LocostUSA.com

Learning how to build Lotus Seven replicas...together!
It is currently October 15, 2018, 5:10 pm

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 57 posts ]  Go to page Previous  1, 2, 3, 4  Next
Author Message
 Post subject:
PostPosted: March 21, 2008, 7:49 pm 
Offline
The voice of reason
User avatar

Joined: January 10, 2008, 4:47 pm
Posts: 7551
Location: Massachusetts
Andrew, I see. Interesting. You could also make something threaded or use a rod end. Look forward to what you find out. Sometimes, the more knobs I have the more confused I get...

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: March 22, 2008, 4:21 am 
Offline

Joined: October 17, 2007, 6:02 pm
Posts: 105
Location: Denmark
Interesting thread!

Another angle - under cornering one would want to introduce negative camber current generation tyres generate maximum lateral thrust when leaning into the turn around -2.5degrees (leaning in like a motorbike). Assuming roll centers lower than CG centripetal affects will be doing their best to roll the car in the opposite direction, so suspension geometry needs to attempt to counteract this by introducing camber in bump. But, here is the contentious part, one could also augment this by introducing camber with rotation of the upright. Hence my feeling that all things being equal, higher caster would be a good thing (caster introduces camber on turn, KPI does the reverse).

Of course all things are not equal, there will be trade offs:

1 Lots of caster will generate a longer trail, unless the wheel center can be moved ahead of the line upright's "king pin".

2 Reducing KPI means less jacking affect.

3 Um anyone want to throw some others in?

It must be possible to compare the contribution to self centering of KPI, scrub radius & trail. I'll have a look in Milliken for suitable formulas.

Any one got a view on this statement: The optimal upright geometry would introduce camber as the wheel is turned, whilst preserving a similar self centering affect as an existing design.

Picking an existing design could be contentious but people rave about Caterham's set up so could use that as the datum.

_________________
Before you judge a guy walk a mile in his shoes. Then when you judge him, you've got a mile head start and you've got his shoes on:)


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: March 22, 2008, 7:41 am 
Offline
Automotive Encyclopedia
User avatar

Joined: December 22, 2006, 2:05 pm
Posts: 5717
HZJ

The bike comparison was strictly from a straight line stability perspective, other wise it's apples to coconuts. My Silverwing's caster is 28 degrees 30 minutes.

A.Moore, do you feel the following is this correct?

1. KPI and caster should be nearly equal.
2. KPI should be minimal, which for practical purposes is around 4 degrees.
3. A very small positive scrub for feel is preferred, and how much depends on the kpi/caster to prevent "flip flopping" pos/neg in turns.

_________________
MV8

https://www.youtube.com/watch?v=Om3C1Ep ... D3E18BB447


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: March 22, 2008, 11:38 am 
Offline
The voice of reason
User avatar

Joined: January 10, 2008, 4:47 pm
Posts: 7551
Location: Massachusetts
Puk:

Quote:
Any one got a view on this statement: The optimal upright geometry would introduce camber as the wheel is turned, whilst preserving a similar self centering affect as an existing design.


Sounds sensible, but my reservation is that the wheel should not require much turning for cornering. If you are in that situation things are already wrong, so pursuing this as a fix is a bit of false goal, so to speak. The more steering angle you require, past a bit, the more clumsy the car will feel. Much too much understeer.

I think negative scrub is fine/preferable for people that don't want to be real race car drivers. Practically though, you might not want to use wheels that produce such a narrow track, on the other hand wheels from FWD cars would be pretty common.

Glad to see this thread. Maybe would be nice to get some plans for homemade front uprights. They would seem a little simpler then the rears that many people build already. Also would like to see how an "optimal" design would look compared to available stock and replica ones.

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: March 22, 2008, 11:46 am 
Offline
The voice of reason
User avatar

Joined: January 10, 2008, 4:47 pm
Posts: 7551
Location: Massachusetts
MV8, understood about the motorcycle example. It's just interesting that although the use such a large number for castor, it is still important for it to be "right", a small change - maybe 2 degrees- made that other motorcycle almost unrideable for me.


Quote:
2. KPI should be minimal, which for practical purposes is around 4 degrees.


