john hennessy wrote:
it would appear that i made the assumption that the toe figure was overall, wheel to wheel, is this not the case, the degree figure is 1.1 degrees.
The toe figures are for each wheel, so for total toe you add them together... or at least I thought it was that simple.
Apparently I made an incorrect assumption too... if you measure your toe the same way I do (front & rear of the wheel & subtract the smaller) then you have to double them again... so in your case you've got a static setting of 1/2".
It seems that the program only calculates the inch toe value as the difference between the axle centerline width & the front edge of a 13" wheel. (plug what you see in any right triangle calculator using 6.5" for one side)... I've never measured toe or bump-steer that way in my life.
Given this wierdness... for bump-steer use a value of 0.06 degree/inch or less... that's the same ballpark.
Either that or remember that you need to double the value you see for each wheel
john hennessy wrote:
in 03, we were racing a 69 camaro at sebring which had come from england, there was masses of toe out and the car was undrivable before we reduced it considerably...
...this may have been due to a change from goodyear slicks to the mandated hoosier grooved slicks.
Extremely likely... in addition to other potential varibles
Most all autocrossers I know (front or rear steer) use static toe out... as do most short-track oval racers & many other race cars... as it helps with the initial turn-in... front-steer cars will use more of it due to the inherently poor ackermann... back when I ran a Camaro front clipped Late-Model I've used as much as near 1/2" out on the little flat 1/4 mile bullrings.... even rear-steer cars (if based on most street-driven vehicles) usually don't have enough ackermann built into them... on purpose... you don't ever want average Joe driver to oversteer if he goes into a corner too hot... so even rear-steer AXers will start with static toe-out... just less of it then the front-steers as a general rule.
Even if they don't know that ackermann isn't just some guy's name, most limited class oval track racers understand from experience... the shorter & tighter the track the more static toe-out they need to run & lap times get better... up to the point the car starts to get loose on entry or feel twitchy under braking.... an AX course isn't Road America, so similar thinking would apply.
john hennessy wrote:
i know that some toe out results in better turn in but we are dealing with autocross, so i want a car that is precice from lock to lock and am willing to sacrifice the turn in for rapid left to right, right to left ability.
Think about what you're saying there for a minute... you state that toe-out helps turn-in, but that toe-in would be better for lock to lock transitions?
A transition from one direction through straight ahead back to the other is no different than the transition from straight ahead into a turn... other than it being more extreme... so how can toe-out help one and impair the other?
This is especially true for a front-steer... a front-steer is already on the losing end of the ackermann battle & only gets worse as you approach the extremes of steering lock
Next time you're at an AX event eyeball the static toe on what most of the top guys are running (ingore anything you're told, use your own eyes)... most of 'em are likely to be obviously toed-out... there's a reason for that.
Now, rear-steer on something like high-banked ovals & road racing where speeds (& tire loading) are higher, can (& should IMO) start with a small amout of toe-in & let the ackermann do it's thing... so long as there's enough of it designed in, & you have a car that's relatively "neutral"... without a lot of inherent understeer or oversteer... this is one of a very few situations where I'd depart from starting with static toe-out.
The thoery behind that has to do with tire loading & slip angles (which is another of those terms that I hate because it draws the wrong mental picture... like "roll center").... "Slip angle" actually has nothing to do with what you'd normally think of as "slipping"... in the case of the front tires, having the tire skid sideways (AKA: "push") has nothing to do with the slip angle.
Basically, "slip angle" is the difference between the direction the steering is attempting to point the tire and the direction it's actually moving in... that slip angle is going to depend on variables like grip, load, sidewall flex, contact patch deformation, etc.
When you have very little slip angle (think: wide high speed turn, AKA a "sweeper") the turn center of the rear axle is actually ahead of the front axle (rear wheels track outside the fronts)... in this case you'd actually
want a little toe-in... which you'd get from the static setting.
