LocostUSA.com

Learning how to build Lotus Seven replicas...together!
It is currently December 14, 2019, 10:02 am

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 22 posts ]  Go to page Previous  1, 2
Author Message
PostPosted: November 12, 2016, 2:34 am 
Offline
The voice of reason
User avatar

Joined: January 10, 2008, 4:47 pm
Posts: 7635
Location: Massachusetts
Here is a picture of a late 60's Lotus F1 car. It shows a reverse wishbone for the bottom control arm, a simple radius rod for the upper control arm and a pair of trailing arms on each side.

Some years later formula cars also started using a pair of simple parallel links for the bottom arm. This has an advantage of completely controlling rear wheel toe during vertical travel. It can be eliminated or by using some caster you can do other things too, like a little toe in for bump.

The beauty here is the simplicity ( it may not look simple at first, but this is an F1 car with a suspension bolted to the back of a 450 HP V8 ). It is a very spare setup. I think the tubing used to make the mounts for the suspension weighed about 2 lbs.

If you look at formula car suspensions, especially from the late 60's onwards you see many permutations of this stuff.

This is not a great picture for an example but if you keep looking you should find many. One thing that may be hard to see is that the upper links are quite short and there is a chrome tube between the mounts for the upper link.


Attachments:
Lotus_49-2.JPG
Lotus_49-2.JPG [ 686.88 KiB | Viewed 1384 times ]

_________________
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  
PostPosted: November 12, 2016, 11:17 pm 
Offline

Joined: March 27, 2011, 7:43 pm
Posts: 38
Horizonjob: I appreciate the simplicity and light weight of the reversed lower a arm, radius rod upper link, and pair of trailing arms per side rear suspension. Here is a photo of the Mclaren M8F rear suspension which uses a pair of lower links in place of the reversed A arm (as used in previous Mclaren M8s):


Attachments:
mclaren rear susp.jpg
mclaren rear susp.jpg [ 102.42 KiB | Viewed 1366 times ]
Top
 Profile  
Reply with quote  
PostPosted: November 12, 2016, 11:25 pm 
Offline

Joined: March 27, 2011, 7:43 pm
Posts: 38
Oops, I hit submit accidently before finishing my post.

Anyway, my first question: is there any downside to using the parallel links instead of a reversed A arm?

Also, I've noticed that most racecars using this design rear suspension angle the side trailing arms going forward towards midline, and often the upper trailing arm is at a different angle towards centerline than the lower one. Why is that? And is there any downside to having these side trailing links go straight forward instead of at an angle towards centerline?

Thanks, I always learn a lot from your posts.

Jack


Top
 Profile  
Reply with quote  
PostPosted: November 15, 2016, 5:13 am 
Offline
User avatar

Joined: March 30, 2011, 7:18 am
Posts: 1554
Location: central Arkansas
There is a fundamental difference between the Lotus and the McLaren pictured above. The Lotus' reversed A-arm means that the suspension geometry resolves to "wide angle semi-trailing arm with camber link"; ie, it has built in toe change in bump.

Back when Lotus started using the reversed wishbone most racing cars were still using Metalastic bushings. The reversed wishbone meant the only toe change came arc of the trailing arms, but the arms were long and the angle wide, so the effect at the wheel was, as far as Lotus was concerned, negligible.

The twin lower links on the McLaren control toe, resolving to "A-arms with wheelbase change."


Top
 Profile  
Reply with quote  
PostPosted: November 15, 2016, 9:16 am 
Offline

Joined: March 27, 2011, 7:43 pm
Posts: 38
I see. So with the reversed A arm design you could design in some amount of toe in during roll or bump by angling the trailing links. Thanks for answering my question.


Top
 Profile  
Reply with quote  
PostPosted: November 15, 2016, 2:04 pm 
Offline
The voice of reason
User avatar

Joined: January 10, 2008, 4:47 pm
Posts: 7635
Location: Massachusetts
Hi JDunn, sorry for the slow reply. TRX gave a good answer.

I think of the parallel lower arms as the best setup for geometry, the drawback is finding a place to mount the rear radius rod. It will take roughly half the cornering load but have little vertical load. The formula car in my avatar uses a reverse lower wishbone which is mounted to the front face of the bell housing. There is no frame available, the lower rails stop at the bulkhead in front of the motor.

An example of what it takes to mount the rear radius arm is the way some cars use a custom bellhousing to provide this extra mount. These custom bellhousings can also serve as the dry sump oil tank on some cars. So it's a big step up in sophistication, which is why you see cars start with one setup and then migrate some time later to the parallel setup.

The parallel radius arms allow you to choose to eliminate bump steer and also allow you to quickly change the car's wheelbase. You can imagine moving the rear wheels an inch forward or back pretty easily and this could change your weight distribution by a couple of percent in just a few minutes without needing to adjust the toe.

Both the parallel and reverse wishbone setup allow further tailoring of bump steer by adjusting the castor of the rear spindles. This could be used to mitigate the bump steer. Usually pretty long trailing arms are used also. I don't remember the number but I did check the bump steer on paper when I choose the reverse wishbone setup for Car9. With 2" of bump travel it seemed minor. Also in the time I drove my formula car with that setup it was not something I ever noticed. It has been obvious in some street cars however.

As to the angle towards the centerline of the trailing arms, that's an interesting question. Much of it is due to just where it's possible to mount the trailing arms, you need some frame there. I think the GT40 uses arms that went straight forward and the only Lotus 7 I've seen with this setup did also. My formula car has much less inwards inclination of the upper trailing arm, but the frame is much wider around the driver than under the driver. It seems though that there was an attempt to mount them I the upper trailing arms ) an additional couple of inches outboard when the mount was designed.

Different lengths of trailing arms will change the caster as the wheel moves up and down. That would be the length fore and aft not counting the inward angle though of the arms, I think.

_________________
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  
PostPosted: November 16, 2016, 5:29 pm 
Offline

Joined: March 27, 2011, 7:43 pm
Posts: 38
Thanks for the reply, Horizonjob. I've been interested in this type of rear suspension for awhile.

I previously asked a racecar designer/fabricator in the UK about this subject, specifically which lower link design is preferable. He said he had changed many early formula cars from the lower reversed A arm design to parallel lower links design (he didn't mention ever doing the reverse). He said both designs have merit, depending on what is desired, and did not elaborate. So thank you for pointing out some things I had not considered, like caster change during wheel movement.

I suspect you are correct about the forward attachment points of the side trailing links often being dictated where the chassis members allowed them to be, paticularly in small formula cars. Although, interestingly, the forward mounting points of the GT40 side trailing links are angled towards the midline, with the lower tailing links at a more severe angle than the uppers. I assume the designers did it for a reason, since the rear of the monocoque seemingly would allow the mounting points to be located anywhere over a large area when it was being designed. Fascinating stuff.

Jack


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 22 posts ]  Go to page Previous  1, 2

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 2 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