Main hoop Bracing support configuration

[carlzxcv]

hello,

I am aadil, a member of frame design for Atraiu racing, Hindustan University,India. We are planning to take part in FSAE Italy 2016.

As of now our main hoop, front hoop, front bulkhead and front suspension points are decided, but while trying to keep the rear suspension points at desired location, a confusion of whether the main hoop bracing supports are legal or not arose.

Please find the chosen configuration's by our team in form of a .png attachment and please advice us on whether the bracing supports are legal or not. In btw number 1 is my personal favorite :P

The rules state that:
1) The Main Hoop Braces must be securely integrated into the Frame and be capable of transmitting all loads from the Main Hoop into the Major Structure of the Frame without failing

T3.13.7 The lower end of the Main Hoop Braces must be supported back to the Main Hoop by a minimum of two Frame Members on each side of the vehicle; an upper member and a lower member in a properly triangulated configuration.

a. The upper support member must attach to the node where the upper Side Impact Member attaches to the Main Hoop.

b. The lower support member must attach to the node where the lower Side Impact Member attaches to the Main Hoop.

NOTE: Each of the above members can be multiple or bent tubes provided the requirements of T3.5.5 are met.

Thanks alot,
aadil

[Ahmad Rezq]

I'am Not familiar with the Chassis Rules.
Regarding the two members to support the bracing lower node back to the main hoop.
Why not to consider the first member the red one and the other members the members in the green ?
I don't know what happened to your suspension points, but it would be better to show them in the sketch

[Bemo]

1 & 2 should be legal (if the tubes are of the appropriate dimensions). 1 is the standard solution to meet that rule. 2 is basically the same if you make the lowest tube attached to the main hoop of the appropriate dimension. That tube together with the red one which is going to the main hoop bracing attachment make basically the same load path as in option 1.

Option 3 is not rules compliant IMO. There is just no frame member going from the bracing attachment point to the lower part of the main hoop.

[carlzxcv]

1 & 2 should be legal (if the tubes are of the appropriate dimensions). 1 is the standard solution to meet that rule. 2 is basically the same if you make the lowest tube attached to the main hoop of the appropriate dimension. That tube together with the red one which is going to the main hoop bracing attachment make basically the same load path as in option 1.

Option 3 is not rules compliant IMO. There is just no frame member going from the bracing attachment point to the lower part of the main hoop.



Thank you for your insight, our suspension team selected number 1 configuration too as the design is allowing our chosen suspension pick up points.

This configuration is not the best as it lacks stiffness in torsion, but in simulation we did get 8000 N/Deg, excluding the hub,brackets and other compliance's. we estimate our hub to hub stiffness to be around 2000 N/deg. In between we are using a fairly low powered engine (around 25-30 bhp after restriction) so we are not expecting lateral acceleration to cross 1.5G for a given FSAE track.

[MCoach]

Why should your low power output have any effect on your lateral acceleration?

[carlzxcv]

Why should your low power output have any effect on your lateral acceleration?

lateral acceleration is dependent on both speed and the corner radius. The formula being (Lateral acceleration= Speed2(mph)/15*Radius(Ft)).

Now according to FSAE rule book,

1) The max straight section is of 60m, with hairpins of corner radius 4.5m at both ends.

so considering a corner entry speed of 30 kmph after braking, the lateral acceleration translates to around 1.54G.

2) The constant turns are mentioned to be between 11m and 22m radius

considering corner entry speed for 22m radius as ~65kmph, the lateral acceleration comes around to 1.5G, but the car will enter corner with brakes lightly on, further reducing speed and thus lateral acceleration.

3) low powered engine means you engine takes more time to build RPM's coupled with a heavy car (280kgs with driver, power to weight ratio being ~90 bhp/ton)

I know this is not a place for excuse, but as a team struggling to clear technical inspections in past International events, our whole concentration is to make a rule-legal, respectively stiff car, which will be a test bench for the team for upcoming competitions.

[Thibault HUGUET]

1) The max straight section is of 60m, with hairpins of corner radius 4.5m at both ends.

so considering a corner entry speed of 30 kmph after braking, the lateral acceleration translates to around 1.54G.

.

