norvalwilhelm

I have been doing alot of suspension testing at high speed lately and run into a wall at 120-130mph. A perfectly smooth car at 100 mph runs into front end vibration around 120. Why? I am looking for the answer.

Pro South sent me an article on pump steer as some of you know that is my goal for this year to achieve Zero pump through relocated tie rod postions both inner and out plus different length tie rods.
Anyway this toe thing has got me thinking.'
This is part of the article, the part dealing with TOE.

TOE-IN INFO
I have a highly qualified suspension book that recomends .1 degree toe OUT reduction, (from stock setting) for every 1 degree neg camber you use.

.1 degree is aprox 1/2 INCH toe out! This is total toe-out across the car, 1/4" per wheel.

Why? -

The front wheels try to turn inward when using neg camber due to camber thrust.

Imagine the tire tilted one degree in at the top. The wheel will try to run around in a circle - Imagine it were on a bycicle and you were leaning it 1 degree.

Both front wheels cancel each other out, so the car stays straight, but the tires are thrusting inward as you drive, and should be toed out slightly to compensate for any "extra" neg camber.



Origonal factory specs for 60's cars were developed for use with positive camber and bias ply tires required more toe in than radial tires do.

When radial tires came out, toe-in specs were reduced by half, so a Camaro with a 1/8" toe in spec was reduced to 1/16" toe in. Many front end shops may have forgotten this fact and set toe to the "book".


Since I do all my own alignment and toe is easy for me with proper toe gages I am going to start working with toe out instead of toe in as normal and do more high speed test. I really changed camber last week to go as far as negative 2 degrees and hated it. I am now set at negative .75 and will try the toe out this weekend.
Everything , everything I read tells you not to let the car bump in and yet we set the toe in.
I will try toe out and get driving impressions, you can really feel a car pushing the front end and if toe out works at normal speed and the feed back is positive I will try it at high speed.
I would like input from anyone about this. Not about my high speed testing but about what you think.
Have a nice evening.
Norval


[Modified by norvalwilhelm, 9:00 PM 7/16/2003]

Jvette73

I thought that the reason for slight toe in is that the forces of the car moving forward pushes slightly back on the wheels. So slightly toe in at still would be precieved to be closer to perfectly straight when moving down the road.

Fevre

Norval

I think you answered a question that I have thought about for a long time but never thought it meant much. In HS physics class the teacher gave a demonstration on centripital (sp?) force. One thing that I noticed was that depending on which way you spun the tire the tire to make it do that :cool: balancing act with the rope, it would allows rotate opposite the direction you spun it. If you spun the tire clockwise it would rotate counter clockwise in relation to the rope. Always wonder what caused that, it must be camber thrust.

Do you think the camber thrust and the opposing force from the toe out would be the same at all speeds. Just curious if maybe your car is in balance at 100 mph but out of balance at 120? This would mean you would need to figure out what your ave speed is and set the car up to be in balance at that speed. Do you think if the forces where out of balance at lower speeds you would feel it as much at say 50 mph as opposed to say 120 mph?

Your tests should be very informative. :cheers:

Matt Gruber

years ago i did some experiments with my 61. i did end up with toe out. But the reason was to make it steer easier as it is 2.75 turns lock to lock no PS.
Maybe an on the car spin balance will help the vibration?

My book says toe out makes a car twitchy at high speed so watch out!(compensate with lots of caster)

Frank75

Norval, how does the vibration manifest itself? Just in the steering wheel, or can you see the hood shake, etc.? There's a fair difference in the amount of aerodynamic forces between 100 and 125 or so - could your hood cutout be causing problems or is this a new condition? Is any bodywork unbonding and maybe starting to shift around? I know these are obvious questions and apologize.

MikeC

Have you seen what happens in toe out condition(worst case scenario)..... shear all wheel studs and have the wheel come off.

Seen it happen.... 6- 1/2" studs on a 4X4 with 35" tires... yes its an extreme situation.....but it doesn't mean it could happen to you..... I'd stay within the specs for a vette.

norvalwilhelm

..... I'd stay within the specs for a vette.

Mike what are the specs for my vet. It came with bias ply tires in some small size, positive camber and little caster.
I went and put 265 tires on the front, run negative .75 camber and 5 degrees caster. Now what happens to toe specs. Do I still run stock specs?
At 100 mph the car runs so smooth, one finger on the wheel deal. I have worked hard to get the steering where it is right now with no play, no following road grooves, and soon absolutely no bump steer.
This thing about falling apart at 120 has got me. If the forces are causing tire scrubbing at that speed I want to find out and eliminate it.
I can very accurately set toe, I put the car on the hoist and turn the tie rods a certain number of flats and the tire moves in or out a predicable amount. I then measure it with a gage that checks half way up the tire both front and back.
I can go for a test drive at normal speeds and feel if the car is scrubbing or not. There is feed back through the wheel that you can feel before high speed. Hopefully.
I am also upgraded to 1/2 inch studs.

norvalwilhelm

Jvette73 I thought that the reason for slight toe in is that the forces of the car moving forward pushes slightly back on the wheels. So slightly toe in at still would be precieved to be closer to perfectly straight when moving down the road.

This is what we are not sure of. When you switch to negative camber instead of positive plus change to radial tires from bias what happens to toe?
What if in fact it is changing the forces causing the tire to toe in instead of the predictable out?

Pete79L82

Norval; I don't have much time this morning but I will get back to you with my thoughts later this evening. I don't care what it says in someones book but toe out will give you more stability at higher speeds. ALL race cars are set-up with toe out.

