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Shortening steering arms

russbost - 4/2/19 at 02:23 PM

I've had a customer ask me if it's feasible to shorten the steering arms to speed up the steering response - bit like fitting a "fast rack" which isn't an option in this case

Obviously it's going to affect Ackerman slightly, tho' I doubt that would cause problems. Just trying to get my head around what else it might affect & wondered if anyone had actually done it & with what results?

Is there any simple way of working out how much length change will produce what ratio of lock to lock change (we'd need to use different length rack spacers to keep lock at same turning circle as current) or is it a case of having to do the basic geometry calcs

Any info & educated comment welcome!

steve m - 4/2/19 at 02:35 PM

It will mean heavier input will need to be applied, as it will have less leverage
a quick rack would be the way to go, and not play with an already perfect ratio on the uprights

nick205 - 4/2/19 at 02:37 PM

If you're asking about shortening the track rods then how would this speed up the steering?

It wouldn't alter the gear ratio between the rack and pinion of the rack so I can't see how it changes the speed of steering. If I'm misunderstanding the question please correct me.

russbost - 4/2/19 at 02:49 PM

nick205, no, not the track rods, the steering arms, (the section that joins the track rod end to the actual hub) they are our own in laser cut mild steel, so no problem in cutting/welding.

Re heavier input surely that would be the same with a quick rack as in both cases you are changing the ratio between input & output?

nick205 - 4/2/19 at 03:44 PM

quote:
Originally posted by russbost
nick205, no, not the track rods, the steering arms, (the section that joins the track rod end to the actual hub) they are our own in laser cut mild steel, so no problem in cutting/welding.

Re heavier input surely that would be the same with a quick rack as in both cases you are changing the ratio between input & output?

Thanks - now I understand what you're referring to and the thought process.

rusty nuts - 4/2/19 at 05:01 PM

Might be better to make or use shorter arms from a different vehicle? Shortening the present arms is likely to involve welding which was and probably still is,an MOT failure unless it is part of the manufacturing process ,if the vehicle in question is a road vehicle Would it be possible to use a smaller steering wheel?

russbost - 4/2/19 at 05:38 PM

As we manufacture the arms in the first place, I can't see how it could ever be an MoT fail, as we would simply have "manufactured" shorter arms - having now thought a little more on this I realise I can maintain Ackerman by locating the track rod end tapers on the same "Ackerman line" back to the centre of rear axle as we used originally.

So far as I can see/think without having any easy way of modelling it it shouldn't affect bump steer & all other caster/camber/KPI angles will be maintained - just trying to make sure I'm not missing something glaringly obvious. It will change the angle in the longitudinal plane of the steering rack by a degree or two, but I'm not seeing anything that would make that a problem

Going for a smaller steering wheel would make the steering feel heavier & would increase the gearing in terms of angular input, but wouldn't affect the no. of turns in going from full lock to full lock, which is what the customer's aim is

Dingz - 5/2/19 at 08:43 AM

I guess the ratio change wii be the same as the reduction in the arm length from the ball joit centre to the king pin? Centre. Ie reduce the arm10% = 10% more effort required and 10% more movement.

Bluemoon - 5/2/19 at 08:43 AM

Some uprights have removable arms is this something you can do? Would allow adjustments latter to satisfy customer?

JAG - 5/2/19 at 08:45 AM

Yeah - higher steering wheel torques required

Also; potential for steering judder increases as you shorten the input arm on the Knuckles. Wheel input loads have a greater output when measured at the Track Rod End location on the Knuckle and can overcome the Driver under some circumstances - leading to Steering Wheel vibrations also known as Steering Judder.

Camber Dave - 5/2/19 at 08:54 AM

The 'included angle' between the steering arm and track rod does affect the ackerman angle.
Rule of thumb
Front rack - Move forward reduces Ackerman (increase included angle)
Move closer to axle C/L increases Ackerman (reduced included angle)

russbost - 5/2/19 at 02:20 PM

Camber Dave, are you sure about that, surely if you move the track rod end location point along the ackermann line between pivot point, TRE & centre of axle the ackermann will remain the same. I guess there could be a fractional change due to the changed angle of the track rod, but that would be very minor, or am I missing something?

Camber Dave - 5/2/19 at 03:35 PM

Hi Russ

I first encountered it on a car I was setting up. It had measurable ackerman but steering arms were cranked to align with the kingpin width.
I did a drawing in plan to understand where it came from.

