What causes heavy steering at racing speeds

[Jac Mac]

Ross, As pointed out earlier, anything you can do to reduce that figure will help. Some suggestions;
1., If you intend to persevere with the current upright consider some hubs that have the stud face closer to the upright/ and if possible move the rotor closer to the upright to facilitate that. This will also mean remounting your calipers---- once you work out the financial cost in time etc it may be more beneficial to simply obtain some uprights along the lines of those shown by Iain or Frank.
2. Changing the sizes of your Inner Outer rim halves to decrease the offset-- bear in mind that you have to maintain running clearance for your Caliper / Rotor and that there is a greater chance of fouling the w/bone's & tierods with this change.
3. All of these efforts to decrease offset means that you will need to fabricate new longer wishbones to reinstate the original track width.
4. Dont forget to check your wet rims for arch clearance with these mods as with the hub face now being out board from its old position they will be closer to the wheel arch.

Jac Mac

 

[Jim Cowden]

Ross

FF would be so light and tyres narrow I would imagine the effect to not be as prononced as a 9" front rim.

Ross - Bloody hell

Tim - you are starting to scare me.

Question
With the ackerman the a line is drawn from centre of the rear axle to the lower ball joint.
As jac mac has explained that I can use the upper ball joint as well.
So to my way of thinking this means that looking from the front of the car I have a projected line from the upper to lower ball joint.
If I land my steering arm on this vertical line height wise and still land on the line to the rear axle centre from the plan veiw this is ok.

Jim

°j°u°s°t°°f°o°r°°u°°p°e°t°e°

 

[Richard]

Hi Richard, I was thinking in terms of overall movement inc' up and down when I mentioned bump steer, I refined it to backwards and forward re effort as that's what was presented to me to try.
Looking at it I was a bit ambiguous...

Here's one you can cut and paste - Δ.

Ross - Bloody hell!

Tim.

Hi Tim
Yes, for sure up / down with / without fore and aft will Δ your bumpsteer.

Thanks for the Δ!

 

[Trevor Booth]

Frank,
ratios are very critical and can drive one nuts, the Trevor version has an eccentric for the pivot of the rocker and the push rod anchor on the chassis. Personally have never explored its use on a 40, will be interested to see how goes on Iain's car. Have found it usefull on high downforce cars and also on softly sprung rear suspension.

Ross N
off set is 1/8 mm inboard of the rim centre so scrub =4 1/2 =114mm. Thats reasonable enough in the circumstance, would be nice if it was less.
What size tyre are you running on the 9" rim

Jac Mac,
I would be suprised if Ross's car is rolling on positive camber, he posted some photos of the front tyre some time ago, it did not look that bad but temps will tell the story (hint,hint RossN)

BTW success is 99% failure

Russ N,
You get longer wheel travel for the same length shock and lower shock velocity + rising rate. This all depends on the geometry of course.

º Ø ß find them at www.asciitable.com/extend.gif

Not all keyboards will support the whole of the table if your running WINDOZE, but DOS and LINUX generally ok

 

[Richard]

Ross
Question
With the ackerman the a line is drawn from centre of the rear axle to the lower ball joint.
As jac mac has explained that I can use the upper ball joint as well.
So to my way of thinking this means that looking from the front of the car I have a projected line from the upper to lower ball joint.
If I land my steering arm on this vertical line height wise and still land on the line to the rear axle centre from the plan veiw this is ok.

Jim

°j°u°s°t°°f°o°r°°u°°p°e°t°e°

Jim, I've forgotton more than I ever knew about Ackerman, but I understand that you draw lines projected along your steering arms until they intersect (if they don't, you have no Ackerman or even anti-Ackerman). The amount of Ackerman you have is usually expressed as a percentage. I think 100% is where the lines intersect on the centre of the rear axle, and this will be your best setup for pushing a car around a paddock. Jury appears to be out as to the value of +ve, -ve or zero Ackerman at speed. Perhaps just a question of personal feel (excuse me).

Given the shallow angle and fairly long length of these projected lines, it probably wouldn't matter if your steering arms were not on the plane from the UBJ to LBJ for ackermann (may be preferable in theory if you have a car with the steering rack to the front of the hub and your steering arm is going to hit the rim or tyre) but in practice, if the steering arm is off that plane it will be really, really bad for bump steer, unless you have some hypercomplex stuff going on with your inner steering uj location.

 

[Trevor Booth]

Jim C
Your figuring is correct. however the line through the upper and lower ball joint is not vertical. Castor takes it off vertical. I would not necessarily be aiming for 100% ackerman you probably wont have enough wheel lock to notice the difference between 75% and 100%. Steering rack positioning can correct it to a degree ( º)

 

[Jim Cowden]

Ok
Thanks Richard and Trevor
I feel ok with what I have done.
All the info can get you thinking.

