Error No. 1
This picture shows a classic design error that all Judges hate, and is considered a \"Mortal Sin\". Every year several cars are presented like this as teams ignore the advice or directions they are given.
The outer spherical bearings are threaded rod ends loaded in bending! The entire mass of the car, plus bump loads, weight transfer and brake torque are reacted to the chassis by bending the threaded shank of the lower joint. This is going to break! GTB! Do not do this!
The upper rod end is being asked to react brake torque in bending. It is also being carried in single shear on top of the upright. These errors are not so serious, but still examples of poor design.
The judges understand why teams do this. It makes camber adjustment easy, but there are better solutions. Teams will argue they have selected a rod end with sufficient bending capacity, but this argument will not hold with the judges. A Rod end with a sufficiently strong shank will be far too big and heavy, and as the thread roots are good stress raisers, the joint will probably crack and break anyway. In any case, we are talking about the Design Competition, and incorrect use of fasteners is not good design.
-1
- 好好学习，天天向上
Some teams may build a set of adjustable suspension arms like this for testing. It enables them to determine optimum camber (and/or caster) angles, but when their testing is complete, they build a more appropriate set of components for the competition.
The inevitable consequence of bending rod end shanks is this!
Design Error Two
Okay, time for this months Design Error. I want you to look at these pictures and see how many design errors you can see. The assembly looks quite good on first view, but a closer look shows many errors.
-2
- 好好学习，天天向上
Okay, where do I start?
1. The rod ends are loaded in bending. (See last months column) 2. They are in single shear.
3. The upper outer hex bolts are threaded into the upright with no positive locking. 4. There is no washer under the hex bolt head to stop the rod end body coming over the
bolt head in the event of a failure.
5. The steering arm is mounted with hex head bolts threaded into the upright with no
positive locking.
6. The caliper mounting plate is sandwiched between the upright and the steering arm. 7. It appears that the brake torque is fed to the upright by having the upright recessed
into the caliper mount
8. The caliper is also retained by the front steering arm bolt.
9. The brake rotor is attached to the hub by use of countersunk hex head bolts with no
positive locking.
10. The assembly restricts access to these bolts so they cannot be checked for tightness. 11. The very short stub axle shows that the distance between the hub bearings is
insufficient.
There may be more (Stress raisers on the edge of the caliper plate recess?) Feel free to find more for me please.
So, what looked okay at first glance was actually a disaster looking for a place to happen. I can promise you that if I see anything like this in Hockenheim next year, I will personally kick the butt of whoever is responsible (Or delegate someone else to do it if the designer is bigger than me)
Remember, I am still available to answer design questions from teams on or
-3
- 好好学习，天天向上
Pat’s Design Errors No. 3
Many Formula teams are not aware that the biggest single force generated in the car is the front brake torque. Unless a team uses inboard front brakes (Unlikely) they have to understand how to react that brake torque into the primary chassis structure via the suspension components.
If you consider in simple terms what happens, it is easier to understand.
When the brakes are applied, the stationary caliper attempts to lock itself to the rotor. The caliper mounts and upright must be properly designed to accept this force repeatedly without failure. This is particularly true of machined aluminium uprights. Similarly, the rotor mount and wheel hub must be designed for the job.
-4
- 好好学习，天天向上
Fig 1. Distorted front upright.
It can be seen where the brake torque has overcome the integrity of this CNC’d upright. A simple diagonal web instead of the horizontal webs seen here might have avoided this failure. But look at the other picture before accepting that advice.
The load path from the upright to the chassis structure should direct and simple. As we discussed in DE1, feeding suspension and brake forces through threaded shafts in bending is not acceptable. Neither is chassis brackets that are flexible or poorly mounted. Judges have a good eye for this and will quickly appraise your design solution.
-5
- 好好学习，天天向上
Fig 2. Failed upright and brake spider.