Conventionally the rack is placed to give straight track rods. This typically gives steering angles of 20 & 23/24 deg.
In your case shortening the steering arms will mean the arm/track rod angle will slightly more than 90 deg
When the rack moves the angular displacement of the uprights will be different than before.

When I got a later edition of Allan Staniforth's 'Bible' it was all explained in a revised Chapter 4.
He had the same experience, and modified his Megapin with success.

Angel Acevedo - 5/2/19 at 03:50 PM

Yes it can be done.
Things to consider.
New hole center is in line with the line that joins the old hole center to the King Pin Axis.
Shorter arm means more effort.
Lock to Lock will be less turns for the same amount of turning radius.
Or same lock to lock turns will yield shorter turning radius.
Ackerman may remain the same provided you shift steering rack back the same amount that the holes have been moved back (Front Steer) or forward (Rear Steer)
Change in ackerman may be so minute that may not be worth it to go through the hassle.

All this is based upon my understanding of books I´ve read but have no Hands On experience, I hpe this may help you make up your mind.
AA

russbost - 5/2/19 at 06:26 PM

Many thanks guys, that's clarified a no. of issues that were giving me some concern, I can now explain to the customer & at the end of the day, it's his choice, he will have to accept that we are going into uncharted territory at his own risk!

I will do some basic drawings to see what looks sensible in terms of amount to move

Will try & remember to report back with results!

SPYDER - 5/2/19 at 08:35 PM

If you maintain Ackerman you will be moving the hole outwards a small amount ie. toward the back of the disc.
Keep an eye on the clearance betwixt disc and TRE.

owelly - 6/2/19 at 12:18 PM

I was swapping steering arms on my buggy to try to sort the geo out....12mm difference in length had a huge impact on the ackerman. Any slight turn of the steering had the hubs all over the place!!

russbost - 6/2/19 at 01:10 PM

Owelly - that would suggest the new arms were not only shorter or longer, but had the TRE taper in a different place laterally, thus making major changes to the Ackermann

CNHSS1 - 7/2/19 at 05:58 PM

Shortening of arms is a common thing in the drift scene (moving ball joint closer to axle line) and you may need to add or move, the steering limit stops to stop the inside tyre going over centre and jamming on lock or touching brake lines etc.

Some uprights (chevette and manta) have a bump in the upright casting as well as a stop in the rack, but not all cars do, so beware when mixing components (like we do!)

As has been said small changes can have big consequences so slowly slowly catchy monkey...

Good look.and like to see the pro
Ject as it progresses

Andy S - 8/2/19 at 02:20 PM

quote:
Originally posted by russbost
Owelly - that would suggest the new arms were not only shorter or longer, but had the TRE taper in a different place laterally, thus making major changes to the Ackermann

It's the angle of the steering arms in plan view that is the issue - as mentioned above - it is like moving the rack forward and changing the angle of the track rods in relation to the rack C/L and the steering arm pivot. It is not in the static centre condition that there is any change or possibly for that matter in bump and droop but in the actual steering of the car. There can be significant changes in the angular positions between the inner and outer pivots on the rack that effect the resultant angles of each wheel in relation to one another that does not follow the form of Ackerman in maintaining the inner and outer wheel angles in the relationship required.

Moving the pivot location on the steering arms and then moving the rack back so that the existing alignment is retained would maintain that balance but there may also be requirement to raise or lower the rack as well to ensure correct alignment in bump and droop due to the outer pivot location moving either in or outboard in relation to the original inner rack pivot location.

Which is why a faster rack is typically the preferred option

Mr Whippy - 8/2/19 at 06:01 PM

This thread is quite funny

russbost - 8/2/19 at 06:04 PM

Mr Whippy, I think you need to get out more!

russbost - 30/4/19 at 12:39 PM

Just thought I would give an update on this.

Ok, so I moved the TRE taper location back down the Ackermann line by around 12mm (I say "around" because I was locating the TRE position by a location already part of the laser cut steering arms as that seemed a lot more accurate than attempting to measure from a hole centre to a new "floating" position). This increased the angle on the track rods (there is already a slight angle from the rack mounting to standard TRE position) by a couple of degrees.

The result - it increased the gearing on the rack from 3 & 1/8 turns lock to lock to 2 & 1/2 turns - exactly the result wanted by the customer. No noticeable difference in general road handling & performance. I did feel that it may possibly have made the car somewhat more susceptible to feedback thro' the steering on very bumpy roads, tho' obviously that's a highly subjective thing with no way easily of objectively measuring it. Steering didn't feel noticeably heavier (front of the car is very light anyway).

Hope this has been helpful & informative!