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Re: Stretton Autumn Meeting. Sunday 3rd November 2019 by StefanN
17 November 2019, 10:51:16 pm

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17 November 2019, 07:34:53 pm

Re: Stretton Autumn Meeting. Sunday 3rd November 2019 by Marek.Z.N
17 November 2019, 06:25:35 pm

Re: Brakes by RhysN
17 November 2019, 03:54:57 pm

Re: Stretton Autumn Meeting. Sunday 3rd November 2019 by TheGiantTribble
17 November 2019, 03:41:10 pm

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le mans '66 by Marek.Z.N
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Author Topic: Stability  (Read 375 times)

StefanN

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Stability
« on: 09 July 2019, 10:06:22 am »
I'm investigating some instability in the Bugatti, so thought I'd try and collect some thoughts and data on key measurements from as many CK's as possible.

I'm going to try and measure:
Weight (kg)  total
Weight distribution by measuring the mass front and rear, with about without driver.
I'm going to measure the weight using bathroom scales, and I'll chock up the wheels I'm not weighing to the same height as the bathroom scales.

Centre of gravity (with and without driver).   I'm going to create a way of tipping the CK in a controlled way until it reaches its tipping point.   Projecting "up" at 90 degrees from the front-back centre line of the CK to the vertical line above where the wheels touch the ground should give a reasonable approximation of the height of the centre of gravity.


So, if you're up for measuring these on your CK it would be really helpful.

I thought it would also helpful to be able to compare side profile photos of our vehicles with a driver on board.  That way we can compare and contrast lots of features of the CK's.   So if you're up for it, would you take a photo of your CK in the following way:
from the same height as your rear axle
from 3m away from the centre line of your CK
at 90 degrees to the centre of the CK
at the mid-point between front and rear axles
with a driver in it.
camera perpendicular to the ground
in landscape format
with a 35mm or equivalent lens (not zoomed in) - this is the typical lens in a smart phone.

I'm going to check toe-in and castor on my CK but they'll really only make a difference with a bit of speed so am going ignore these in this discussion.

Any other thoughts on stability?

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Slack Alice

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Re: Stability
« Reply #1 on: 09 July 2019, 01:25:13 pm »
Tyre pressures!   I searched for ages last night on the American site for references to tire (!) pressures and found absolutely nothing. I had too many other problems at Stretton to test this but it must be a vital aspect.

RhysN

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Re: Stability
« Reply #2 on: 09 July 2019, 01:57:56 pm »
Ironically, probably the best handling of the 30+ CKs I have driven is the Peugeot Quadrilette of Doug Varey. Seat is not over low.In fact not below the chassis rails.
If you go to www.gittrevillegp.com and then the various "Cars" sections, most have side views as you request Stefan. Bloody Mary is a great car (my favourite really) but I don't think it handles as well as the Quad.
I would not take too much notice of the US site for real learned outcomes. Most of those who really know don't post there any more, haven't for a few years. There are exceptions to that comment. Tyre pressures, I recommend 30 ish on the rear as a start point, toe, 1/8" toe in is a good start point, then experiment.
Stefan, I disagree about toe and castor only being at speed, they apply very early. Castor, if it's "darty" try more, if it's heavy to steer, try less. I have gone as high as 12 degrees, very stable in a straight line but very hard work to turn in. I have, inadvertently, gone as low as about 3, frightening and no straight line stability. Every vehicle is different, but start points and the ability, and desire,  to make one change, easily, at a time and make notes, is what will tune it to how you like.

StefanN

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I've still got to double check my front castor but thought these might be interesting.   With a fixed front eye on the leaf spring, flattening the spring pushes the axle back.  The two videos show the same thing form different view points.  The image is two frames from the side video and shows that the castor changes by about 4 degrees over the full range of movement - leaving 1 degree of castor at full deflection.
https://youtu.be/vn-YvbWhbLY
https://youtu.be/n6wii1lW0AY

Its interesting to see almost all the deflection is in the front part of the leaf.  Is this inevitable with this configuration?

RhysN

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Re: Stability
« Reply #4 on: 09 July 2019, 03:59:07 pm »
Yes, that's normal that the deflection does that with a more heavily arched spring, I'm unsure about the flatter ones. In the days when the inspiration cars had this sort of springing there were a number of methods to either make use of the characteristic, or do the best negate the effects. Options were; sliding trunnions, shackle at the front or rear, or both with control linkages, and I'm sure there were other options.
While it's an interesting exercise, somewhere along the line the "science" might become baffling :)
Looking a bit closely at the side elevation video I wonder if your shackle is actually hitting the frame and then something else happens. The shackle should be only angled back a little to allow full travel, but not so much that it goes into forward of vertical on droop.
It also looks as though it goes into negative castor to me. Your still photo seems to confirm that.
Now contemplate what is trying to happen in roll!
« Last Edit: 09 July 2019, 04:05:27 pm by RhysN »

StefanN

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Re: Stability
« Reply #5 on: 09 July 2019, 04:04:38 pm »
Looking a bit closely at the side elevation video I wonder if your shackle is actually hitting the frame and then something else happens. It also looks as though it goes into negative castor to me. Your still photo seems to confirm that.
Now contemplate what is trying to happen in roll!
I did check and its not hitting the frame and I also thought it looked like negative castor but its just a trick of the camera angle.

