Solid Rear

[Kyle Kapa]

Hello All,

This will be our first year at attempting to design, build, test, race ect....

Anyway.

I am considering the option of a solid rear axle (similar to something you would see on an ATV). Mainly to keep cost down and simplicity up. Has anyone tried this?

Other than the obvious, any pros and cons?

Thanks

[Denny Trimble]

Yes, teams have done it. Figure out your chain tensioner (or mount your engine to the swingarm moped-style and I'll buy you some beers).

[Faterooski]

Lehigh University did it last year, but I don't really remember how they finished up. Their website has some pretty good pics of their swingarm setup though, check it out.

http://www.lehigh.edu/~insae/formula/

[Christopher Chow]

Chain tension? Have your swingarm pivot near, preferably on, the same axis as the output of the tranny.

Beer me.

[rjwoods77]

My buddies 600cc sprint car has rediculous suspension articulation. It is a solid axle with 4 link and panhard control. The amount of chain tension change is really shocking. But he uses this simple chain tensioner and it works awesome.

http://www.hyperracing.com/product.asp?prodid=08-600

The standard used to be a scissor style tensioner but they got snart and use a single side tensioner. Make sure you tension on the slack side. Something else to note. If you do a concentric drive system(drive sprocket on same axis of swingarm pivot) it will help chain tension problems tremendously. It also reduces the chains effects on your suspension system.

By the way. Just had to add. OMG. 12 square feet for 250 bucks.

http://www.hyperracing.com/product.asp?prodid=30-100

[Denny Trimble]

Just make sure your chain system can handle roll (misalignment of the axle relative to the chassis). Short chains don't like this as much as long chains (micro sprints). And with spring-loaded tensioners on the "slack" side, what happens when your klutz of a driver downshifts without matching revs perfectly?

[rjwoods77]

Originally posted by Denny Trimble:
Just make sure your chain system can handle roll (misalignment of the axle relative to the chassis). Short chains don't like this as much as long chains (micro sprints). And with spring-loaded tensioners on the "slack" side, what happens when your klutz of a driver downshifts without matching revs perfectly?

Use a cvt and it wont be a problem. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[Z]

Originally posted by Kyle Kapa:
I am considering the option of a solid rear axle (similar to something you would see on an ATV). Mainly to keep cost down and simplicity up.
Kyle,

I think this is a feasible option.

Most ATV's (Quads over here) with a solid rear axle only allow the axle to move in bounce - ie. remaining parallel to itself, with no "axle-roll" relative to chassis - this being done with a simple "swing-arm" type suspension. If you copy this then your car is very likely to have lots of oversteer (be very "loose") and there is not much you can do about it. Better is if you allow the axle to move both in "axle-bounce" and "axle-roll", with the stiffness of the "axle-roll" springing easily adjustable.

There is no rule saying you have to use a chain. Many agricultural Quads use a shaft and crown-and-pinion final drive - neat and tidy and no problems with dirt. A very short (~4"/10cm) driveshaft, basically two CV's back to back, will allow for movement between engine and axle. A single cylinder or V-twin engine mounted with the crank longitudinal could work here. Likewise something with a CVT - eg. Dearborn's car last year, or a Yamaha Grizzly with the engine turned sideways...

Main thing to work on is packaging it in as small a space as possible. I reckon you want the driver's back as close to the rear axle as possible.

Also the solid axle isn't just cheap and simple. It is (IMO) actually a performance advantage - you can have soft springs and the camber angle of the wheels never changes! http://fsae.com/groupee_common/emoticons/icon_wink.gif

Z

PS. And while you're at it you may as well fit a beam axle to the front - same performance advantage!

[Garlic]

Solid axles would have been hell on last years course-- way too bumpy. Depending on the new course it might not be a bad idea.

Problem is integration, and keeping weight down to a reasonable level. I suppose an ATV engine would be the best way to accomplish that.

I can't see how it'd be a performance advantage.. negative camber is a good thing to have; even in racing classes they do thier best to get as much as possible from a solid axle. It's not real easy to do either.

However it might not be much of a disadvantage as our tires aren't as camber sensitive, as say, radials. And in accel event having no camber gain in squat would certainly help.

[jack]

Originally posted by Z:
Also the solid axle isn't just cheap and simple. It is (IMO) actually a performance advantage - you can have soft springs and the camber angle of the wheels never changes! http://fsae.com/groupee_common/emoticons/icon_wink.gif

i fail to see how this would differ from an a-arm setup with soft springs on the dampers and a very stiff ARB. am i missing something?

[Z]

Garlic,

"Way too bumpy" is a relative concept http://fsae.com/groupee_common/emoticons/icon_smile.gif. On really rough roads (eg. most around here) a solid or beam-axle does have a disadvantage from its extra "unsprung weight". But they can make up some of this disadvantage by softer springs. Circuit racecars with independent suspensions typically have very stiff springs and, in a sense, the whole car acts as if it is "unsprung".


Jack,

An a-arm setup with soft springs and "a very stiff ARB" will also need a very stiff ARB at the other end of the car to maintain neutral handling balance (ie. to keep roll-moment-distribution around 50:50). Two stiff ARB's keep all four wheels coplanar (ie. front and rear axles parallel to the car's floorpan).

So with stiff ARB's all four wheels can NOT move in twist (an ARB is actually an anti-roll-&-anti-twist-mode-bar). So any small twist in the road surface results in a big change in wheelprint loads, and hence a big change in handling balance. The twist in the road can actually determine whether the car understeers or oversteers in a corner!

It is a good idea when calculating wheelprint loads during cornering to also add in the effect of a non-flat road, with a slight twist either one way or the other. Big surprise!!!

Again, the main advantage of beam-axles over independent suspensions is that the beam can have much softer springs, especially in the roll/twist mode. This makes a big difference on anything other than a perfectly flat track.

Z

[Garlic]

Relative to any circuit track you can think of. The lot was very irregular and in high speed corners as well.

It's true that typically independent suspension cars are typically stiff... FSAE cars are not typical and in FSAE Detroit the good cars were very softly spring. Sometimes they were stiff in roll but you can see the pitch of the cars is quite high. And ride heights confirm this.

Having driven the course I'd say a solid axle would be a very poor choice. A kart would not make it on that track.

However at the proving grounds in 2006, the solid axle might be very viable. Still would not suggest it's the best option, but if you are looking for something different and simpler it could definitely be competitive.

[Z]

Kyle,

In my first post above I said;

"with a simple "swing-arm" type suspension ... your car is very likely to have lots of oversteer"

Actually, if you use a solid-axle the lack of any "differential" action will move the handling towards understeer (the solid-axle always wants to roll straight ahead). If you are lucky these two effects (stiff rear anti-roll + "spool") might just cancel out and you get neutral handling.

But I don't think you would get that lucky. I have a hunch that you will get understeer on corner entry from the spool, and oversteer on corner exit from the stiff rear anti-roll. This is not good because you get the same US/entry + OS/exit from the yaw inertia of the car (regardless of suspension or differential type).

Anyway, a softer roll motion of the rear solid-axle can cure the OS/exit (as would a low horsepower engine?), and there are other ways of curing the US/entry due to the spool rear axle.

I still think it is a feasible idea for a first year team.

Z