yorkshireracer wrote:
Hi guys, eventually got a (relatively) free bit of space so i'm starting to get a few ideas together for a mid engine 3 wheeler with bike engine. Plan is to use it for various race events, drag racing, sprinting, hill climbs.
I ' ve been building bikes for years but this will be 1st car so i'm looking for plans which anyone can recommend. I' m especially looking for front steering / suspension info.
Any recommendations or pointers would be appreciated.
Hey, nice to see another reverse-triker on the site. though there are a few of use who have built/are building reverse trikes, there is really rather a paucity of data regarding front end geometry for best handling (though as with all design solutions, "best" is a relative term).
I completed a scratch built revese trike (though front-engined) last january and have been driving it about 100 miles a week since march, its been a blast.
A proper double A-arm suspension works really well. as long as your toe settings are right, and you achieve for zero bump steer within your planned suspension travel, you can bomb over all sorts of bumps without any sort of stability issues. one can learn alot through mimckry, so look carefully at the front suspension design of the CAN-AM spyder, and Modern morgan three wheeler (don't bother with the older sliding pillar design, its a quaint relic, much like the old V-dub double trailing arm front ends). You can also find detailed parts layouts for the Campanga T-rex. That one might be most edifying of all.
there actually are many reverse trike designs out there, from which you could learn. ultimately though, you'll just have to pick a design and go for it. you would even do well to swipe one of the "book" locost front ends and use that. its proven, pretty much. In fact, given that you've never worked on a car design before, that is absolutely my advice: take the dimensions and geometry from the Haynes "book" locost,and either buy or build the upper and lower Kinetic control arms for Miata spindles, (I'm not shilling for Jack, its just a solid design that's easy to fabricate).
KB has a point about weight and balance, though its not nearly so dire as he claims. Since you're planning for a rear mid engine, you can absolutely bias the weight toward the rear tire and load it sufficiently to get good grip, but you'll have to make sure your rear tire is wide enough for the power you're planning to put down. making the rear tire aproximately equivalent in width to the two front tires is a decent rule of thumb though. that said, I think you should avoid a superwide rear tire with really low sidewalls. here's why: unlike cars, reverse trikes control roll stiffness at the front only, and -beyond deisgned in geometric mitigations mentioned below - this means either stiff springing, a stiff anti-roll bar, or both. but no matter what you choose, the vehicle will experience some roll, and this will tend to rotate the tire to the outside edge of its tread, lessening effective contact patch area. this must be lessened as much as possible, and the best way to do that, IMO, is a medium-width radial tire with a - minimum - 60 aspect ratio. The thing is gonna roll, let the sidewall flex at the rear to soak some of that up.
fore-aft locaion of the CG has little effect on tippiness (EDIT: umm, actually, yeah, it kind of does, see Acerguys post below. Far be it from me to gratuitously misinform
), though CG height and roll center location do. Comparitively, a CG far above the roll center will roll/tip more, and CG below the roll center will roll less. on a car, it is usually accepted that for best handling, the roll center axis should incline upward from front to rear, with the rear roll center higher than the front. this is fundamentally impossible on a reverse trike, because the rear roll center will always be at the intersection of the rear tire centerline and the ground. designing the front suspension such that the front roll center is enough below ground to maintain the automotive roll center axis paradigm will only exascerbate tippiness.
in my case, since I chose a 6 inch ground clearance as best for daily driving, I have both a really high roll center at the front, and I've sprung the thing rather stiffly. Roll is minimal. in the interest of full disclosure, I do experience camber loss in roll, but the thing still rails around turns in a most pleasing style.
its important to remember that you're not designing either a car or a motorcycle, and the reverse trike requires applying a different set of design rules to best fit its specific dynamics.
But, ultimately, the best advice, really, is to look at successful reverse trike designs, and mimic those. have fun!