front end issues

chuck smith · 08-14-06, 10:41 PM

I have noticed my CAV MONO14 wanderes a good bit at speed (BF Goodrich radials) and seems really sensitive to crosswinds or large trucks passing by. The steering wheel really jumps when you hit a small choppy bump and the front end seems to get really light at 80-90 mph. I have the standard front A arms with Spax coilovers,any thoughts? thanks; chuck smith

Mark Worthington · 08-14-06, 11:17 PM

First thing that comes to mind is front-to-back rake. The back should be at least a half inch hogher than the front, more like a full inch.

Eric · 08-15-06, 01:00 AM

Sounds like posssible alignment and/or geometry issues. Possibly some bump steer as well.

Simple things first, you could simply have a toe out condition at the front.
Front wheels should have a small amount of toe-in, or at least be at 0 toe. Toe-out at the front can help the car turn in quicker, but it also makes the front end very nervous as you describe.

What is your castor setting at the front?
what is your KPI and scrub radius? What offset are the front wheels?

How did you set up the steering rack, and was it checked for bump steer?

Also, are the settings equal side-to-side? sometimes one side can be out and make the front a disaster.

Thats what i would check first if I were setting up a track car. Same for the road.

Hope it helps
Eric

David Briggs · 08-15-06, 01:15 AM

All of these comments are good, I took my suspension completely apart and fiound it did not have the correct bolts and washers attaching the front hub to the billet for the ball joint. I run about a 16th of toe front and rear, 1/2 camber in the front 1/4 in rear, front upper control arms slid back almost all the way(castor) and the top of the rear hub tilted back( level placed on lower control arms with rear of the arm being 1/4 inch lower than front.
Ride heght 4" front 4.5" rear, I have also boxed in the lower part of the radiator(in front and sealed the back against the nostrils(all air goes thru radiator and over the top of the nostrils, be careful not to have the shocks too firm. I foyund considerable binding in the rear suspension, at full droop the sway links were bind badly. I opened up the clevis a little so that the heim joint would have full movement.

CliffBeer · 08-15-06, 03:23 AM

Chuck, I have CAV mono #8 and she's rock steady up past 100 in side winds and with uneven pavement. There's definitely something off based upon what you have described.

Some basic stuff to check would be the steering rack - make sure there's no movement in the rack mounting, or, in the ball joints etc. Then, check the A arms - the mounting bushings and the ball joints, etc. Also, check the wheel bearings. There shouldn't be any slop in the wheel bearing clearance. Take a look at the wear pattern on the tires also - look for anything strange. Example would be wear patterns on the outside or inside of the tire patch as this could indicate excessive toe out or toe in. Ensure your tire pressure is good and consistent across all tires.

The above will ferret out 90% of the typical problems. If you don't find anything based upon the above checks then repost and we'll take it from there. Good luck!

Canuk40 · 08-15-06, 11:21 AM

Hi Chuck,

All good advise and any CAV properly dialed in will be strait as a bullit as fast as you care to go. Where it can get tricky is the effect of tire diameter on ride height and suspension geometry particularily in the front.

We run 225/50 front tires to get the ride height down without cranking down the coilovers and running the suspension all the time at angles that would normally occur only in bump. However, most cars have larger diameter front tires meaning that if you want say 4" ride height you're pushing your luck on remaining free travel in the suspension. If your front tires are 25" in diameter go to 4.5" minimum front ride height, at 26" diameter 5" is the lowest. The rear ride height should be .5 higher (no more) than the front. A little bit of wedge helps keep the air pressure down under the car, too much looks goofy and upsets the rear end geomoetry.

For street use (and occasional lapping) what works the best is .5 degree negative camber front and rear, 1/16" total toe-in front and rear.

If the car is darty or hunting the crown or camber in the road and gets pushed around in sidewinds, it's all alignment my friend. These cars work great set up properly.

If you go to our Revised Reversed Lower A-Arms on the back, it will correct the rear castor angle, giving more anit-squat geometry, fix the wheelbase so the tire is in the middle of the wheel arch and eliminate all the friction from the rubber bushes in the stock A-Arms, plus they look cool

Please feel free to call me on the toll free Tech line: 866 278 4840

Cheers

chuck smith · 08-15-06, 12:17 PM

Thanks guys,I will look into these issues. chuck

chuck smith · 08-15-06, 07:21 PM

where do I measure the road height from,the lowest point on the front and rear of the frame?

chuck smith · 08-15-06, 08:42 PM

My toe in appears to be excessive,.75. ride height appears ok,tire pressures were low 26lbs.

Gary Gibbs · 08-15-06, 09:08 PM

My guess is bump steer. Need to really work it to get it close with a "lowered" car.

chuck smith · 08-15-06, 09:33 PM

how do I go about that?

Gary Gibbs · 08-15-06, 09:51 PM

First check the bunp steer to see how bad you might be. Then you are likely going to need to lower the rack (not very easy) or raise the tie rod end. The last setup where I was able to raise the tie rod end was accomplished by drilling out the upright to accept a bolt for the tie rod connection (1/2") and using a rod end (heim joint) for the tie rod end with a threaded adapter (to attach to the rack). Then you can add shims/washers under the rod end at the upright to get the best compromise bump steer. You will likely not get "zero" for all the range and thus will have to trade off the results. I tended to set the front end closer to zero under bump and sacrifice a bit under droop. Thus the tires with more load (bump condition) will have a little less toe change. The rack needs to be a few inches longer (at the in board pivot points) to get near zero bump throughout the range. The result is the car is much more stable at high speeds and under hard high speed braking.