As turn radius decreases & tire loading increases, the slip angle gets larger... the turn center of the rear end moves back toward the rear axle & the rear tire's track transitions to inside the fronts... if you've ever seen an ackermann diagram you know this requires some toe-out... which you get from having the ackermann designed in.
The "gotcha" is that when you're generating high slip angles (on the outside front vs the inside) you don't need (or want) 100% ackermann in the steering geometry.. the difference in slip angles between the inside & outside tire is creating some of that for you already... as the outside tire is operating at a much higher slip angle than the inside... you may have turned the outside front wheel 3 degrees but it's operating like it's only at 2-1/2 or less.
This is why a lot of oval track cars with sticky tires and relatively tall sidewalls actually work well with very little (or even reverse) ackermann... the tires are generating it anyway... a heavy car on a track like Martinsville or NHMS can have the RF operating with a slip angle of up to 5 degrees.
One of the better explainations of this concept I've read published online was by Mark Ortiz...
http://www.auto-ware.com/ortiz/ChassisNewsletter--August2011.htmWith front-steer however, it would be very difficult (if not imposiible) to build enough ackermann into the geometry to warrent static toe-in... unless we're talking about your average grocery-getter where we want Mom to feel nice & comfy when she starts to make a lane change on the highway or hit an off-ramp.... Mom likes mushy.
If you doubt this, go into Wishbone & keep increasing the "Y" value of your outer tie-rod pivot (for a front steer) until the program says your static ackermann is a perfect 100%... now look at the diagram & ask yourself if you could physically put it that far into the wheel.... I doubt it.
If you go back to your base numbers, you probably notice that your static ackermann percentage for a front-steer, is likely a single digit number at best.... more than likely it's a negative value... nature of the beast... If you want to try racing a front-steer car with static toe-in, God bless you & I wish you luck... but my money would be on handling only slightly better than your average school bus.
The design of any front-end has compromises built into it somewhere.. all the way up to and including F1... there's no such thing as the theoretically "perfect" front suspension in practice... the trick is knowing what compromises are better made than others.... which is why there are guys that get well into 6-figures for doing it... and there's no computer program, regardless of how much it cost, that can tell you that.
And what looks good on paper doesn't always work so well out on the track... compromises exist because they have to... just like theoretically perfect ackermann doesn't work anywhere except a parking lot.
Quote:
In theory there is no difference between theory and practice; in practice there is.
At some point you have to stop the mental masterbation & build it... otherwise you'll just be tweaking a "paper car" forever... chasing prefection that can never come.
There have been plently of races won with front-steer, rear-steer, pro-ackermann, reverse-ackermann... you name it... it's all about picking the best set of compromises that works best with the specific package you've got.
john hennessy wrote:
well they were $21.00 for the four delivered so i thought it would be worth a try, i will build a simulator to test the rate.
i will be using pushrods and bell cranks to achieve 2" of suspension compression at ride hight with a further 2" of available bump, the "secret" will be in the bell crank ratio.
alas, i have no idea of the front corner weight as i don"t have a car to weigh but i would assume the front unsprung weight will be around 200 - 250 lbs. but there are other factors to take into account such as push rod angle and the outside location of the push rod to the lca and the chassis pivot position.
Using those as springs
can work... our F500 cars use a piece of rubber 2" diameter by 1" thick as "springs" & no shocks either (all as per the rules)... but, if you're not going to use shocks then remember that you'll want to build in some friction points somewhere as a dampeners.
Also, since you're using what is effectively a rapidly rising rate spring, it's true that the motion ratios & the bellcranks will be key... but remember too that they're a double-edged sword... the motion ratio can reduce the amount of compression travel of the "spring" per inch of wheel travel, but this also "preloads" the spring more which uses up some of your available travel from the get-go.... you can't just go from 1:1 to 2:1 on the bellcrank and expect double the wheel travel and/or to be cutting the wheel rate in half as the spring now has to support twice the weight, it's going to be preloaded more.