CycleKarts GB

Forums => Tech Forum => Topic started by: StefanN on 09 July 2019, 10:06:22 am

Title: Stability
Post by: StefanN 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?
Title: Re: Stability
Post by: Slack Alice 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.
Title: Re: Stability
Post by: RhysN 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.
Title: Re: Stability - change in castor angle and spring movement
Post by: StefanN on 09 July 2019, 03:42:07 pm
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?
Title: Re: Stability
Post by: RhysN 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!
Title: Re: Stability
Post by: StefanN 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.
Title: Re: Stability - Centre of Gravity
Post by: StefanN 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!!
Title: Re: Stability
Post by: Chris Brown 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.
Title: Re: Stability
Post by: StefanN 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?
Title: Re: Stability
Post by: StefanN 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)?
Title: Re: Stability
Post by: Chris Brown 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.
Title: Re: Stability
Post by: RhysN 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.
Title: Re: Stability - Bugatti side view and weights
Post by: StefanN 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
Title: Re: Stability
Post by: TheGiantTribble 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!
Title: Re: Stability
Post by: ChrisS 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!
Title: Re: Stability - checking castor
Post by: StefanN on 10 July 2019, 12:35:32 pm
I needed to double check the caster on the Bugatti.  Not 100% accurate as without dismantling the kingpin I had to judge the angle by eye but the castor looks to be correct.
Title: Re: Stability
Post by: ChrisS on 22 July 2019, 07:59:30 pm
Hi Stefan, I've got some data re the ERA;

Wheelbase - 1665mm
Track front - 930mm
Track rear  - 920mm
Tip angle   - 57 degrees (pic taken at 55)
Overall weight  - 105.4kg
Front  - 35.4kg
Rear   - 70.0kg



Title: Re: Stability
Post by: StefanN on 22 July 2019, 10:13:54 pm
ooo the ERA is looking very nice.

Thanks for the numbers and photos Chris that's really helpful.  I put the picture of your CK alongside mine and made sure they were scaled to the same size and aligned using Powerpoint.   The Bugatti sits marginally higher because of the bigger tyres, but I'm sitting at least half a head higher than you.  We're not exactly the same height but even so that's quite a difference.   When I compare the ERA to the Bugatti with the new lower seat, we're sitting at about the same height.  I'm now desperate to get it onto a bit of tarmac somewhere to test it out and to see what difference it has made to handling.
Title: Re: Stability
Post by: ChrisS on 23 July 2019, 08:17:23 am
Yes it would be great to find some tame local tarmac for the occasional test and tweak between meetings, any ideas anyone?
Title: Re: Stability
Post by: RhysN on 23 July 2019, 08:49:34 am
I have gone and spoken with local industrial estates, NEVER mention race cars. Take something like the information flyer (Stefan has that) to give them a background.
I'm interested to know why the focus on tarmac though. These things are capable on all surfaces.
Title: Re: Stability
Post by: TheGiantTribble on 23 July 2019, 09:51:23 am
Re places you can try CK's. Locally I have

A neighbour has a padock I'm free to use.
Just down the road is a new building estate, weekends there is no building going on the site is empty, lots of flat tarmac and it's not officially a road yet :-)
School car park, unfortunatly our local High school looks their gates, they started this before I finished my CK so no blaiming Bill :-) But you might have better luck.
Local farmer, might not mind depending upon time of year...so not during lambing season
And lastly, get to know some of your local boy racers, they often use shopping centre car parks out of hours, now they might take the mik a bit, equally they
are petrol heads and will love the work that goes into a CK. And a lot used to be gokarters so the recognise and understand the clockwork we use.
I wont admit to driving mine on the road, but it was scary as hell, even passing a parked car you realise just how small and venerable these things are!


Ok back to posts about stability.
Title: Re: Stability
Post by: StefanN on 23 July 2019, 10:20:27 am
I'm interested to know why the focus on tarmac though.
For me a smooth surface will make it easier to observe what is happening on the CK without the added dimension of a random surface.  Once I'm happy with it on smooth then I might move onto rougher surfaces.
Title: Re: Stability
Post by: ChrisS on 23 July 2019, 11:14:13 am
Thanks for your suggestions guys, like Stefan, I think a smooth surface will be better for making subtle changes.
Title: Re: Stability
Post by: Chris Brown on 23 July 2019, 11:58:35 am
My view too tarmac makes it easier to judge how you're getting on with development. I can run mine around the perimeter of the paddock, but it's not big enough to get up any speed, nor is it either FLAT or LEVEL, not even in Welsh terms. As for our road, there's not much traffic, but it tends to be big tractors not hanging about. Knowing my luck I'd meet our six monthly police patrol.
Title: Re: Stability
Post by: RhysN on 23 July 2019, 01:51:04 pm
I get all the comments about getting some comparison, but as before, they are capable elsewhere and what works on tarmac might not be the solution elsewhere. Back in the days when vehicle dynamics and testing was a part of my life, the preferred surface was pea gravel on a hard surface. It shows the characteristics without having to have the speed.
Of course, if you make more than one change at a time, do you know which one was the one giving the outcome?
Title: Re: Stability
Post by: StefanN on 02 September 2019, 10:46:04 pm
So, without access to a test track, I committed the sin of making multiple changes at the same time to try and improve handling:


- I set the toe-in to about 5mm - it was about 15mm
- I moved the front leaf rear shackles to a nearly vertical position hoping to get a more even bend in the spring and hopefully less change in caster
- Lowered the driver position by about 100mm and moved pedals further to allow more space to straighten leg - both will have lowered the centre of gravity
- Played around with tyre pressures settling on 40psi at the back and 24psi at the front

Well between them, the handling was massively improved at Stretton and not at all sketchy in the way it had been.   My gut feel is that lowering the centre of gravity had the biggest impact followed by the tyre pressures.
Title: Re: Stability
Post by: RhysN on 03 September 2019, 07:06:48 am
for next time, can I suggest you try about 2 mm of toe in?
Title: Re: Stability
Post by: StefanN on 03 September 2019, 08:39:18 am
I can try....but
a) if Iím honest I donít think I have that much precision in my set-up
b) contributing to a) is too much play in my yoke set-up.  Iím working on that.
c) why 2mm...on what basis?  What would I have noticed while driving at Stretton that would suggest 3mm too much toe-in?   Iím keen to learn.
Title: Re: Stability
Post by: RhysN on 03 September 2019, 09:41:45 am
Stefan, only reason it's a metric conversion as near as I can from lessons learned where imperial 1/8" was the best by most of the Gittreville.
Title: Re: Stability
Post by: StefanN on 03 September 2019, 10:40:21 am
Ah got it.  I rounded 1/8th (3.18mm) up to 5mm.   Once I sort the play I'll try bringing it in a little and see if I can feel a difference.  I've no idea even how stable it is - ie once I set it to 5mm does it stay they and are my measurements consistent.  Lots of to improve on here.
Title: Re: Stability
Post by: ChrisS on 05 September 2019, 04:53:45 pm
I bought cheap rose joints for the build and they're already showing an amount of play, hard to measure but probably responsible for 2-3mm slop in tracking. There seem to be a wide variety of materials and prices on the market, what are you guys using?
Title: Re: Stability
Post by: RhysN on 05 September 2019, 05:58:30 pm
I bought cheap rose joints for the build and they're already showing an amount of play, hard to measure but probably responsible for 2-3mm slop in tracking. There seem to be a wide variety of materials and prices on the market, what are you guys using?
https://www.mcgillmotorsport.com/ there is a range at very good prices.