Why will you not be able to have a corner speed of 32 kph in the hairpin (1.78G) ?
I hope your engine can move your car at 32 kph before 60m of straight line. ^^

[carlzxcv]

Why will you not be able to have a corner speed of 32 kph in the hairpin (1.78G) ?
I hope your engine can move your car at 32 kph before 60m of straight line. ^^

well sir my respects to you, if you can negotiate a 4m radius hairpin at 32kmph throughout, but my technique of driving involves coming up to a corner at whatever maximum speed my car allows, brake very late and continue braking lightly while negotiating the corner. This effectively slows down the car at mid corner but allows me to have a very clean fast exit, whereas allowing me time to set the car for next corner.

so the 30kmph speed is not due to cars limited accelaration capabilities, but the real time speed which the driver will corner(a very inexperienced driver, with an average car set up)

[Thibault HUGUET]

My question was why do you chose 30 as corner minimal speed and not 32 for instance (at the same point, using the same driving technique)?
This small speed difference gives a big difference in LatG and definitely, engine power doesn't come in the story... But setup, car's design and driver, yes !

[carlzxcv]

My question was why do you chose 30 as corner minimal speed and not 32 for instance (at the same point, using the same driving technique)?
This small speed difference gives a big difference in LatG and definitely, engine power doesn't come in the story... But setup, car's design and driver, yes !

Fair point, okay lets consider 1.5G cornering effect on car:

1) Roll gradient is set at 1deg/G, so 1.5 degrees of roll.(set up is such that, camber change in roll is around 0.2 deg)
2) Lateral load transfer of 80%

considering 2G cornering force (tires will cave in before the car hits 1.8G as we do not have an aero package):

1) 2 deg of roll (camber change of around 0.4 deg)
2) Lateral load transfer of 90%

I don't see a situation where my design can go critically wrong. 1.5G was chosen as a threshold because the inner wheel will be lightly loaded, causing a large difference in slip angles of left and right wheels. But after the car is built, the team is going to experiment with toe-out, ackerman percentages etc, to find a solution which lets us utilize our tires better.

[Z]

Carlzxcv,

Your grasp of the simplest racecar Mechanics is clearly lacking.

It is fairly well accepted in FS/FSAE that a Team choosing a small engine can more than compensate for the lack of power by having very high lateral acceleration. It is an approach that has worked well for many (Globally!) winning Teams.

But your lack of Mechanical commonsense is quite common.
~o0o~

What worries me more is this:


Option 3 is not rules compliant IMO. There is just no frame member going from the bracing attachment point to the lower part of the main hoop.

Bemo,

In Option-3 there very obviously are "Frame Members*" that provide a "properly triangulated" load path from the "lower end of the Main Hoop Braces" down to the "node where the lower Side Impact Member attaches to the Main Hoop".

(* "NOTE: Each of the above members can be multiple or bent tubes provided the requirements of T3.5.5 are met.")

Not obvious???

Z

[carlzxcv]

Carlzxcv,

Your grasp of the simplest racecar Mechanics is clearly lacking.

It is fairly well accepted in FS/FSAE that a Team choosing a small-engine can more than compensate for the lack of power by having very high lateral acceleration. It is an approach that has worked well for many (Globally!) winning Teams.

But your lack of Mechanical commonsense is quite common.
~o0o~

Z

hello Mr. Z,

1)Agreed my grasp of race car dynamics and mechanics leave a lot to desire, may b that's why i am trying to work hard to get an average set-up car on its wheels so that i can go round and round in circle's and understand the so called mechanics better :).

2) The teams which you mentioned who have won competitions with small engines, according to my research have a power to weight ratio of 130-150 bhp/ton, whereas i only got 90 bhp/ton.

3) we do not have an aero package, and according to tire data given in avon tires website, The tires can only pull 2G's when they have a load of 150kg's on them (which will, for our car mean to ride on two wheels up in air, so that outer rear wheel will have 150 kgs load on them)

4) so can you please direct me as to where i am wrong in assuming 1.5G average cornering force for our car, but the design still having enough left to take on 2G's (provided the tires stick)

[Z]

Carlzxcv,

You are digging yourself a deeper hole.

* "Bhp/ton" has NO influence on lateral acceleration capability.

* If Avon say their tyres "can only pull 2G's when they have a load of 150kg's on them", then how many "Gs" do you think those tyres will pull with a load of only 100 kg on them? Or 50 kg? Or 200 kg?

* If you design your car with an assumed working load of 1.5G but "still having enough left to take on 2G's", then you can expect many failures. Well, assuming you get said car built in the first place.

Z

[carlzxcv]

Carlzxcv,

You are digging yourself a deeper hole.

* "Bhp/ton" has NO influence on lateral acceleration capability.