In this case, not only has the upright failed catastrophically, but the brake rotor spider has also failed
If you have a composite chassis structure, be prepared to show the judges proof of the effectiveness of your design.
The problem at the rear of the car is usually not so critical as most cars feed the brake torque and the engine torque into the chassis inboard of the suspension, and if the brakes are mounted outboard, the torque generated is usually only about one third of the front wheel torque.
Design Error Four
Well, not really an error in design, rather an error in project management.
-6
- 好好学习，天天向上
This is the famous 2001 WWU 554cc V8. It used Kawasaki cylinder heads on a bespoke block with all internals and systems made by the team.
The engine drove through a carbon fibre cased gearbox housing Honda gears and selectors. The transmission was also designed and built by the students.
The engine made a mockery of the costing formula for engines and raised an enormous amount of interest at the event.
But it all fizzled out to naught. So much effort had gone into the power train that the chassis left much to be desired.
Then the engine could not be started at the event and so did not compete. Under the stresses placed on them by the project, the team had neglected to fit any form of starter! Probably the best example in FSAE history of a team totally forgetting the intent of the competition, still, they will always be remembered for their achievement!
This months Design Error FIVE
More teams are making their own wheels. Usually they make or buy rims and then design and make an aluminium centre.
This seems a pretty straightforward task, yet many teams obviously do not understand the loads applied to a wheel on a racecar.
The shear loads across the interface between hub face and wheel is sometimes passed through the root of a thread on the wheel studs. This is particularly common where teams use high tensile bolts or Allen screws as wheel studs.
The wheel should be centered on a spigot on the hub face. Some teams assume that using conical nuts will centre the wheel for them. Conical nuts will introduce stresses in a light alloy
-7
- 好好学习，天天向上
wheel centre that may well cause a total failure of the wheel. This is a very public way to have a DNF and might well cause someone to get hurt. People have been injured by wheels coming off cars in competition.
Wheels before and after failure
All the best suspension calculations are useless if wheel flex permits the to assume a positive camber attitude on the track under cornering loads. This picture shows the effect of wheel flex on camber.
So, until next month, good luck with your car and team building exercises.
This months Design Error six
Just to be different, this month I am going to talk about a piece of good design. This is not a Design Error, rather it is a very nice solution to hub and wheel mount design, although it may have one flaw.
The reason for posting this is that I have been very critical of some hub designs. Having thread roots across the shear face between the hub and wheel, no spigot to centre the wheel on the hub and using tapered wheel nuts seating in aluminium wheel centres are all evidence of poor design.
I don’t know where this hub design comes from, but I like it.
-8
- 好好学习，天天向上
As can be seen, the shear load at the hub face is taken through a small shank on the wheel stud. The wheel nuts are sleeve nuts designed to tighten against the steel hub face whilst applying a measured amount of clamping force to the wheels. The flat washer prevents the nuts galling the wheel centre (especially important with magnesium wheels).
The flaw? What about a centering spigot for the wheel? Well, I hope that is machined into the wheel and sits into the recess in the hub face. If the wheels are being centered by the wheel nuts, then this design is not so nice.
Another nice feature is that the tripod recess is machined right through the live hub. This allows the driveshaft length to be as long as possible, therefore reducing angularity losses, whilst at the same time accepting any driveshaft end plunge caused by suspension movement.
That is enough for this month. Hopefully teams will be back at work getting ready to build their new car. Good luck to all and I look forward to seeing you all in August.
This Months Design Error Seven
Obviously, my Design error of the month has to relate to a misunderstanding of one of Newton’s Laws, in this case the Third Law.
-9
- 好好学习，天天向上
Look where the bell-crank and spring loads are being reacted! Judges look at a design like this, they can see the load paths and they eliminate the car from the Design Finals!
Now take a look at this load path. Isaac Newton turned in his grave just a couple of minutes before the frame broke.
-10
因篇幅问题不能全部显示，请点此查看更多更全内容