LocostUSA.com

I have started mocking up my rack and ran into a problem. With the rack all the way forward my tie rods don't line up straight to the spindle mounting point. Is this a OK or do I need to notch the vertical front tubes of the frame to move the rack forward.

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Speed has never killed anyone, suddenly becoming stationary... That's what gets you.

That's good. Aft of a straight line provides a degree of ackermann.

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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

Thank you. I was worried that having the tie rods angled to the outer front would cause problems.

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Speed has never killed anyone, suddenly becoming stationary... That's what gets you.

I intentionally built my rack mount that way on the spitfire v6. They come just the opposite from the factory.

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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

While we're on Ackerman angle, I have 2 questions that relate to this rack mounting subject.
1. Is there a standard method to measure Ackerman?
2. How much is right for our cars?

100% is shown in the pic and has the steering arms at an angle to a rear steer spindle and the rack straight across where the arms would point to the center of the rear axle (which is not normally the pinion center). Many are stuck with arms that are what they are but moving the rack longitudinally can change the effective Ackerman.

To determine the 100% Ackerman turning radius of a car, as shown in the pic, put the cars rear axle center along a wall, set a laser pointer on each wheel (strap on high mount outrigger for the outside wheel and tilt the laser down) then turn the steering wheel to see if the pointers ever align or get close.

My spitfire was designed with anti-ackerman and usually engine swaps don’t make moving the rack back an option. Some actually move it further forward for clearance. I’ve moved my rack (and engine) as far back as practical but still need to design the steering arms and their angle.

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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

I think I've said this before, but so long ago that I had to look it up again ....

Staniforth (Competition car Suspension, fourth edition, Chapter 4, Ackermann Steering Angle, pp 64-73) discussed Ackermann at length but to cut to the chase and recommended a default case of an included angle of 75 degrees between the track rod and the steering arm for a front steer rack and 105 degrees for a rear steer rack. He shows how to derive this, noting about a plus or minus 2 degree variation across a wide range of cars, and implies that the 15 degree difference from 90 degrees (plus or minus depending on rear or front steer) is good enough .....

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Warren
Isuzu Pickup/SR20DE, +401 COLD frame
Build Log: viewtopic.php?f=35&t=11601

Thanks for bringing Staniforth up again. This is only the seventh time in this decade you’ve brought it up and I still completely forgot. I have this book too. Probably the best reference for Ackermann there is.

The best angle of the steering arm shown in the previous pic for rear steer with a straight rack/tierod extends forward of the rear axle center line for front steer. Splitting hairs a bit, but this angle is through the virtual center in the overhead view of the spindle axis of rotation rather than just the LBJ so caster and KPI play a role in finding the actual angle. The range of common race car wheelbases result in 100%/full ack with an arm 13-17 deg outboard of a longitudinal line parallel to the wheelbase. This space is usually occupied by the brake rotor. All oem spindles I’ve seen have the arms 90 deg or less to a line across the track. 90 deg arms are for rack and pinion applications and less than 90 deg are box type steering with multiple links.

Staniforth’s reference of a 15 deg steering arm is outboard. A straight rack/tierod would form a 75 deg angle between the arm and tierod to reach 90 deg. An angle greater than 90 reduces ack. It does take much forward offset of the rack to reduce the ack to the point of being anti-ack or toe-in when turning.
If the front steer arm is moved inboard 15 deg to zero deg (parallel to a line through the wheelbase) the 75 deg angle must be maintained by moving the rack aft, longer steering arms (slower effective steering ratio/greater turning radius for given rack travel), wider rack with shorter tierods (higher mount to offset bumpsteer) or some combination. The angle between the steering arm and tie rod should still be 75 deg. As you can see, this can be impractical and one downside for front steer systems.

It’s been brought up a few times that slip angles from the loaded outside tire at speed in a wide turn make ack a waste of time but Staniforth points out that ack always helps, it just helps the most during tight low speed turns. Scrub is wasted traction.

