Tire Grip & Load Data

[Birkin42]

I'm hoping someone out there may have some tire grip or suspension load data that I can use as a sanity check for the rear suspension that I am designing for my car.

 

 

 

What I'm building is a DeDion setup that is very much like the Caterham setup with the Watts link on each side so information from one of these would be bonus.

 

 

 

What would be most useful is the grip force at the tire for accel, braking and cornering. I'm planning to use Yoko A048's, but as long as I know the tire that was used for the data, it still will be helpful. I am in contact with a local FSAE team that can likely supply this data for their pervious car. Though the cars are fairly similar in power to weight, they differ in many other ways so again I need to take this with a grain of salt.

 

 

 

I suspect this will be hard to come by for someone with a 7 type car, so what would still be very helpful is logged data from something like a G-Tech with a few useful spec's on the car like torque, gear ratios, diff ratio and tires.

 

 

 

Jack

[slomove]

 

I suspect this will be hard to come by for someone with a 7 type car, so what would still be very helpful is logged data from something like a G-Tech with a few useful spec's on the car like torque, gear ratios, diff ratio and tires.

 

 

 

Jack

 

 

 

Sounds like a good starting point but I guess you should not design for regular driving loads only. Most parts fail when somehow abused and I think you should allow for hitting the occasional curb. I am not sure if there is an analytical way of doing that without a big research department. The safer way may be to take and modify a known good design.

 

 

 

Gert

[locostv8]

It would seem that you will be doing enough fab and mods that you would be be better using an IRS. A good starting point for that would be the Rorty plans using either MK VIII or Mazda pieces for the mechanicals.

[slngsht]

A good starting point for that would be the Rorty plans using either MK VIII or Mazda pieces for the mechanicals.

 

 

 

As long as you leave Mazda's rear out of this http://www.usa7s.com/aspnetforum/images/emoticons/biggrin5.gif

[locostv8]

http://www.usa7s.com/aspnetforum/images/emoticons/Smiley_surprised.gif

[Birkin42]

I serious considered doing a double A-arm IRS but decided against for a number of reasons. First the stock Birkin chassis has practically no structure in the back to mount the inboard suspension pick up points from. Supporting the diff is hard enough. I was also worried about the chassis torsional stiffness given the lack of structure to hang the suspension off of. Another thought was that the car is designed for a beam axle (live axle) in the rear so I would have less of a chance on making a mess out of the handling. Finally, though I am not really a traditionalist, I thought I would prefer to keep it a bit more in line with the Seven's nature.

 

 

 

It was my plan to do my own thing in terms of suspension and though I have looked at some of the available designs, I would prefer to do it myself. Part of the fun.

 

 

 

For sure I need to consider impacts and such, though like you said, it may be even more challenging to come up with those loads. At least I know what the load differences will be between my design as compared to the stock Birkin and I can determine the strengths of the stock parts, so I should be pretty safe judging the design by that method.

 

 

 

This is where the design is currently at. I'm sure it will get tweeted a bit as I finish it off.http://www.usa7s.com/forum/uploads/20070112_145922_frame_&_rear_su.jpg

[Ivan]

Hi Jack

 

I really like your design and think it’s real neat!

 

 

 

Just one thing puzzles me from the drawing – the fore and aft Watts links restrain the tube to almost pure vertical movement whilst the lower A arm seems to force the tube into radial movement, this will cause torsion and bending moments in the tube or am I miss-reading the drawing or is there a detail that’s not visible on the drawing.

 

 

[Birkin42]

The axle is only really supported at 3 points, center bottom and the pivot of the watts rockers on either side, therefore there should be not binding or twisting.

 

What the watts links & rocker do is provide a near vertical path for the rocker pivot, which in turn means that when the car is in roll, it doesn't steer the axle like all other trailing setups for live or DeDion axles.

 

Since the top of the axle goes through a vertical path and the bottom goes through a traditional arch, the axle rolls about rocker pivots a bit and hense the wheel axis moves about a bit, but it is the same on either side irrespective of vertical compression or roll.

 

I liked the basic layout of it when I saw it on the last gen Chaterham's. I think it was/is an option on some of their chassis. When I first saw it I couldn't figure out why they were doing that, but eventually figured it out.

 

I've also mounted the wheel bearing housing on adjusters so I can adjust toe as well as chamber without changing out parts.

[Ivan]

Thanks for the reply - I get it now http://www.usa7s.com/aspnetforum/images/emoticons/smile5.gif.

[Ivan]

Hi Jack

 

What type of bush will you use for the Watts linkage pivot - a roller bearing or a bronze bush?

[Birkin42]

Was going to use spherical rod ends for all the connecting links but due to space limitations was planning to use IGUS iglide Q flanged bushings which have a PV rating of 16000 and a max surface pressure of 14500 psi. This is many times higher then commercial bronze bushings and is also lower friction and can tolerate running dry, though I intend to grease them.