Damn, that's pretty trick. cool!

 

Dave - did you have to shorten the steering shaft?

 

Dave- are you using SN95 spindles? My understanding is that these improve the Ackerman. I was considering making a test bracket to mount the tie rod end in various positions and test the effect this would have on Ackerman and bumpsteer.

Bob

 

David and John,

Very nice work, as usual. You guys continue to impress me with the thoroughness of your chassis tuning. Is there any advantage to removing the rear rack mounting brackets and bolting the rack between the front mount brackets and the crossmember with a set of shims to allow adjustment? I suppose that would reduce the tow out when you turn in. So a better question might be how much ackerman is desireable for these short wheelbase cars? Again, good job guys!

Mike

[ August 02, 2002, 11:03 AM: Message edited by: MikeBraddock ]

 

Will this cause an excess side load on the rack? Interesting approach. Cheers Richard.

 

David and John, you guys are too much! Looks like a fairly easy modification. I don't understand how it would effect ackerman, but do understand how it would necessitate realignment for toe. It seems ackerman would be controlled at the spindle and its relationship to the centerline of the balljoints. But that's why I am in healthcare and not racing!

Great job!

 

I went back and spent some more time analysing my lap times against the racers I benchmark myself against. The time improvement is more close to 1 second. I changed NOTHING else on the car and am now .5 sec off a guy that is normally 1-2 seconds faster than me. Last years BSP nation champion. The same goes for a number of other racers I compare my times too. I looked back about 5 races and compared top times in about 3 separate classes so I am confident the rack change made the difference.

For those not familiar.... 1 Second is HUGE in autocross for lap times under 60 seconds.

The ball joint steering knuckle relation is also important when concidering ackerman. However, moving the rack back behind the steering knuckle centerline will also change ackerman. Its the combination of the two that will give you the best effect.

Im guessing we could probably use even more ackerman effect. Im not sure how much is enough. I do know it causes quite a debate amoung race tuners though.... and I am quite certain any toe-in during steering input is QUITE BAD when it comes to racing.

I started the discussion with John because two guys with high power rwd light cars also "fixed" their ackerman and gained approx 1 second on less than a 60 second course.

David

 

the arguments for the amount of ackerman desired in racing are complex.

ill be doing the same mod myself. 0.5 seconds is the difference between 1st and 7th place in my class.

-james

 

It is my understanding that one reason for mounting the rack in front of the spindle is to give the steering understeer with bump and the the opposite is true with the rack mounted behind the axis of turn. Do you notice this? Is it important if you do?

 

David and John,

Is the reason you are changing the Ackerman geometry because the Mustang geometry is setup for a longer wheelbase than the FFR and the Ackerman needs to be increased to compensate or has the change been necessary for mustang owners, too?

I always thought that a line drawn from the tie rod end of the steering arm through the centerline of the ball joints should line up with the center of the rear axle. So if the wheelbase gets shorter, the steering arm needs to be further outboard for front steer vehicles such as the FFR.

I love this engineering stuff!

[ August 02, 2002, 01:08 PM: Message edited by: Larry Cornish ]

 

I like this engineering stuff too.

What spindle are you using that accomodates a tie rod mount behind the axel?

 

David and John, you guys are da bomb. John H's been telling me about this ackermann mod for a long time and I could never visualize it in my mind...man, what a simple direct sanitary improvement. What are the negatives, if any, for this mod if the car is used extensively on the street, as well as autocrossed heavily? I can dial in more caster, as I'm running P/S so steering effort isn't a problem. Any other points of concern? Thanks, and good luck with this improvement.

 

Way to go David! A second? That's huge! I thought it felt good. Guess I improved as a driver less than I thought!

I'm glad David verified my impressions of this simple mod. I changed a lot of other things at the time I did it on my car, and I couldn't sift out the effect of just this change. We had spent a lot of time talking out the options and it looked like major frame mods or spindle re-design were going to be needed. I was still hung up on the old school anti-Ackermann theories until David pointed out that people were getting good results with dialing in a lot of positive Ackermann on powerful, fat tired racers without aero effects.

I'll touch on JohnP's question about negatives first. The only serious one is that the tie rod is closer to the springs and you may have to run another steering restrictor to prevent contact. I don't, but then Xena doesn't see much street time any more. The mount is stiffer than the original, and the out of plane loads on the rack ends are only increased by small amounts. You do have to re-adjust your toe and bump-steer settings. The bump steer progression becomes elliptical rather than nearly linear, but can be minimized to the same levels as before.

Some "Z" is introduced into the intermediate steering linkage, but I haven't had any problem with angle dependent torque variations with my manual 15:1 rack and David hasn't mentioned any with his power rack. The "z" could be completely eliminated by re-designing the steering column lengths and pillow block mounting, but that's not on my priority list.

The question of how much Ackermann progression is needed is very complex, as James said, and a little controversial in racing engineering circles. Colin Chapman, and others, proposed that the unloading of the inside tire in a turn moved the peak slip angle of that tire toward center enough that anti-Ackermann (dynamic toe-in on turning) was needed. This was very successful on the Lotus formula cars. It's also the reason you see all those mechanics pushing those tiny cars around the pits in pictures of that time. It really made them quite hard to move at low speeds and lateral loads. Now there is some question as to whether this is only true for the relatively skinny tires and low downforce of that era, as others are moving back to adding Ackermann progression and having success. The situation is also complicated by the dependency of the location of the turning center on design and driver input factors that determine the dynamic balance of the car in the various turning phases. The back-of-envelope estimate that is bandied about is between 100% and 150% Ackermann progression for cars with our characteristics. This mod moves us toward that goal, taking us from about -20% to about 60% but we might benefit from mods to the spindle to allow even more Ackermann progression.

But, I think I'm going to worry about scrub radius a bit before I go after this again.

cheers,
John

 

FYI, not only did David take a second off of his time but I did also. I am now the fastest woman in our region on Kumho's and I am closing on the Hoosier & slick wearing ladies also. In raw time I consistently beat the Hoosier cars (C4, C5 and Z06 vettes in BSP, SS and AS) and narrowly miss the fastest two ladies driving slick wearing vintage vettes.

I love the way our car handles and am in awe of David and John's ingenuity.

 

hi john,

another really big difference from the negative ackermann days is the difference in tires. our modern, street legal DOT tires dont resemble ANYTHING anyone was tuning with even 20 years ago.

one reason this is important is that the ackermann effect helps determine the relative slip angles on the 4 tires during cornering. if you look at davids autocrossing videos, he doesnt use ANY slip angle, and so would benefit from closer to 100% ackermann.

-james

 

Sounds like something that would be fairly easy to do while I replace the rack.

What size is that long drill bit and where did you get it?

are diffrent bolts needed?

 

you will also not be able to use the stock PS lines i think as the fittings end up under the x member.

the other concern i had was the increased angle to the steering shaft.. make sure the u-joints wont bind.

-james