But if some/many FWD cars have negative scrub, doesn't that imply they could have zero KPI? Sigh, head starting to hurt...

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: March 23, 2008, 10:13 pm 
Offline
Always Moore!
User avatar

Joined: November 9, 2007, 3:40 pm
Posts: 3940
Location: Pittsburgh, PA
Miatav8,MstrASE,A&P,F wrote:
A.Moore, do you feel the following is this correct?

1. KPI and caster should be nearly equal.
2. KPI should be minimal, which for practical purposes is around 4 degrees.
3. A very small positive scrub for feel is preferred, and how much depends on the kpi/caster to prevent "flip flopping" pos/neg in turns.


1. For a starting point, yes. We're lucky enough now-a-days we don't need to rely on string computers and regular desktops and laptops with the right program can tell us way more than we want to know about a suspension. Based on this, I like to start with about 5 degrees of caster and 1 degree of KPI and move from there and let packaging drive the design for awhile.

I find it very interesting to set suspension points then actually roll and pitch the car on the screen to where it should be in turns and see what the tire actually does and make changes from there.

2. Yes. I'd say the less the better for a performance application as long as it cannot go negative from deflection, adjustments, etc.

3. Yea I'd agree with that statement also. Rack ratio and steering arm length also plays a big role in how easy/difficult the car is to steer and whether you have good feedback or a heavy steering wheel.

_________________
-Andrew
Build Log
Youtube


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: March 24, 2008, 6:30 pm 
Offline

Joined: October 17, 2007, 6:02 pm
Posts: 105
Location: Denmark
Quote:
It must be possible to compare the contribution to self centering of KPI, scrub radius & trail. I'll have a look in Milliken for suitable formulas.


Searched Milliken in vain - it must be possible, but my physics isn't up to figuring it out and I guess if Milliken don't quote a formula then the great and the good haven't found deriving one important. What my search did throw up were several statements to the affect that except at low speeds the the self centering affect of KPI was negligible compared to that of trail. Also advice to keep scrub radius positive for a rear wheel drive car and to allow KPI to be driven by desired scrub radius.

This leads me to believe that minimal KPI is possible and unlikely to be a bad thing, and that the actual value would be driven by the desire to achieve small positive scrub radius and packaging considerations.

_________________
Before you judge a guy walk a mile in his shoes. Then when you judge him, you've got a mile head start and you've got his shoes on:)


Top
 Profile  
Reply with quote  
 Post subject: Upright Design
PostPosted: March 24, 2008, 7:41 pm 
Offline

Joined: October 17, 2007, 6:02 pm
Posts: 105
Location: Denmark
This is a quick sketch of a "universal" upright that I've been toying with, a single design for all 4 corners. It is designed around:

    Bolt on hub (Ford Mondeo)
    Bolt on ball joints (GM Chevette / Austin Maxi / VW Golf)
    Track rod end and upper ball joint in same plane to remove bump steer
    Track rod end and upper ball joint mount on a demountable arm, with shim(s) sandwiched between it and the upright allowing camber changes without upsetting tracking (Porsche 962 design)
    Upright from steel rectangular hollow section


Lets see if the image shows up:


Attachments:
File comment: Steel Hollow Section Universal Upright
Upright01800.jpg
Upright01800.jpg [ 123.44 KiB | Viewed 4972 times ]

_________________
Before you judge a guy walk a mile in his shoes. Then when you judge him, you've got a mile head start and you've got his shoes on:)


Last edited by Puk on March 25, 2008, 10:04 am, edited 2 times in total.
Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: March 24, 2008, 8:10 pm 
Offline

Joined: March 22, 2008, 7:36 pm
Posts: 1
Location: Australia
Hi I'm new here. I've recently reduced the scrub radius on my Locost from around 80-90mm to about 16mm by machining up some new hubs, the Ford Cortina uprights I'm using have about 5 degrees of KPI it has reduced the kickback through the wheel and made the steering lighter, I had previously shortened the rack to eliminate the bump steer so all in all it's been very worthwhile, if I had a bit more self centering it would be perfect.

Cheers, Graham


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: March 24, 2008, 10:21 pm 
Offline
The voice of reason
User avatar

Joined: January 10, 2008, 4:47 pm
Posts: 7551
Location: Massachusetts
Puk, thanks for the info. That seems to put KPI in perspective.

