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PostPosted: September 22, 2010, 12:31 am 
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Any pointers? Does anyone here use these kind of rack? I need a 360 degree lock to lock steering.


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PostPosted: September 22, 2010, 9:53 am 
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Variable rate steering works by using a power steering pump that provides less assist at low speeds. The actual steering ratio does not change, only the steering effort.


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PostPosted: September 22, 2010, 11:01 am 
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While certainly most cars simply use a variable-assist system, there are in fact variable ratio systems as well. Years ago the idea used standard style power steering systems, but designed them such that the ratio changed in conjunction with the steering angle. More recently there have been systems like BMW's 'Active Steering' that are actually capable of varying the ratio based on the speed of the car, and can even make small steering angle adjustments for the driver in conjunction with the stability control system...I'm not familiar with exactly how they achieve this, but I'm sure that there are reasonably good explanations available somewhere on the web.

That being said, you'll have to be more specific when you say a 1-turn lock to lock. Unfortunately that doesn't mean much without knowing how much the rack needs to move. You'll want to figure out the inches of rack travel per rotation of the steering wheel/pinion. Since you want one turn lock to lock, then you simply need to determine how many inches the steering rack needs to move to get the wheels to turn their desired full range of motion. If the rack that best suits your needs has a larger than necessary range of motion, then you just need to set some type of steering 'stop' on the rack. If there are no off the shelf racks quick enough for your goals, there are also steering 'quickeners' available...Which still may or may not be able to get you exactly where you want to be, if you actually want this fast of steering while using mass production car uprights. Depending on the design and goals of your car, I'm also a bit skeptical as to whether or not one turn lock to lock will really be desirable.

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PostPosted: September 22, 2010, 1:01 pm 
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Its possible to do variable rate manual steering racks. I found two ways to do that already.


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PostPosted: September 22, 2010, 1:03 pm 
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Well it's not variable rate, but to get quicker steering, is there anything significantly wrong with making it a bit simpler by using a chain-sprocket pair to change the ratio somewhere on the steering shaft? If you can fit it that is.

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PostPosted: September 22, 2010, 4:19 pm 
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foxk56 wrote:
Well it's not variable rate, but to get quicker steering, is there anything significantly wrong with making it a bit simpler by using a chain-sprocket pair to change the ratio somewhere on the steering shaft? If you can fit it that is.


You could also do exactly like you have in your avatar and just use two gears that work together, thus reducing a failure point.


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PostPosted: September 22, 2010, 4:20 pm 
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flat4 wrote:
Its possible to do variable rate manual steering racks. I found two ways to do that already.


Why not list out a quick definition(1-2 sentence) of each the methods then?


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PostPosted: September 22, 2010, 4:24 pm 
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flat4 wrote:
Any pointers? Does anyone here use these kind of rack? I need a 360 degree lock to lock steering.


Why do you need 360º lock to lock steering?


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PostPosted: September 23, 2010, 12:38 am 
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If you need really a fast steering ratio, one trick is to use what's called a "steering doubler". A doubler with a 2:1 ratio will spin the steering shaft twice as fast at the steering rack as you spin the steering wheel. It's probably not a good idea for street use unless you have a huge amount of experience (and the requisite muscle memory) driving cars far more twitchy than anything being sold as a production vehicle. If your reflexes are set to "typical car" then in a panic situation you will likely over-steer the front wheels, loose traction and just end up skidding in a straight line.

Bonus points: Both the new Honda and Toyota Prius power assist systems are very light weight, use electric power, and provide their assist to the steering shaft. People are starting to hack the electronics on these systems to create custom variable rate power steering assists for non-oem projects. Great possibilities for running huge offsets or really wide tires with a steering doubler and without feeling like a trip to the corner store is a workout at the gym. Very handy for endurance racing applications. There's a very good chance I will look to retrofit something like this once the car is up and running.

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PostPosted: September 23, 2010, 1:19 am 
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dilbert wrote:
flat4 wrote:
Its possible to do variable rate manual steering racks. I found two ways to do that already.


Why not list out a quick definition(1-2 sentence) of each the methods then?


1. use a screw to move a cam plate that actuates the steering arm. straight cam is fixed rate. curved cam will be variable rate.
2. use a bevel gear to drive a screw that then moves the steering arm. the fixed pitch screw is constant rate variable pitch screw will be variable rate.

i prefer number one.