* If Avon say their tyres "can only pull 2G's when they have a load of 150kg's on them", then how many "Gs" do you think those tyres will pull with a load of only 100 kg on them? Or 50 kg? Or 200 kg?

* If you design your car with an assumed working load of 1.5G but "still having enough left to take on 2G's", then you can expect many failures. Well, assuming you get said car built in the first place.

Z

Mr Z,
If digging a deeper hole results in me understanding some concepts better or get a new view point, I’ll gladly dig a hole every day.

1)The vehicle with more Bhp/ton will have a higher corner entry speed, provided the driver utilizes the car’s potential. Let’s compare two cars:

a) A 250 kg car equipped with Cbr 600rr engine, churning out 60 horses.
b) A 210 kg car having KTM 390 engine, giving a power output of 30 horses.

Which car do you think will have a higher cornering speed and thus greater lateral acceleration?

2)I do not have data for 50Kg’s but according to Avon, a 75kg vertical load can generate maximum force of 1250 N, a 150kg load will give a cornering force of 2300N (my apologies for mistaking a conversion and telling you it can pull 2G’s at 150 Kg's), 225Kg (2930N), 300Kg (3830N).

3)I never said I designed my car keeping a working load of 1.5G, I said I designed a car capable of taking a lateral load of 2G, but assume that the car would not reach that high value of Lateral acceleration. What makes you think that the car would not be built? (shall we wager :))

[Adam Farabaugh]

You're thinking about the cornering problem ack-basswards.

Imagine a Bugatti W16 in a geo metro (extremely large power, not great handling). Say the tires on the metro can let you ballpark handle 0.5G lateral. Would you go into a 10m radius turn at 120 MPH (~50 m/s) just because you have a giant engine? (Hint: A = V^2/r = 250 m/s^2 ~= 25G!)

Don't go farther without understanding this point...

[carlzxcv]

You're thinking about the cornering problem ack-basswards.

Imagine a Bugatti W16 in a geo metro (extremely large power, not great handling). Say the tires on the metro can let you ballpark handle 0.5G lateral. Would you go into a 10m radius turn at 120 MPH (~50 m/s) just because you have a giant engine? (Hint: A = V^2/r = 250 m/s^2 ~= 25G!)

Don't go farther without understanding this point...

Hello Mr. Smarty pants,

You are my hero as you provided me with the perfect opportunity to present my point.

I completely agree with you on your example, the car will be an abomination to drive, let alone race with.

Now consider a BMW M3, arguably one of the best handling cars of the world. Present it to a team of best engineer's from all around the world, and then fit it with a 100cc motorbike engine.

will it corner at the G's it is designed for or will it crawl across the track?

Do not comment a single word further, without understanding this point.

[Jay Lawrence]

carlzxcv,

You're being a bit of a jerk to people who are actually trying to help you.
In your example, there is absolutely no reason why the M3 wouldn't be able to pull the same lateral G's as it was designed for. In fact it may pull more (I imagine a 100cc engine will have less mass than the S55).

What you need to be concerned with is whether or not your '90hp/ton' is enough to accelerate your vehicle to about 50km/h and keep it near there on a FSAE track. Hint: it is.

[carlzxcv]

hello Jay,

If a M3 with a 100cc engine can pull more lateral G's than it compared to its stock engine, Then I think you are right about me being a jerk and a jackass for not understanding one bit about vehicle dynamics. I guess time for me to give up chasing motor sporting and find a dumb desk job.

But as i am a jerk, one last pitch to prove my point:

I took a design spec sheet from google, and started calculating lateral weight transfer. Lets call this car A and our car as car B.

Car A:
Engine: Yamaha R6 (60Bhp)
wheel base: 5.5 Ft
wheel track(front): 4.08Ft
C.G. Height: 1Ft
Roll center to C.G.: 0.78 Ft
front roll center height: 0.22 Ft
rear roll center height: 0.24 Ft
Weight at front: 316lb
weight at rear: 386lb
Roll gradient: 4.6deg/G

Calculated Total lateral load transfer at 1.8 G: 90%

Car B:

Engine: KTM 390 (30Bhp)
wheel base: 5 Ft
wheel track(front): 4.265Ft
C.G. Height: 0.833Ft
Roll center to C.G.: 0.916 Ft
front roll center height: -0.083 Ft
rear roll center height: -0.083 Ft
Weight at front: 246lb
weight at rear: 370lb
Roll gradient: 1deg/G

Calculated Total lateral load transfer: 80%


Now consider a hairpin bend of radius 4m at the end of a 60m strech, Can you please tell which car will generate more cornering force.