Jim
°j°u°s°t°°f°o°r°°u°°p°e°t°e°

 

[Russ Noble]

Trevor,

I may not be interpreting Iain's photos correctly, or I may have my theories completely cocked up, but what I am seeing is an arm the approx midpoint of which pivots on the wishbone, the inboard end is attached to the chassis by a link, the outboard end is attached to the shock. The movement of the shock will be magnified (approx) by the ratio of the pivoting arm. In other words the shock in this case looks as though it will have about twice as much travel. Or is the photo misleading and the system is different? Or are my theories well and truly up the creek?

Cheers

 

[Ross Nicol]

Damn Trevor I thought I had that figure correct.Tyre size is a 265 - 17" Dunlop slick.
Ross

 

[Ross Nicol]

Scrub Radius is not far from that of your car Trevor, yours 110mm, mine 114mm.

Ross

 

[Bob MacDonald]

Hi to All ---Ackermann Angles !
I wondered about Ackermann angles on the 40 some time ago.
After some thought and lots of trig I came to the conclusion that most texts/books have been a little lazy in lifting the Ackermann stuff from previous publications without real consideration of the evolving changes to the steering mechanisms.
The concept of the steering arms projections meeting at the rear axle was approx true back in the worm and wheel days of steering gear where a fully floating track rod was employed.
The geometry associated with the present steering racks with an anchored centre section and articulated ends is all together geometrywise another ball game.
With present day articulated racks the steering arm projections will have an intersection closer to half way between the axles. If I remember correctly, on the 40 it's more like 60% from the front axle. The exact point will indeed depend upon the particular layout in each vehicle, --.rack position, rack mid section length, steering arm length, KPI, wheelbase, track --to quote a few that come immediatly to mind.
I had intended to modify my GTD steering to give some, if not full ackermann angle, (havn't got around to it yet of course), and in consequence I knocked up an exl sheet giving a simplfied 2/3' ish dimensional (more 2 than 3) model since it seemed a lot less work doing the investigations at the keyboard rather than spannering change after change.
I could be totally wrong of course but the maths didn't seem to lie.
Prehaps someone with deeper understanding of the subject will point out the errors in my conclusions.

Re Power assisted steering -- I think I have sussed the situation, happy to expand but this reply is already a bit long so will forgo at present.

Bob MacDonald

 

[Frank Catt]

Bob, nice to see you enter this one. I for one totally respect your scientific knowledge and mathematical approach, and I am sure we can all learn a lot from your input. Please give us some more detail of your work, Frank

 

[Malcolm]

Bob, can you just send Frank a copy of your spreadsheet, I think he asking for it!

 

[Ross Nicol]

I've taken photos of my front tyres but I've left the download cable at home. I'll post them tonight for the panel's perusal mmm that rhymed quite well.The wheels are still on the car and uncleaned so a good chance for post race observation.

Ross

 

[Chris Duncan]

""Scrub Radius is not far from that of your car Trevor, yours 110mm, mine 114mm.""

Too much scrub radius. Should be less than 25mm. 0 is optimum. As light as these cars are in front, if the steering is heavy there's problems.

It is hard to find an upright that gives you zero scrub with a 235 Halibrand tire wheel setup, and 255 is worse. One reason I scratch built mine.

My setup.

4 deg King pin
3 deg Caster
3 deg Ackerman
0 camber
20mm scrub
235's
2.7 turn 240Z rack
13 in steering wheel

easy to park, good turn radius
one handed steering at any speed
tracks nice, light and quick like a go kart.

If changing uprights is too much trouble, Frank Catt's setup may be the way to go.

 

[Ross Nicol]

How wide are your rims Kalun? and what is their offset? I have 205 road tyres on 7" rims and they are fine for the road but it's a totally different story on the track with 9" rims and sticky slick rubber.

Ross

 

[Chris Duncan]

8" wheels with 1/2" outer offset. The most available with these wheels.

Suspension is always compromise. You start with the wheel tire setup, the most outer offset available, then with the standard race caliper with pistons on both sides of the rotor, that takes up a lot of room. It's hard to get to zero scrub especially with anything off the shelf. Look at lots of the supercars now too, LOTS of outer wheel offset to the point there's no outer rim lip, don't think anyone makes a Halibrand or BRM with no outer rim lip. When you go inboard with the rim you run into clearance problems making the a-arms narrower or reducing the turn radius. And don't think turn radius is not important in racing, just wait until the back end slips out and you need max recovery steering angle. ....auuugh!!!

 

[Richard]

Hi Kalun

Are you sure a scrub radius of zero is optimum? Sure, for the lightest steering, but my understanding is there should be a least some (15mm - 25mm) for "feel" (that subjective element).

I see you've also gone relatively light on for KPI, camber and castor. I guess it really is the results of the whole package working together that count.

 

[Chris Duncan]

Richard,

Feel or tracking can always be increased with caster, scrub is not usually adjustable. Scrub causes other problems besides increased effort, it side loads the suspension.

Too much king pin doesn't allow the contact patch to stay flat to the road when turning.

 

[Ross Nicol]

Yes I have to say most race cars today seem to have wheels with the centre near the outside rim. Now my question is - would this be solely to reduce scrub radius?

Ross