StefanN

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Re: Stability - Centre of Gravity
« Reply #6 on: 09 July 2019, 04:06:31 pm »
I wanted to get a feel for where the centre of gravity is on the Bugatti so very simply I found some level ground and tipped it up until is was at the balance point.  It tipped to about 50 degrees.  Obviously added a driver changes that significantly, but it doesn't feel like there's anything inherently wrong with the front/back axis CoG.

But there's no avoiding it, the big comfy cushion that I didn't get around to slimming down really does have to go...just look at the difference in driver position!!

Chris Brown

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Re: Stability
« Reply #7 on: 09 July 2019, 04:23:40 pm »
One thought, if I remember correctly you've got wider tyres than the others. Part of the problem with the Morgan was too much grip on the back. While the others could drift round corners I just tipped, I was going to fit a 120/70 with a more blocky tread, then I spotted a 3.00-12 NOS Metzler for £10, so I'm fitting that.
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StefanN

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Re: Stability
« Reply #8 on: 09 July 2019, 04:24:55 pm »
the best handling of the 30+ CKs I have driven is the Peugeot Quadrilette of Doug Varey. Seat is not over low.In fact not below the chassis rails.
If you go to www.gittrevillegp.com and then the various "Cars" sections, most have side views as you request Stefan. Bloody Mary is a great car (my favourite really) but I don't think it handles as well as the Quad.
So what is it about the Quad that makes it handle well?

StefanN

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Re: Stability
« Reply #9 on: 09 July 2019, 04:29:17 pm »
One thought, if I remember correctly you've got wider tyres than the others. Part of the problem with the Morgan was too much grip on the back. While the others could drift round corners I just tipped, I was going to fit a 120/70 with a more blocky tread, then I spotted a 3.00-12 NOS Metzler for £10, so I'm fitting that.
Interesting challenge, so how can I reduce the grip without changing the tyre (because I like the look)?

Chris Brown

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Re: Stability
« Reply #10 on: 09 July 2019, 04:42:30 pm »
The only way I can think of to reduce grip without changing the tyres is to increase the pressure. That way you reduce the contact patch, but also reduce the "springing" effect of the tyre.

As for the seat cushion, mine is 2" of blue upholstery foam, over 1/2" of EVA foam. The upholstery foam gives the comfort, and the EVA stops you "bottoming out" on the seat base. The depth when you're sitting on it is probably no more than 1 1/2".

The EVA I used was children's garden kneeling pads, 4 for £1 at a local boot sale.

RhysN

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Re: Stability
« Reply #11 on: 09 July 2019, 06:03:50 pm »
I think the handling of the Quadrilette comes down to Doug being very analytical, making small changes, one at a time, and analysing what has changed, if it's an improvement trying a bit more, etc. The car has been around for years, and always a learning exercise.(As are all his cars).
In my motorsport background, tyre pressures are used to change grip levels, sometimes it's the opposite of what Chris has said, sometimes not.
I don't have foam except sometimes very little.

StefanN

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Re: Stability - Bugatti side view and weights
« Reply #12 on: 09 July 2019, 07:01:21 pm »
I've no idea how accurate the weight measurements are but:

WhoFrontRearTotalwheelbase
Stefan Bugatti41kg (32%)89kg (68%)130kg (100%)1675mm
« Last Edit: 11 July 2019, 11:07:06 am by StefanN »

TheGiantTribble

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Re: Stability
« Reply #13 on: 09 July 2019, 08:34:53 pm »
The only way I can think of reducing the grip, is to reduce the surface area, maybe
by cutting the individual blocks to be smaller, or maybe a grove down the centre might be easier.
Think Jackie Stewart cutting his tyres at the Belgium GP, although that was to increase water
dispersion IIRC.
Of course cutting them will also spoil the look!

ChrisS

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Re: Stability
« Reply #14 on: 09 July 2019, 08:57:45 pm »
I'm afraid I've already stripped mine down to work on the boat tail, so I'll have to catch up on all the weighing and tipping in a few weeks.  In the meantime, for info, my Gemini front axle is set vertical at rest (no driver) which means I should have the 5 degrees of castor built in by Gemini, and I found it very stable in a straight line, but as Rhys said, this means it takes a lot of effort to turn it in. Now I've got some laps under my belt, I plan to make some controlled changes, either that or I'm going to have to start going to the gym!