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.