One quick question. On downward tire movement (body moving up) does your car bump steer in or out?

norvalwilhelm

Fevre Just curious if maybe your car is in balance at 100 mph but out of balance at 120? This would mean you would need to figure out what your ave speed is and set the car up to be in balance at that speed.

My car is in balance at 100 and keep wondering why it goes out at 120? Sure would hate to figure out my average speed. I am becomming like George and want to cruise at higher and higher speeds. With my 308 rear end and .68 overdrive my car idles at 50 mph so it really likes to run at 70 or better. It is not a big deal to start changing Toe and see how it works. Being my own alignment guy it cost nothing to change and put back.
I will start dialing more toe out in the car and take test drives to find out how it likes these new wettings. I will post how the car feels and how it feels to me.
I think with the 17 inch tires and their short stiff side walls they do not experience tire squirm nearly as much as my 15 inch. This alone might force me to go to 17 inch rims.

norvalwilhelm

Matt My book says toe out makes a car twitchy at high speed so watch out!(compensate with lots of caster)

Everything I read warns not to let the car bump to toe IN. Roger my stock car friend says the same as you. Don't let it toe out but my suspension books say don't let it toe in. Who is right?
I will have to try it or it will bug me until I finally give in and try it like I did with the 460 springs last weekend. Springs in and out in the same day. Now I know I don't want them and I can forget it. I am still wondering about MOnty Carlo springs. Tried them once and liked them.

norvalwilhelm

Frank75. It is just a feeling I am getting. At 100 the car feels extremely smooth and when cruising along and not watch the speedometer you quickly creep up to 100. YOu look down and suddenly realize that is what you are doing. If you push it a bit harder the car seems to resist, as if something is starting to push against the car. It is not a lack of power and the wheel doesn't start shaking but the whole car seems like it doesn't want to do this. Maybe my wife put a curse on it :) :)

The Money Pit

I was thinking that toe was designed to assist in self centering the steering.Like if you let go of the steering wheel on a straight section of highway,the car will still track straight.Each wheel pushes slightly against the opposite to achieve a balanced condition.
I'm also thinking the vibration at speeds is the result of worn suspension parts,or wheel balance.I had a set of wheel adaptors on for a short time that would run smooth till around 60 mph,then shake your molars out if you went faster.The new adaptors run true way past 120 mph.Maybe you could try a different set of wheels for a quick test drive.

Matt Gruber

i'd have to agree with pete,
while my book warned it would be twitchy i tried it and it is NOT twitchy. gotta watch those ivory tower books! lol

i just had this crazy idea; CHAIN YOUR FRONT END so it can't lift at high speed FOR A TEST ONLY.
bump steer will be the same as at low speeds. Of course a "bump" in the road will cause the tires to lose contact with the road :eek: so find a smooth road for the test.

norvalwilhelm

Pete79L82 I was hoping to hear from you as well as redvetteracer.

I don't care what it says in someones book but toe out will give you more stability at higher speeds. ALL race cars are set-up with toe out.
One quick question. On downward tire movement (body moving up) does your car bump steer in or out?

This is what I have also read but don't know from experience. I am of coarse worried about tire wear on a street car.

I am afraid my car still bumps in. Last week I got a proper bump gage with a single dial indicator on the front and wheel on the back but still haven't committed to totally taking the front end apart for moving the tie rod inner mouning location and tie rod lengths. I do have the out bump steer studs, I also have longer tie rods plus a reamer to start making new inner tie rod locations and trying to reduce bump.
Even with a little work my bump down is reduced to .015 at 2 inches down, which is good but bump up is still 3/8th inch at 3 inches rise, not good.
But without changing the drag link and tie rod lengths I don't know if it will improve much for now. I will address this in the fall for sure and not accept anything less then a few thou.

norvalwilhelm

Money Pit bite your tongue :) :) Worn parts on my car. I seem to be putting new parts in daily. Would not hesitate to replace anything if I even suspected a worn part. I am under my car almost daily shaking, studying ways to get around things. Warn parts I don't think so.

As for centering a wheel I though Caster did that not toe. I have 5 degrees caster in mine.

norvalwilhelm

Matt my suspension book also says that toe out is better then toe in. I wouldn't know quite how to chain my front end down but will have a look under there this weekend. Limiting straps to prevent lift?

Matt Gruber

i've seen drag racers do this for wheels up launches :D
bolt a piece of chain with a turnbuckle to the frame, and to the lower control arm. carefully adjust it to control max movement.
and for fun, try for a wheelie :eek:

The Money Pit

I don't doubt you're a parts changing guy Norval,I've rebuilt my front end twice in eight years and believe it or not I blew a tie rod in between. I was doing 65 mph on the way to the airport to fly out on a buisness trip and hit the brakes to slow for a turn and the front wheels went in two different directions.
I take great pains to maintain my Vette too,but stuff does wear out when you drive the heck out of a car like I do! :cheers:

gkull

Norval - You don't want either. Zero toe is the best. Toe in gives the vehicle self correcting steering factors. Zero toe makes the car start to hunt and follow road grooves. Toe out depending on how much can make a car nearly unmanageable for all roads other than smooth race tracks.

Toe out does allow the front end to turn in quicker to a turn. I've tried as much as 1/8 out on each front. I didn't like it and had it returned to Zero.

As for balancing your tires. I always do high speed spin balancing with stickon weights on the inside of the wheel. Just having your car parked too long causes the tires to go out of round which causes odd shaking feelings. Some manufactures radial tires are more susceptible to that out of round problem.

It's time for you to get Z-Rated tires :lol: I went out to cruise night last night.