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PostPosted: September 23, 2010, 1:25 am 
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gtivr4 wrote:
flat4 wrote:
Any pointers? Does anyone here use these kind of rack? I need a 360 degree lock to lock steering.


Why do you need 360º lock to lock steering?


I want both my hands on the steering wheel when going around a curve.


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PostPosted: September 23, 2010, 1:49 am 
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erioshi wrote:
If you need really a fast steering ratio, one trick is to use what's called a "steering doubler". A doubler with a 2:1 ratio will spin the steering shaft twice as fast at the steering rack as you spin the steering wheel. It's probably not a good idea for street use unless you have a huge amount of experience (and the requisite muscle memory) driving cars far more twitchy than anything being sold as a production vehicle. If your reflexes are set to "typical car" then in a panic situation you will likely over-steer the front wheels, loose traction and just end up skidding in a straight line.

Bonus points: Both the new Honda and Toyota Prius power assist systems are very light weight, use electric power, and provide their assist to the steering shaft. People are starting to hack the electronics on these systems to create custom variable rate power steering assists for non-oem projects. Great possibilities for running huge offsets or really wide tires with a steering doubler and without feeling like a trip to the corner store is a workout at the gym. Very handy for endurance racing applications. There's a very good chance I will look to retrofit something like this once the car is up and running.


Well i dont drive much around public roads yet. I usually take public transport to go from point A to point B. The last thing i drove was an R1 which is in tiny little pieces in Singapore. Besides that i only drove go karts until they banned me for being too old to be spending too much time in one. Singapore can be boring. Muscle memory is not a problem.

I need my cg to be lower than usual and i want the rack to be parallel with the top a arm. it needs to be light weight or it can be heavier if the housing does more than just keep the rack in proper orientation.I might use the steering rack housing as a structural cross member in front of the engine. That will save me the weight of the mounts and universal joint if i can keep the steering shaft straight. Only a "custom" steering rack will make that possible.


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PostPosted: September 24, 2010, 9:01 am 
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All his stuff needs to go to the Theory forum.

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PostPosted: September 24, 2010, 10:08 am 
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flat4 wrote:
flat4 wrote:
Its possible to do variable rate manual steering racks. I found two ways to do that already.

1. use a screw to move a cam plate that actuates the steering arm. straight cam is fixed rate. curved cam will be variable rate.
2. use a bevel gear to drive a screw that then moves the steering arm. the fixed pitch screw is constant rate variable pitch screw will be variable rate.

I don't understand what you're describing. (Not that that bothers me...)

If you just want to reduce your steering wheel degrees, one of the simplest ways is to shorten the steering arms on the upright. Each inch of rack travel would then turn the wheels more.

If you really want a variable ratio of steering wheel degrees to knuckle degrees, and you're set on doing something exotic to achieve it, I'd take a look at some bizarre link geometry. We've all got an inner and outer tie rod end. Add a middle tie rod end to the mix with a floating link to the control arm. Something like a commercial door closer, maybe. I'm not smart enough to design such a thing, and I'd be terrified to drive it over 5 mph, but maybe it would be fun to design. File this one under "bad ideas".

-dave "about 1.5 posts from the 'In Theory' section and counting..." hempy

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PostPosted: September 24, 2010, 10:21 am 
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Location: St.Thomas Ontario
Even then there are two types of variable rate, would you want the steering wheel to wheel turn angle to increase as the steering wheel turns?
From straight up to 90deg left the wheel turns 10 degrees (I just made up numbers), then from 90deg to 180deg(straight down) the wheel turns to 30degrees, so the rate doubles as you move past horizontal. This could be done mechanically some how. The key is that it has to be a very smooth increase.

OR

The rate at which the wheel moves is relative to your speed. So when below 20mph it doesn't take much steering wheel input to turn the wheels a lot, but at 60mph it takes a lot more rotation of the wheel to move the wheels the same amount. This would be because the higher amount of rotation required at higher speeds would give you more detail on small steering inputs. You should never need to go to full lock or probably even half of full lock at 60mph. This would probably have to have some kind of electronic control to configure it.
EDIT: The S2000 does this, I think...: http://www.wikipatents.com/US-Patent-5503239/variable-gear-ratio-steering-device


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