[Pat Clarke]

Aadil,

When you are deep in the sh*t, the very first thing you need to do is STOP DIGGING.
You are adding nothing to your credibility with sarcasm.
It's time to stop squawking and start listening!

And as I have told you twice already, behave yourself!

Pat Clarke

[carlzxcv]

Aadil,

When you are deep in the sh*t, the very first thing you need to do is STOP DIGGING.
You are adding nothing to your credibility with sarcasm.
It's time to stop squawking and start listening!

And as I have told you twice already, behave yourself!

Pat Clarke

okay sir,

Sorry you all are correct and i was wrong to put forth my point, without any concrete proof.

Thank you for your time, no offense meant :)

[Pat Clarke]

Aadil,

You just can't help yourself.

No, you were not wrong to put forth your point. That is the intent of a forum.
What has ruined your presentation has been your sarcastic insistance that you know it all, when as Z pointed out a few posts ago, "Your grasp of the simplest racecar Mechanics is clearly lacking".
You need to accept that as fact and start listening to the help people are trying to give you and kill the smartass attitude!

Pat Clarke

PS,

An old song, especially for you... https://www.youtube.com/watch?v=bqIzAjqtkF0

[carlzxcv]

Aadil,

You just can't help yourself.

No, you were not wrong to put forth your point. That is the intent of a forum.
What has ruined your presentation has been your sarcastic insistance that you know it all, when as Z pointed out a few posts ago, "Your grasp of the simplest racecar Mechanics is clearly lacking".
You need to accept that as fact and start listening to the help people are trying to give you and kill the smartass attitude!

Pat Clarke

PS,

An old song, especially for you... https://www.youtube.com/watch?v=bqIzAjqtkF0

aaahhhhh Beatles, nice selection :P

I always grew up with an attitude where i wont take a no for an answer, or change my decision, just because some one said so.

All i wanted was to get a proper answer, which showed the fault in my ways, i presented my points (in a aggressive, smart a** kind of way). Though wrong (obviously, i am no top dog), i did deserve a small explanation at least.

Not going to make a bigger fool of myself, again. So thank you gentlemen for your patience and time. I'll concentrate on making a rule legal car and let vehicle dynamics be saved for another day :)

this will be a better suited song for me:
https://www.youtube.com/watch?v=7S94ohyErSw :P

[Dylan Edmiston]

Instead of thinking about an autocross style course, think of a skid pad. As long as you have enough power to keep the car moving, how will power affect your performance in a skid pad? Same weight, same tires, same suspension, etc... Just different power output. All that will differ between the two setups is the percent of maximum power being used, but each setup will require the same amount to keep the car moving. The only way more power will help is if the radius of the turn is large enough such that your low power engine can not get you up to speed (not really an issue on FSAE courses).

[carlzxcv]

Instead of thinking about an autocross style course, think of a skid pad. As long as you have enough power to keep the car moving, how will power affect your performance in a skid pad? Same weight, same tires, same suspension, etc... Just different power output. All that will differ between the two setups is the percent of maximum power being used, but each setup will require the same amount to keep the car moving. The only way more power will help is if the radius of the turn is large enough such that your low power engine can not get you up to speed (not really an issue on FSAE courses).

thank you :), the skid-pad case is really interesting, does the same apply to autocross and endurance too?

A Little bit off-topic but won't the single's need more time to reach the same power output when compared to an I-4(assuming same car and set-up), wont this hurt timings and consequently points?

[CWA]

does the same apply to autocross and endurance too?

That is a question you should be able to answer yourself chap. Have a stab at getting a lapsim-type-tool together and make some predictions. Or get your hands on OptimumLap somehow, it was free to get hold of last time I tried. A process like this should be what is driving your engineering decisions.

[carlzxcv]

That is a question you should be able to answer yourself chap. Have a stab at getting a lapsim-type-tool together and make some predictions. Or get your hands on OptimumLap somehow, it was free to get hold of last time I tried. A process like this should be what is driving your engineering decisions.

will do, thanks for the help :)

[MCoach]

thank you :), the skid-pad case is really interesting, does the same apply to autocross and endurance too?

A Little bit off-topic but won't the single's need more time to reach the same power output when compared to an I-4(assuming same car and set-up), wont this hurt timings and consequently points?

I don't think the string of victories around the world of singles trumping I-4s has consequently hurt points. OH! BURN!