Your picture didn't show up...

I saw you enquiring of bonding Aluminum on the UK forums. I think a helpful thing to get a bond on clean aluminum is to wet sand the surface with mixed epoxy resin. That way you don't get anymore air contamination. Obviously also solvent wiping with paper towels and never wiping twice, always using new paper etc....

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: March 25, 2008, 3:39 am 
Offline

Joined: October 17, 2007, 6:02 pm
Posts: 105
Location: Denmark
Horizonjob - picture resubmitted should be ok now.

Thanks for the tip regarding bond prep. Keep them coming!

_________________
Before you judge a guy walk a mile in his shoes. Then when you judge him, you've got a mile head start and you've got his shoes on:)


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: March 25, 2008, 9:38 am 
Offline
The voice of reason
User avatar

Joined: January 10, 2008, 4:47 pm
Posts: 7551
Location: Massachusetts
That upright looks doable, but likely to be very heavy. Flat plate is not very friendly that way. There's someone selling an upright like that on the UK forum - it is aluminum billet and hollowed out to save the weight.

The tradeoffs are difficult, home construction is different then even limited production and would likely vary according to the skills and available tools of each builder.

The rears seem harder to come by, I see a couple of choices for fronts at race cars parts places, but no rears - they seem more specific to each design...

Keep at it. See if you can get a look or buy an old aluminum race car to answer some of your practical monocoque questions. Maybe a Ralt RT1? Thirty year old cars should not be too expensive. I think you could make a stiff and relatively corrosion free chassis, but not cheaper or lighter then the traditional locost.

Power is much easier to get then when the locost was designed though, so adding a little chassis weight is hardly an issue. Keep in mind that aluminum is designed to fatigue limits, unlike steel. So a wheel, for instance, has every rotation counted as a load cycle. That's why race wheels are lighter, they don't turn as often as a road wheel.

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: March 25, 2008, 9:54 am 
Offline

Joined: October 17, 2007, 6:02 pm
Posts: 105
Location: Denmark
Sorry Horizonjob - that rectangular section should read hollow section. I'll edit the original post to reflect that.

The same design would be used on all four corners - as the Lotus Elise design.

On the chassis front I'm taking inspiration from an old Lotus design - Formula Lotus. It wasn't really a monocoque but a twin spar, each side being aluminum honey comb "plank". I built a few when I was a student on a year out from college. Once I've got a chassis sketched out I'll start a new thread to try and harvest the collective knowledge. Weight wise it was similar to the comparable Formula Ford steel space frame chassis, but much easier to build (no welding). The Formula Lotus chassis developed a reputation for being a great car to crash in.

The major elements of the chassis were bolted together, not bonded, but there are now structural adhesives that cure at room temp and are tolerant of less than perfectly clean surfaces. Hence my interest in that area.

Cheers,
James

_________________
Before you judge a guy walk a mile in his shoes. Then when you judge him, you've got a mile head start and you've got his shoes on:)


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: March 26, 2008, 2:56 pm 
Offline

Joined: January 23, 2008, 9:55 pm
Posts: 25
Location: CR, Iowa
Here's a similiar spindle design that I plan usung.
Image
Image
Image
In place of the turned spindle pin I am planning on using a bolt on hub and bearing assymbly from a late model Chevy Malibu.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: March 26, 2008, 6:24 pm 
Offline

Joined: October 17, 2007, 6:02 pm
Posts: 105
Location: Denmark
Hi Lurch - your design is encouragingly similar to the one that I'd envisaged. Can I pick your brains?

    What is the total weight of the upright and bolt on hub coming out at?
    That is an interesting top ball joint, where is it from?
    What is the KPI (it looks like it could be zero)?
    Does your design allow for camber and/or caster to be adjusted?


If it is of any help I have a Mondeo bolt on hub that I can measure up for you (I don't know if they would be any easier to obtain than the Chevy Malibu one that you are planning on).

Cheers,
Puk

_________________
Before you judge a guy walk a mile in his shoes. Then when you judge him, you've got a mile head start and you've got his shoes on:)


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 57 posts ]  Go to page Previous  1, 2, 3, 4  Next

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 4 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
POWERED_BY