streamliner chute tether

[neandethal]

Hi Guys,
I have used the search function extensively but virtually all the posts about chute tether points are for cars. Im hoping to bring my motorcycle streamliner to speedweek next year after several years building and developing a prototype.
My question is the height of the tether, i have read dire warning of it being too high, too low and causing rear end lift or front end dive, i am assuming as a starting point being in line with the swing arm spindle/gearbox sprocket/ crankshaft would do??
any views, photos etc appreciated. There arent many reference photos on the net. Thanks Ian

[Elmo Rodge]

You want to determine your center of mass. Basically, you weigh both ends (at the tire) then elevate one end and weigh again then trig it out. There are calculators out there which makes this easy. I did this on my car and I am really happy with the way my 'chute hits. With a motorcycle 'liner I wouldn't take chances with this. 
Wayno

[Stainless1]

Ian, some where on here is a picture of the Ack Attack hanging from a crane to confirm they found the correct spot.  I think it needs to be at the vertical center of gravity. 
With that said... I think the center of the bike or just above the swingarm is a good starting point.  If you are testing you could have multiple points which it the approach Ack took.
Be mindful of the type of chute you will use... and how it will fly...
Max ran down the road tossing chutes out to see how they flew... that is on here somewhere as well.

[neandethal]

Great info boys. Have found a formula on line although jacking rear up will be fun, it'd all doable and adds some science to the option of a good guess. Many thanks, will hold off welding tether point in until I'm happy. Will build in adjustment to the bracket. Cheers for help

[Stainless1]

Have you consulted with a/the chute manufacturer... I'm kind of a crossform or triform type of guy... ribbons seem to hunt a little... just my opinion and experience.
Maybe consult the engineering firm of Pork and Pie   

[neandethal]

Follow up question.
Looking at the "x-ray" photo of killajoule it looks like they mounted one tube above and one tube below the tether point. I'm assuming it doesn't matter where as long as at the centre of gravity line??

[Stainless1]

Tube location doesn't matter, attach point does.

[kiwi belly tank]

Pulling on the vertical CG point is critical on a liner, especially a light one & then you only have two wheels. The further back from the rear wheel axle center the attachment point is mounted, the more leverage arm you are creating & that will induce yaw when it hits. That can upset a 4 wheel liner so I expect it would be a holy-F moment in a bike liner.
I prefer cross form chutes on a liner because they're less likely to hunt but they will if they're too big. They will tend to spin if the panels are really long also.
I would suggest you get in touch with some bike liner guys just because of what you're building. 
  sid.

[kiwi belly tank]

Something I noticed in you pic is the vertical front cage hoop. You might want to run that by tech, I don't think that is allowed.
  Sid.

[neandethal]

Construction of chassis has been communicated and passed by tech at every stage of construction.
Had long discussions with Stroud before purchasing the chute so hopefully tapped into their wisdom. There ain't that many bike streamliners to talk to!! Last 3 I've seen didn't make it far enough down course to need a parachute. Am in regular dialogue with Anders Jonsson of Sweden with his aluminium clad streamliner (now a sidecar).
Very happy to talk to anyone but to some degree chassis is built.
For added info, this is a 100cc bike so speeds will be below 200mph.

[kustombrad]

I noticed nobody has commented on tether length or chute size in regards to a 'liner. I'd definitely want a smaller chute and longer tether to keep from "unloading" the set of the car...

[kiwi belly tank]

Since it's a bike-liner there aren't many people with hands on experience & quite possibly none of those here on the forum.
As a streamliner (4 wheels) guy with more than 20 experience, I know what I don't want when it comes to chutes. Brad is correct about "small & long" & you'll get that info from any of the chute manufactures. The critical choice is attachment point. On Betsy that point was right at the back of the transaxle at vertical CG centerline. Any further back from that point & you start to create a leverage arm & the longer that arm gets, the more likely it is to create yaw when the chute hits. The worst case of this I've seen was Bobby Moore with the Maro Special. I had discussed this with him previously & at that time he said it felt good so far. The incident happened the first time he ran 350-ish & it turned right 90 degrees. To this day I'm amazed that it stayed on it's wheels didn't fall over. That was the last run he ever made on the salt & he sold the liner. His wife was running a pretty stout El Camino & they stopped running that too.
So many other streamliners I see are built this way & when I ask them about it I get the same answer. "Seems ok so far & Jo-Blow hasn't had any problems & he's gone 900mph!
On my new liner I have a 9ft tail that tapers to a point & the chute attachment point is right behind the rear diff. I've achieved this with an aero tunnel that starts about 6ft further forward at the back of the engine.
It's going to be so much easier to upset a Bikeliner with any kind of chute leverage arm than a 4 wheel liner.
 PM sent.
  Sid.

[Tman]

Since it's a bike-liner there aren't many people with hands on experience & quite possibly none of those here on the forum.
As a streamliner (4 wheels) guy with more than 20 experience, I know what I don't want when it comes to chutes. Brad is correct about "small & long" & you'll get that info from any of the chute manufactures. The critical choice is attachment point. On Betsy that point was right at the back of the transaxle at vertical CG centerline. Any further back from that point & you start to create a leverage arm & the longer that arm gets, the more likely it is to create yaw when the chute hits. The worst case of this I've seen was Bobby Moore with the Maro Special. I had discussed this with him previously & at that time he said it felt good so far. The incident happened the first time he ran 350-ish & it turned right 90 degrees. To this day I'm amazed that it stayed on it's wheels didn't fall over. That was the last run he ever made on the salt & he sold the liner. His wife was running a pretty stout El Camino & they stopped running that too.
So many other streamliners I see are built this way & when I ask them about it I get the same answer. "Seems ok so far & Jo-Blow hasn't had any problems & he's gone 900mph!
On my new liner I have a 9ft tail that tapers to a point & the chute attachment point is right behind the rear diff. I've achieved this with an aero tunnel that starts about 6ft further forward at the back of the engine.
It's going to be so much easier to upset a Bikeliner with any kind of chute leverage arm than a 4 wheel liner.
 PM sent.
  Sid.

Sid, we were sitting on the return road loading up #416 when Bobby "stopped by to say hi" THE NEXT COURSES RETURN ROAD! That thing was so outta shape it was crazy. He literally came to a stop like a skipping lawn dart. By the time I got back to teh truck and grabbed an extinguisher, safety was on the scene. As he came to a stop the ass end of the liner was in the air and it landed nose first.  When I went over to talk to them a couple pits away from us they were not real talkative. Surprised nobody came over to take statements from us?

[kiwi belly tank]

Yeah that sure was a sure was nasty moment. I got to see the whole right side of the liner from the left side of the course before it disappeared into a white cloud & when the cloud settled I was not expecting to see it in one piece. I went over to see him also & he looked at me & said "you were right!". I had previously shown them a better way to pack the chutes to reduce line burn & had given my opinion on the attachment location.
  Sid.

[Stainless1]

As I recall Pork Pie told them when they brought the car out for the first time that their chutes were in a bad place.  He said the faster they go the worse they will act...
But what the heck does a crazy German know...