You may be considering one point, on corner entry, what in your mind seems to be Spa Francorchamps high speed track where the cars are power limited before entry, in fact it's more likely that average corner entry speed is in the 15 - 35 mph range. The corners dealt with in FSAE also will turn your thinking on it's head. Lateral acceleration is awesome, but the turns happen so quickly it's more of a yaw and stability limited event for autocross and endurance to a point. Steady state cornering is nothing but a fleeting dream. The faster you can get through the transition stages of into and out of a corner, the less steady state matters. Dylan is spot on above.

I hope that the terms yaw, yaw moment, and stability are not new to you. If they are, please read up, they'll be very useful.

As a counter point to your bhp/ton having an influence on cornering acceleration. I would like to propose that power output has no effect on potential cornering ability of the vehicle. However, the mass change that you are taking into account when comparing vehicles is most of the difference.


Now consider a hairpin bend of radius 4m at the end of a 60m strech, Can you please tell which car will generate more cornering force.
No one other than the FEA guys care about cornering FORCE because it is not a normalized value. A semi truck navigating that corner will generate a whole lot more FORCE than that puny little M3!
However, who gets around it faster? Why? Is it because the M3 generates more acceleration? OF course!

Now. I have this really cool thing I built. It's called a drift trike. It's a lot of fun. Here's the catch: it has 3 wheels and a 5hp motor, yet it will out handle a car on small radius corners. Can you think of why this is possible? I'm interested to hear your conclusions. Don't be shy. :)

Everyone is here to help each other. Take advantage of what they have learned, but please don't insult them.
And with that, I leave you with a gift of an on car video demonstrating what I mean about fleeting steady state from two of the best drivers I've ever known from FSAE. Also, it's powered by a single. ;)

https://www.youtube.com/watch?v=MQwQsAhAjVY

[CWA]

Some real good advice from lots of experienced people here. I know the horse has been flogged quite enough, but it is not quite dead, so further to these comments and my own previous comment, I would like to add an exercise I recommend to understand the real importance of horsepower during an event:

Pick the peak horsepower levels of two engines you are considering, a higher and a lower. Instead of plotting the usual engine torque vs rpm graph, you should make a similar graph for the full vehicle. If power = torque x RPM for an engine, power = 'tractive force' x 'vehicle speed' for a car. Plot out the 'tractive force' that would be created by a car with each chosen engine at a range of speeds and compare curves. What you have here is the actual tractive force that will reach the tyres at all vehicle speeds with an engine of chosen power level fitted, but with a gearbox of infinite ratios allowing peak power level to be maintained at all vehicle speeds. Real life will not be too different, at least not different enough to negate the usefulness of the exercise, go with it for now.

Check your units and format your graph; have tractive force plotted against vehicle speeds up to say 50mph. Seems reasonable as I'm sure you won't be exceeding 50mph for much of an event.

Now pay attention to those force values. If in Excel, write an IF function capping tractive force outputs to below say 3000N. After all, let's assume each car has the same tyre and can only output the same amount of tractive force (we have assumed each car is a point mass (no weight transfer), is of equal mass (1500N) with a tyre mu of 2). You should be able to see the speeds up to which the engine with more power has NO advantage over the other; at these speeds BOTH cars are traction limited. This is a very useful first step, and should visually depict for you in quantifiable amounts what everyone above is trying to explain to you.

Another key thing to picture here is that you have so far only compared tractive force outputs against the grip capacity of a set of tyres during straight line acceleration. If your tyre has to give out some of that grip force as cornering force because the car is still exiting a corner, the threshold speed below which both cars are effectively equal will increase again. Try and incorporate a friction circle and a generic FSAE-event-corner into the above plots to see this if you can't picture it from my words.

So you can see the speeds above which the engine with more power is giving you an advantage. And you can roughly quantify this advantage; at a chosen speed, engine 1 can output X newtons, engine 2 can only output Y newtons. Now the next step is to quantify how often you will be in those upper speed ranges during an event, and to then make the whole-lap-performance comparison between the benefit of the higher-power output engine at these higher speeds, and the benefit of the lighter lower-power engine in every single corner you get.

Which is the best engine option? We'll leave that to you to work out, but I dare say you'll find a big clue in MCoach's video.

A good lap-sim should bring you to the same conclusion. What I've described above are effectively the first-steps of the generation of a simple lap-sim. And you can be sure that other teams have used this approach to arrive at the decisions they made; don't forget how many successful teams now turn up at events with singles.