Kingpin offset Vs. upright design

[Wolverine_1987]

Hello guys,
I wonder if anyone can help me on a simple compromise between the kingpin offset and upright design simplicity however it seems a pain in the ass. We were just about to finalize our suspension design but a stupid mistake was to pop up.. we're done with our chassis design and fabrication except those wish bones brackets however their positions are predefined. The problem is when i started to design the upright i found the wishbones to be quite short (or not long enough)to reach the upright at a reasonable kingpin offset (scrub radius) and wheel offset that satisfies a suitable packaging for the wheel assembly inside the wheel as well as acceptable steering effort, on the other hand just elongating the wishbones to reach the upright at its position is quite difficult cause they go unparalleled they just get further from each other as they get inside the wheel which can cause them to interfere with the inner rim surface and also just bending the upright to reach the wishbones spherical joints wont solve the problem and we still have a big kingpin offset plus fabricating the upright will be a great great deal..... any suggestions ??

[Wolverine_1987]

Hello guys,
I wonder if anyone can help me on a simple compromise between the kingpin offset and upright design simplicity however it seems a pain in the ass. We were just about to finalize our suspension design but a stupid mistake was to pop up.. we're done with our chassis design and fabrication except those wish bones brackets however their positions are predefined. The problem is when i started to design the upright i found the wishbones to be quite short (or not long enough)to reach the upright at a reasonable kingpin offset (scrub radius) and wheel offset that satisfies a suitable packaging for the wheel assembly inside the wheel as well as acceptable steering effort, on the other hand just elongating the wishbones to reach the upright at its position is quite difficult cause they go unparalleled they just get further from each other as they get inside the wheel which can cause them to interfere with the inner rim surface and also just bending the upright to reach the wishbones spherical joints wont solve the problem and we still have a big kingpin offset plus fabricating the upright will be a great great deal..... any suggestions ??

[The_Man]

I am not very clear on why you can not extend the wishbones. I understand that your IC of the right suspension in to the left, and for some reason you can not or do not want to take it further to the left. You can may be change the king pin inclination slightly to get your scrub radius down if it is really that much of a problem. Other thing you can do is to actually consider reducing the track if the wishbones are not falling short by too much.

A good solution I think will be Multi-link suspensions. The Eindhoven team had them, its pretty neat actually, one of the ways to look at them is that you have your wishbone(rather a control arm now) as 2 separate links that goto the uprights Now the kingpin is a really a virtual point(the IC of the 2 links it changes with bump and steer); it can be anywhere even inside your rim. It will be hard to implement this as a quick fix though and if you have not yet manufactured your uprights you can give it a shot.

[Adambomb]

Welcome to the hell that is suspension packaging!

Honestly, you have the problem defined pretty well, and unfortunately I don't have any "magic solution." Suspension packaging, with regards to steering clearance, is a lot more difficult than it would really appear to be, given the large steer angles the car needs to be capable of to run on the small radius turns we encounter in competition.

It really is a trade-off between scrub radius, a-arm to wheel clearance, wishbone mounting points (in top view), KPI, etc.

Not sure what your time frame is, but if you just need to get it done without redesigning the whole system, the best solution (IMO) would be to just go with the larger scrub radius, as that is the solution that would get your car onto the track. Steering may be a bear, but the car will drive. Then next year you can design that problem out of the car!

[Wolverine_1987]

The problem is that if we tried to keep things unchanged we will have to design a stupid-looking upright and even more difficult to be fabricated plus the kingpin offset will much higher than 10% of our tire tread width (a safe percentage for good steering effort)..... we thought about reducing track but things will get shitty to the suspension guys cause the IC will be slightly changed and the steering rack mounting will have to be changed as the IC position is changed to avoid bump steer....

[exFSAE]

But nce the tires are rolling... how much effect is scrub radius going to have on your steering efforts (bumps notwithstanding)?

[Wolverine_1987]

Our scrub radius is 47 mm (at zero camber) which i am trying to reduce it to about 20 mm (since our tire is only 7.2" tread width), we are working on lotus suspension analyzer where measuring the scrub effect on the steering effort is not of the program potentials (or may be), and according to bump steer we have it within safe margins

[The_Man]

If you are looking for a quick fix, may be it is a good idea to go ahead with the higher scrub. High scrub is not as bad as bad as you think it is, specially when you are not too much concerned about tyre temperature and its distribution(I assume since you are a relatively new team). The scrub changes significantly in the dynamic and adding camber and tuning the caster slightly will can help if your driver is having difficulty steering. If it is too bad you can always go for a higher steering ratio or even a bigger steering wheel which I assume is simpler fix than doing the suspension geometry again at this stage.

[Wolverine_1987]

Alright guys, i think it's better to leave everything as it is cause we are running out of time.... Thanks and wish us luck http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[RollingCamel]

Btw, we are having a 12:1 ratio rack and pinion so i believe it gives us extra tolerance to scrub radius on the price of quick steering.

Plus, we don't have time Ahmed!!!!!!!!!!!!!

[Adambomb]

Yep, FYI we got 7th at MI in 2006 with a car that had 60 mm of scrub, a much quicker steering ratio (more like 4.5:1), and something like a 225 mm diameter steering wheel. Getting through a fast slalom was a workout, but we still did well with the car (probably mostly due to having a fast driver that year). Just make sure your drivers spend some time with free weights to get ready for endurance. http://fsae.com/groupee_common/emoticons/icon_wink.gif

In the end, I'd guess at least 90% (maybe 95%) of the cars out there have at least one compromise this severe, so having to "live and learn" with it isn't the end of the world. Good luck!

[RollingCamel]

Well we are in a dilemma between 2 designs of the uprights .One has a flat body along the kingpin then has the bearing housing reaching towards the wheel center in an effort to get the bearings closer.

The other design is v-shaped allowing the bulk of the upright to be nearer to the center.

Advantage of the first design:
Easier and faster to manufacture since we only need 2 axis.

Disadvantage:
the caliper mounts could be far and therefor we should either extend the mounts or use a stiff steel bracket. Either way we will have more bending on the mount web.

Also, the extended bearing housing wall should be thickened to be stiffer against the moments.

Advantages of the second design:
Bearings inside the bulk will insure stiff housing.
caliper mounts nearer.

Disadvantages:
more complicated and would take and cost more to machine.
The steering point which is already far gets far i.e. More compliance and fatigue.

Ahmed would you post some pics. My internet connection is cut and i'm using my stupid mobile phone.