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Bump steer plot or values. Manoeuvre test. 
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Part built GTM

Joined: Sat Oct 22, 2016 12:41 am
Posts: 59
Location: Brisbane Australia
GTM: Spyder
Post Bump steer plot or values. Manoeuvre test.
Wonder if anyone has any documentation or links relating to either topic? Things that might be useful:
- any documentation or test results that indicate the bump steer characteristics of the Libra or Spyder
- any information about Type Approval of Libra or Spyder.
- does Type Approval include any assessment of the manoeuvrability of the car?
- any manoeuvrability test results (eg lane-change test) for Libra or Spyder


Tue Apr 09, 2019 1:12 am
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Part built GTM

Joined: Sat Oct 22, 2016 12:41 am
Posts: 59
Location: Brisbane Australia
GTM: Spyder
Post Re: Bump steer plot or values. Manoeuvre test.
I went ahead and measured the bump steer on my Spider and created the following plot.

Image


Sat Apr 13, 2019 12:26 am
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Joined: Sun Oct 19, 2014 10:10 pm
Posts: 805
Location: Bristol
GTM: Spyder
Post Re: Bump steer plot or values. Manoeuvre test.
That doesn't look bad to me - what did you conclude?

Obviously zero would be nice but around 0.15 degree in real life bump changes seems reasonable?


Sat Apr 13, 2019 8:05 am
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Part built GTM

Joined: Sat Oct 22, 2016 12:41 am
Posts: 59
Location: Brisbane Australia
GTM: Spyder
Post Re: Bump steer plot or values. Manoeuvre test.
Yes apparently some negative (toe-out in bump) bump steer is desirable. I lifted the following from a post by Maurice Gloster at Team GTM.com. I hope he doesn't mind the copy and paste.

"Looking through my old bump steer measurement spreadsheets, a quick summary of what I did is.
Front:
1) original as built bump steer was ~ -17 deg/m (ie. toe-out into bump & v v high!)
2) trying different shims under the knuckle steering arm gave a ratio of 2.3 deg/m bump steer change per 1mm shim (with raising the steering arm reducing the amount of -ve bump steer)
3) increased castor (for improved steering feel & returnability) to ~8deg which improved bump steer to ~ -6 deg/m (with no shims on steering arms), which is much better.
4) swapped steering arms over and bump steer reduced a little more to ~ -5 deg/m.
In general you need some -ve bump steer for roll understeer, and with less roll on a Libra you generally need more than a normal saloon car. Eg. Aston & Porsche have values up near -10 deg/m, whereas Jags are around -6.5 deg/m."


I roughly checked the castor on mine to be about 8 - 9* so my bump steer at about 4*/m seems about right (no shims under steering arm).


Sun Apr 14, 2019 11:59 pm
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Joined: Sun Oct 19, 2014 10:10 pm
Posts: 805
Location: Bristol
GTM: Spyder
Post Re: Bump steer plot or values. Manoeuvre test.
Not sure if its linked to that but in the box of bits that came with my car (it was built 2003-2004, I bought it in 2014) there was a pair of steering arms that look like they had been fitted but not used and then replaced. Holding them up against the ones on the car they look very similar but slightly different hole offset and a slight upward kink in the ones on the car. I wonder if a slightly different version of the steering arm was rolled out around the time it was built to cure some geometry issues.


Mon Apr 15, 2019 7:27 am
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Part built GTM

Joined: Sat Oct 22, 2016 12:41 am
Posts: 59
Location: Brisbane Australia
GTM: Spyder
Post Re: Bump steer plot or values. Manoeuvre test.
Sounds like it.

A kink which changes the height of the tie rod end will change the bump steer characteristics. The simple ways to change bump steer are shims under the steering arm and castor changes. I am happy to run a bit more castor. The other effects are stronger self centring, better turn in and better front grip in low speed corners.

Different hole positions will change the ackerman by moving the tie rod end left or right. Because they are "bent" in plan view, the steering arms can be swapped left and right to change the ackerman significantly.


Last edited by Spider on Tue Apr 23, 2019 8:27 am, edited 1 time in total.

Tue Apr 16, 2019 12:58 am
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Looking like a GTM

Joined: Wed Feb 09, 2011 1:12 am
Posts: 161
Location: Derbyshire
GTM: Libra
Post Re: Bump steer plot or values. Manoeuvre test.
I have some information some printed off the gtm web site when under rdm ownership this said that rover xpower chassis development team where involved with significant changes to suspension hardware and geometry in 2004 this was when the pillow balljiont rear suspension was introduced .Maurice glovers car was featured in complete kit car magazine in the write up it said he worked as a suspension engineer for jaguar and was involved with changes to the libra suspension so this maybe was when the steering arm were changed.


Mon Apr 22, 2019 10:19 pm
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Part built GTM

Joined: Sat Oct 22, 2016 12:41 am
Posts: 59
Location: Brisbane Australia
GTM: Spyder
Post Re: Bump steer plot or values. Manoeuvre test.
Thanks Mark.
Is there some way I can I get a copy of that information?


Tue Apr 23, 2019 8:25 am
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Looking like a GTM

Joined: Wed Feb 09, 2011 1:12 am
Posts: 161
Location: Derbyshire
GTM: Libra
Post Re: Bump steer plot or values. Manoeuvre test.
so for delay with reply i don,t have computer skill to do it but have given it to someone who will try to scan it then email i to me then i will try to email to you


Sun Apr 28, 2019 10:28 pm
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Part built GTM

Joined: Sat Oct 22, 2016 12:41 am
Posts: 59
Location: Brisbane Australia
GTM: Spyder
Post Re: Bump steer plot or values. Manoeuvre test.
Great, thanks Mark.

Success - the transport department has approved my Spyder for registration. Should be on the road in a week or two.


Tue Apr 30, 2019 6:35 am
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Joined: Sun Oct 19, 2014 10:10 pm
Posts: 805
Location: Bristol
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Post Re: Bump steer plot or values. Manoeuvre test.
Spider wrote:
Success - the transport department has approved my Spyder for registration. Should be on the road in a week or two.


Great news! Was going to say just in time for the better weather but isn't that the other way around down your way?


Tue Apr 30, 2019 1:13 pm
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Part built GTM

Joined: Sat Oct 22, 2016 12:41 am
Posts: 59
Location: Brisbane Australia
GTM: Spyder
Post Re: Bump steer plot or values. Manoeuvre test.
We always have "better weather" down here. :D

Having said that, it is a bit showery this week and these are not wet weather cars. BTW, has anyone made inner mudguards or similar protection for the back of the headlamps?


Tue Apr 30, 2019 11:56 pm
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Looking like a GTM

Joined: Wed Feb 09, 2011 1:12 am
Posts: 161
Location: Derbyshire
GTM: Libra
Post Re: Bump steer plot or values. Manoeuvre test.
sent email 05/05/2019 hope you have got it


Mon May 06, 2019 10:03 pm
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Part built GTM

Joined: Sat Oct 22, 2016 12:41 am
Posts: 59
Location: Brisbane Australia
GTM: Spyder
Post Re: Bump steer plot or values. Manoeuvre test.
Email received and files successfully downloaded. Thanks Mark, there is lots of good info there including some I hadn't seen before. I hope you don't mind me posting them (cropped) here for the benefit of other members?

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Tue May 07, 2019 1:53 am
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