transfers the rest to the suspension, tires, etc. If no, the chassis transfers everything on the suspension. Tune accordingly to achieve desired result.

Same exercise works with the wheels on one side.

But I think most important is cross-chassis flex (twist) - diagonal load transfer. This is more complicated to tune and simulate, and I prefer to not use it, because I believe it inevitably leads to chassis tweak.

I guess the moral of the story is do your own thing, and don't worry if everybody else is far from your setup as long as your car is on pace.

Last edited by niznai; 02-28-2013 at 07:53 PM.
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Old 03-01-2013, 01:12 AM
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EDIT - already covered above
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