CliffBeer · 08-16-06, 02:15 AM

Chuck, did correcting the toe and the tire pressure alleviate some of the undesirable characteristics?

What size front tires are you running?

No offense to Gary but I wouldn't go changing the rack position just yet. The CAVs, when set at the proper ride height and with the correct dimension tires, are stable and don't exhibit excessive bump steer. If the concept of "bump steer" isn't clear to you it's basically (in layman's terms) the undesirable change in toe/steering caused by excessive angles of the steering link from the rack to the hub when the front suspension goes through movement (up or down). The problem is common to all cars and is accentuated when cars are lowered and the angle of the link become more extreem (farther off horizontal). The CAVs look about right to me when at the proper ride height and with the proper dimension wheels/tires.

Take another look at the rack and all the links and ball joints. Get the car up on jack stands and look for any slop/free movement. You'll find it if it's there.

chuck smith · 08-16-06, 12:21 PM

I have not tackled the toe in yet. What pressures would you run? I hope to avoid the rack changes if possible,however I may have to do that. thanks again. chuck

Gary Gibbs · 08-16-06, 02:39 PM

Quote:

Originally Posted by CliffBeer

The CAVs, when set at the proper ride height and with the correct dimension tires, are stable and don't exhibit excessive bump steer.

I do not agree with this statement. With small diameter front tires and 5" of ride height the bump steer will be better; however, with the original front end equipment the bump steer is non-zero for most any ride height. With a lowered car (e.g. 4.25" ride height and 245/45-17=25.6" tires) the bump steer can be .2" at 2" suspension travel. To get close to zero bump steer you want the inboard tie rod pivots to be near the line drawn between the inboard upper and lower pivots. In other words when viewed from the front of the car, if you draw a line between the upper suspension inboard pivots and the lower suspension inboard pivots the inboard tie rod should lie along this line. This is not 100% true but it will get you close. Optimal pivot point is slightly outboard of this point. You still need to adjust the angle of the tie rod relative to the angle of the suspension arms (thus the outboard shims). The original CAV rack pivots are inside of this "line" resulting in toe change with suspension movement. If you don't believe me just measure the bump steer and report the results.

chuck smith · 08-16-06, 11:20 PM

Well, I fixed the toe problem and it was really off and the car now drives much better,not perfect but I am definitly on the right track. I do not doubt that there is some bump steer considerations so after I get the allignment correct I will address that. Thanks again. chuck

Canuk40 · 08-25-06, 01:53 PM

Hi Gary,

Quote:

Originally Posted by CCX33911

With a lowered car (e.g. 4.25" ride height and 245/45-17=25.6" tires) the bump steer can be .2" at 2" suspension travel.

Yeah, getting optimal bump steer usually creates packaging or design modifications if it was not a priority from the onset. One would like to believe the original GT40 was optimised in this regard, however all the replicas have issues.

I'm interested to know if the toe change of .2 was in or out and in bump or droop. Bump steer gets "ugly" if it causes changes in the intended path of the car or makes the steering "nervous" and unpredictable.

My observation in driving the CAVs quite quickly on the track and carefull observation of steering feedback on the road, is that bump steer does not become an issue. It may not be optimal however it doesn't get "ugly".

About the toe change in bump: Say we were turning right - if it was a toe out number on the left front wheel the car would run wider than planned on a bump in a corner. If the inside wheel (in droop because of weight transfer)went toe in at the same time the two toe events would compound the sensation of understeer although the intended turn path would resume after the suspension damped the bump (assuming you're not in the kitty litter). If the inside wheel went toe out instead there would be some cancellation of the effect. There would be additional scrub and heating of the tires and loss of speed. Just an example to keep the grey matter going

In a strait line on level ground, if both wheels undergo bump or droop, even through larger angles, the the toe changes cancel each other out. It's one wheel bump steer adding to the turning radius that will bite you in the behind. This goes for rear end bump steer too. Some cars run sunstantial toe in at rest and dial more in to help drivers maintain the intended path through a corner. Very comendable in a mini van or family hauler, sports cars need to be predictable and stable without numbing the chassis responces.

And what if the bump steer is less than the slip angle of the tires or suspension deflection of the bushings? What about Ackerman? Oh my! I'm rambling again. Very interesting stuff though...

What is absolutely true is that the CAV chassis is stiff enough to tell you exactly what's going on with those four little contact patches of rubber on the road. It responds very well to proper alignment, shock valving and suspension upgrades that reduce friction and misalignment associated with the rubber bushes in the stock suspension.

So carry on Chuck, the car will only get better and better

Cheers

Gary Gibbs · 08-27-06, 07:51 PM

The toe change was out on bump and in on droop. With the tie rod long (or rack pivot to pivot length short) the system will tend to out in either bump or droop. However, I was able to get near zero change in bump with slight toe out in droop. Car is very good under breaking and turn in. There is a little noticeable uneasiness when the front in rises under hard accel. It is still quite manageable.

Canuk40 · 08-31-06, 12:31 PM

Quote:

Originally Posted by CCX33911

The toe change was out on bump and in on droop. With the tie rod long (or rack pivot to pivot length short) the system will tend to out in either bump or droop. However, I was able to get near zero change in bump with slight toe out in droop. Car is very good under breaking and turn in. There is a little noticeable uneasiness when the front in rises under hard accel. It is still quite manageable.

Hi Gary,

That's about what I expected. About the uneasiness in the front end, is that on the exit of a corner or strait line? Rear bump steer or anti squat may be contributing to that.

Cheers