Steering

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spegru
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Steering

Post by spegru » Thu May 09, 2019 1:00 pm

This video gives a rather good overview of bump steer between 9:50 and 11:20
https://www.youtube.com/watch?v=oh535De4hKg
So it's not the position of the steering ball joints being in line that matters it's whether the track rods are the correct length and joints (and therefore the rack itself) are at the correct height.
I reckon this means that steering arm length is not relevant to bump - but track rod length definitely is so shortening those will probably cause problems

The rest of this video series is also rather good



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Post by spegru » Tue May 14, 2019 9:58 am

I wondered how to measure Ackerman and found this https://www.longacreracing.com/articles ... kerman.pdf
Typically, Ackerman is measured by turning the right front 10 degrees to the left. If you have Ackerman, the left front will travel further than the right front. A typical amount would be three degrees in 10 degrees of steering. To simplify, moving the right front from zero through 10 degrees of steering will cause the left front to move say 13 degrees.

Now, I actually have some calibrated steering turntables . So I had a go at measuring the Ackerman on my SE6a this weekend.
I had two goes, using both 10 and 20 degrees (each way) turning angle - and I could not detect any Ackerman effect at all.
ie 10 degree turn on one wheel was still 10 degrees on the other and 20 on one was also 20 on the other

hmmm. Does anyone else have any measurements or specifications?



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Post by rebel alliance » Tue May 14, 2019 11:12 am

Put it on full lock then measure tyre centres front and back with a tape



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Post by gtcse8 » Tue May 14, 2019 1:16 pm

I would have not expected a great deal of Ackerman on a Scimitar as the factory spec`s state the toe in and out is Parallel and the camber is 0-1 +.
King pin inclination is only 9 and the castor 2.4 or 11 on pas would not really effect it.


Mark Wilson. See the Beast on youtube under" RELIANT SCIMITAR CONVERSION LOL.,Se5,Se5a, Se6a, Two Se6b`s,1 & 1/2 GTC`s, SST 1800Ti & not a lot of sense

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Steering

Post by CarlosVanDango » Wed May 15, 2019 9:58 pm

spegru wrote:
Tue May 14, 2019 9:58 am
Now, I actually have some calibrated steering turntables .
interesting :idea: - like spegru says try it on full lock and see what you get then at points going back to straight - i think you will find it will be progressive, it will be interesting to get a base/standard reading but may be only noticeable with more lock - there must be some ackerman i would have thought with the track rod ends being inboard of the trunnion and ball joints that would cause some effect

i have actually this evening collected a full SE5a front suspension complete with steering rack that had been cut out of a scrap car - so i can use that to play with then swap the whole lot over when ive done, and i'll be able to get a measurement of the SE5 rack too :D



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Post by spegru » Sun May 19, 2019 6:46 pm

I just did a more accurate full lock Steering turntable test on my SE6 PAS rack, and the results are as follows

Left Wheel
Max Left 32 degrees
Max Right 29 degrees


Right Wheel
Max Left 31 degrees
Max Right 30 degrees

So aside from the fact that it seems that my rack is not quite centred, it also looks like there is virtually no Ackerman angle at all, or at most 1 degree

I wonder what the effect on driving feel would be if some Ackerman were added, using cranked steering arms?



Incidentally, I do not think that Caster, Camber, or King Pin Inclination have any bearing at all on Ackerman, as that's only determined by the steering arms and rack position



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Post by gtcse8 » Sun May 19, 2019 6:59 pm

Most Scimitars tend to try scrub the front wheels on full lock because they were not engineered with ackerman.
The Scim`s do not need an expensive parallelogram type of steering.
I don`t think cranked steering arms will make any difference at all.

Working on lots of different cars has shown me that cranked arms is only used to allow the steering arms to clear other components.


Mark Wilson. See the Beast on youtube under" RELIANT SCIMITAR CONVERSION LOL.,Se5,Se5a, Se6a, Two Se6b`s,1 & 1/2 GTC`s, SST 1800Ti & not a lot of sense

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Post by CarlosVanDango » Mon May 20, 2019 10:01 pm

Interesting results spegru it looks like we have only 1 deg then - i've been looking more closely at the front end ive bought and the steering arms arn't actually that far same level as the trunnions i don't really know why they don't have much ackermann but i would think if reliant didn't think it necessary then it's not something i'm going to try to add, but it may be because the rack is in front of the centre of the axle to gain more ackermann angle the track rod ends have to be further out not inboard - so it was possibly they couldn't do it as the brake disks were in the way??

this is the front end i've got my plan is to use it to see if i can get a quickrack fitted and possibly replace the trunnions with ball joints - i can do all this without taking my car off the road
IMG_7558.JPG
IMG_7558.JPG (168.84 KiB) Viewed 653 times



spegru
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Post by spegru » Tue May 21, 2019 10:11 am

To be honest 1 deg from full left to full right is so small that it can be considered a rounding, measurement or tolerance error. It's effectively zero.
As to why that appears to be case and considering that Reliant bought Triumph suspension and that that design goes back to the 40s, I doubt that they 'decided' not to have any at all!
However I just realised that although cranked steering arms would be easy to manufacture, it would have the effect of shortening the track rods themselves - so we would enter into the realms of bump steer concerns
Could get tricky...

I'm very interested in the quick rack question, and also ball joints btw
Last edited by spegru on Fri May 24, 2019 8:13 am, edited 1 time in total.



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Post by CarlosVanDango » Tue May 21, 2019 6:45 pm

i've been doing a bit of research and it might be designed to be like that it's called anti-ackermann and basically due to the outside wheel needing more slip angle on a higher speed corner due to being loaded up more, but it is a very complected subject and loads to go through

if you did crank the arms to gain some ackermann you would need to crank them out towards the tyres

if i could find a steering rack to do the job and it was wider i would be ok with widening the track to match it rather than shorten it so as to not introduce any bump steer.

i'll post up any info i find :D



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