I don’t know why we don’t see more rear steer locosts. There always seems to be plenty of room in front of the engine behind the suspension on these builds, rear steer RHD racks are much more common/cheap (typical fwd rack), it would slightly lighter (less intermediate shaft), steering would be slightly lighter (less intermediate shaft to rotate) and would help reduce the polar moment of inertia.

Easy enough to make a cardboard “string” computer to visualize with multiple pin points for rack position, tie rod length, and arm angle.

_________________
Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

I appreciate the discussion of Ackerman. I’m “revising” my front suspension and still selecting a rack so this info will help when I get to the point of our OP, magoins.

I was doing some measuring yesterday and have no ackerman angle at full lock. It is parallel. I maybe looking at it wrong but moving the rack forward should move the outer turn tire straighter and inner tire more giving me some ackerman.

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Speed has never killed anyone, suddenly becoming stationary... That's what gets you.

The percentage of Ackermann is typically measured at a 20 degree turn. Lay a straight edge against the tire on the flood, and mark a line straight head on the flood for both wheels, then turn 20* and mark a 2nd line, and compare angles right wheel to left wheel. If you have 50% Ackermann or better you should be OK. Unless you are driving on a go-cart track, almost all your turns are only going to be only a few degrees, not counting parking lots and some very tight auto-crosses. It is probably more important, depending on car use, to have the steering rack rod end at less then a 7* angle fore/aft, to the steering arm, so the fore shortening with suspension movement, do not impact bump steer on the road and/or race track.
If you are going to notch a frame tube, over kill the reinforcement.
Davew

@davew

Thanks for the practical technique idea for measuring Ackeramnn, Dave. I may use that when my front suspension is finished.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

Your steering arms have virtually no Ackerman, and possibly a slight anti-Ackerman. It's hard to tell for sure from the pics, but either the amount added from your rack position is within your measurement tolerance, or the amount added is just enough to compensate for the anti-Ackerman. Moving it forward will give you some amount less Ackerman, and moving it back will give you some amount more.

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-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

Justin
If given only a choice between Ackermann or bump steer, You need to direct your effort to bump steer. The longer tie rod required for correcting Ackermann, will mean a narrow sweet spot for bump steer. If you have ever driven a car at high speed with bump steer, it is very unnerving have the car dart to the side, when you corning and/or going down a straight way with an undulating road surface.
High Speed Stability Vs correcting Ackermann, which will only give you a very slight improvement in slip angle.
I would say most Seven driver just might exceed the speed limit once in a while It's your call.
davew

Dave,
Why choose if you don't have to?

I would bet that most Locosts are built with double wishbone suspensions having the two control arm inner pivot axis being parallel to both each other and to the centerline of the car. In this specific situation, the bump steer is controlled entirely by the front view geometry, with effectively zero consideration for how far forward/aft any given pivot is.

Now if some 3D geometry is added, then yes, there will be some small additional bump steer effect from a fore/aft rack location. But seeing as this is rarely anything more than a small amount of anti-dive on these cars, this effect is in a distant third place in magnitude to the front view effects of rack length/height to causing or correcting bump steer. Kind of like build/measurement tolerance effects on bump steer. I doubt it would even be noticeable unless the rack width and height were already near perfect as well. And even if that is the case, a corresponding rack height adjustment should be able to effectively eliminate whatever perceptible bump steer effect might have been imparted. It should be easily enough be measured and functionally minimized with mockup testing before finalizing the rack mounts, as has been detailed in other threads here in the past.

That being said, I also agree that minimizing bump steer is typically more important than small changes in Ackerman. So I would probably put more effort into ensuring that is good than worrying too much about not being able to reasonably get much Ackerman out of the parts you're working with. The easy answer is to place the rack back as far as practical considering the chassis, tie rod length limits, and inner joint angle limits. Ideally, at least until the inboard tie rod (as steered) is in-line with the rack at max rack travel. Then adjust rack height as necessary to minimize bump steer effect from all of the combined contributors. If there is appreciable bump steer that adjusting the inner (rack) or outer tie rod end height cannot fix, then there are probably bigger problems than fore/aft rack location.

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-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie