Birkin42

My guess is that the frame shots are in fact solid model CAD images. My reasoning being that parts like the unsupported ends of the half shafts are nicely oriented with no visible support. I know with SolidWorks you can produce some very realistic looking models if you take the time to set up lighting etc. Someone must have spent a fair bit of time modeling all that detail.

How true a lot of these are. My wife still doesn't understand.

I would think mounting the accelerometer to the car is good for compression but maybe attached the axle may be better for rebound. Ultimately you should be able so see the effects of both in either location, but how readable will the data be?

Nice to see another Canuck into sevens. Turns out we're not far from each other as I live just outside of Cambridge. Building your own chassis or starting with a kit of some sort?Birkin422007-02-21 14:09:50

I'm surprised the front spring rates are so low though I am more familiar with Birkin geometry then Caterham. Your car has double wishbone front with the shocks canted at close to 45 degrees, and the rear is a DeDion axle with the shocks nearly vertical? Do you know your corner weights? If you are close to 50/50, I would expect the front to have stiffer springs then either the 170 that you say is stock or event the 250's you bought to try out. A Birkin runs stiffer then this on the front and has the engine mounted a bit further rearward then a Caterham as I understand it. You say your rears are progressive 150. Is the 150 the start rate or the finish rate? Normally progressive springs list 2 rates like 150/200 for example. Lowering the car may be an option for when you are running slicks and then bump it back up when you run on the street when lean is less of an issue. This may be more of a pain then it is worth though. Also, lowering the car will reduce the roll somewhat, but likely the amount of roll you lose will be less then how much you lower it by so your exhaust grinding issue will likely be worse, not better. If you want to do some analysis on your spring rates, you can download a catalog from Eibach. The catalog you want is complete Motorsports catalog. In the back it tells you how to calculate wheel rate for your front suspension as well as for your rear suspension. This isn't the end all be all, but it might prove helpful in understanding where you're at and where to go. Hope this helps.

What springs are you currently running front & rear? Are these stock? If not, what were the stock rates? Have either/both the front/rear sway bars been changed? Does your car have a rear sway bar? All of this would be helpful to know. In the end, if you have more roll then you like, you either need to increase roll stiffness, lower the center of gravity or lower the roll center of the car. The last 2 are major changes so increasing the roll stiffness is likely the best option. To increase roll stiffness you can either increase the spring rate, install a stiffer sway bar or increase the leverage on the sway bar. The last 2 will have the same effect. In making any of these changes you will need to maintain the same front to rear ratio of roll stiffness to keep the same understeer/oversteer. In deciding whether to add more roll stiffness through springs or the sway bar will depend on what your thoughts are on compliance of the car driving in a straight line including brake dive, squat under accel and compliance over bumps. If the car moves too much under these situations, add spring stiffness. If you are happy with these, add sway bar stiffness. Hope this helps.

I have a fair amount of experience tuning shocks/forks on 2 wheels (motorcycles & mountain bikes) but not a much with 4 wheels since most cars don't offer a lot of adjustments on the shocks. I plan to fix that on my Birkin with a nice set of Ohlin's shocks though. I have read up on a lot of shock technologies and tuning methods over the years as well, though I don't claim to be an expert. Just here to offer an opinion. Anyway, I believe there aren't many procedures offered for a few reasons. One, settings are largely based on personal preference on what characteristics you put higher priority on as all shock setups are a compromise. Two, most adjustment on shocks with multi-way adjustment have overlap so there is interplay between the adjusters. Three, different car setups, suspension geometry's and tires can produced different results with a given change in the shock settings in some cases. Four, there are many types of adjustments available on shocks these days, from rebound only, to 5 way dampening adjustment. I've even seen shocks which attempt to adjust both rebound and compression with 1 adjuster. Five, there always seems to be the latest trend in tuning shocks, particularly for racing where you need to work around rules and there are aero considerations as well as mechanical grip. One day you should have fast rebound for good grip and the it should be high rebound to such the car down for less aero drag. Six, a lot of people don't understand the setup of dampers and are afraid to make adjustments so they only run the factory recommended setting. I was amazed at how many motorcycle racers I came across how did this after spending a pile of money on a shock with a bunch of adjustments. A lot of excuses for a situation that like you seems to be a lack of information on. In terms of adjusting your shocks, this somewhat depends on what adjustments you have. Assuming you have low speed adjustment of rebound and compression, it is best to start with rebound. Adjust it a click at a time until the rebound is to your liking. If it is too fast, the car will feel bouncy as you drive and you won't feel connected to the road. If it is too stiff, you will lose traction, the ride can become harsh, and the car can start to pack down or ride low in its travel (not always a bad thing if you talk to the NASCAR guys). You final adjustment will come down to feel versus traction. Then move on to compression. The compression adjust mostly affect low speed motion of the car like brake dive, squat and roll. It also affects ride harshness, though this is mostly the high speed compression dampening which usually isn't offered on shocks unless you are paying big bucks. You can use the compression to set up things like turn in over/under steer and other transistion situations. Depending on the shock technology will determine how useful the compression adjuster is in tuning the handling characteristics of the car without making it ride harsh. Hope this proves somewhat helpful, or at least interesting.

I have talked with a bunch of different shock manufacturers in trying to sort out which I want to go with. They all state that for a pressurized shock, it doesn't matter and the difference in mass on either end isn't a big deal. For twin tube shocks that aren't pressurized, it is important that they are mounted body down/rod up and not be canted over more then 45 degrees. I believe the stock Birkin shocks are twin tubes and thus defines their orientation. For mine, I was going to mount them body up since they will have a remote reservoir connected via a hose from the top of the body. Let us know how they work out for you. Jack

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.

I don't think that anyone intended this to be the best bike against the best 7, but it's pretty obvious to me that the guy on the bike backed off a number of times to avoid passing. I'm assuming the guy in the 7 never backed off since he was always in front. It would be interesting to see a fast 7 with an experienced driver up against a current litre class bike with an experienced rider. The car would definitely have the advantage in the tight stuff but the bike should be guicker in the fast stuff. The Ring is such an interesting mix of tight and fast stuff that it would interesting to see. One of these years I'm going to have to make back there with a decent car or bike. I was there many years ago with a pretty sad diesel Jetta. Would barely do 140 kph. Pretty hopeless.

In terms of dampening vs. spring rate, I agree that higher spring rates with less compression dampening feels better and handles better. Vehicles with low spring rates that try to compensate with high dampening always seem harsh but blow through their travel on anything significant.

On the front suspension of a typical 7 with out board shocks, the shocks are canted over so far that leverage ratio is actually fall rate. A progressive spring would help to counter act this. The rear suspension is of course as linear as it gets. My opinion is that progressive springs are a nice compromise for the street and some track use, but prefer standard springs for competition use. I guess it really depends on what you are looking for. For the progressive springs, are they offering you a choice of how much the rate changes? Another way to look at it is a progressive spring is similar to a standard spring with a tender spring. I've seen these used on comp cars.

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.

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

The Birkin dimensions are near identical to the original Lotus. Wheel base: 88" Front track: 49" (53" with wide track option) Rear track: 53" Overall width: 64" Overall length: 138" (I believe including spare tire) Cockpit width outside: 40" All dimensions are rounded. This is an interesting photo showing how small a Birkin is compared to more typical cars and trucks on the road. http://www.usa7s.com/forum/uploads/20070112_063319_IMG_1700.jpg

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

The V8 Busa end he is planning on using in the dp1 is from Hartley Enterprises and the estimated cost for the engine is $28k. Definitely not cheep, but a lot better then the PowerTec option from a cost perspective. Yes you can build a domestic V8 to some very high power levels for that kind of money, but if you want to keep weight in check for a very light car, that really isn't a great option. His V8 dp1 will be under 900 lbs. Pretty good for a car with 375 hp. As intriging as it is, the cost will stop most people cold I'm sure.

Check out the dp1 on this site http://www.dpcars.net/ on someone who decided he'd like to design and build his own bec mid-engined car.

If it is like up here in Canada, you will need to show a bill of sales for all major items used to build the car, whether that be a kit or a home built when you go to register it. Unless you have a method of getting fake receipts so that it doesn't look like all the parts came from one source or car, you will likely have problems registering the car. I've heard in Canada they impound the car, then you have to get it out of the country, and then pay the storage fees and fines. To avoid these problems when I brought in my Birkin roller into Canada, I needed to have the car further disassembled, shipped in pieces and all items itemized as car parts, not as a reciept for a dismantled rolling chassis. This worked fine with no hassles from customs. Like in the US, you can't bring in a roller or a whole car unless it was previously registered and is at least 15 years old, or is on the approved list of vehicles that can be imported. There are no current 7's that are on the approved list and haven't been for some years. Hope this helps. Jack

I guess I'm about middle of the road at 43. First fell in love with Seven's after I read a test report on a Caterham in Road & Track back in 1985 I believe. The more I read up on the car, Lotus as well as Colin Chapman, the more I admired it raw simplicity while still be attractive. Had my first drive in a Birkin back in 1998. This just reinforced my desire to have one, though at the time there were few options in Canada. Got a Birkin roller in 2005 and have been working on it when time permits. Can't wait to drive it. Hopefully that won't be too far away. Jack

No matter what you find the liquid is, I'd recommend you flush and change your oil. Whether it is water or gas, some of it may have made it past your rings and contaminated your oil. Running it like that is a good way to wreck the rest of your engine. Glycol from anti-freeze is particularly hard on bottom end bearings from what I understand. Good luck.

There are so many things that affect torque/power curve & efficiency of an engine. Displacement is just one factor. The higher technology engines tend to use more rpm's to get the air flow so they can keep the size and weight down. Look at an F1 engine with about 800 hp from 3 litres and 8 cyliners run at 19,000 rpm. A MotoGP bike engine is about 245 hp from 990 cc's, but they need a bit more smoothness in the power curve since the bike isn't much over 350 lbs. In the case of an air cooled Harley, it's biggest problems are the rotating interia (very long rods and very heavy crank), poor cylinder head geometry caused by the layout & push rods, and getting rid of heat (particularly from the rear cylinder). Most air cooled engines can be tuned to make a lot of hp, but not for very long. There are some pretty fast HD drag bikes for example. I last road a Buel back in 98 and I was pretty disappointed. I hit the rev limited before I thought I was even in the power band. A current at that time Japanese 600 would eat it for breakfast, and it was the top of line, fastest Buel they had on offer. I would think that some of the newer liquid cooled models have a bit more potential, but you have to keep in mind who HD is selling bikes to and what they want. They want a cruser bike with a wide torque curve. Big hp, other then for braggin rights, isn't really important. Japanese and Italian sport bikes on the other hand are looking for big power from the smallest, lightest package. They live or die by the results on the track. Hense next year there is always lighter or more powerful bikes coming out. These are great fun to ride in a sporting manner, but aren't always as flexible and as easly to ride as a torquey motor when just crusing. In comparing them to car engines, most car engines are like the HD. If you want a screamer like a sport bike engine, look at a VETC Honda with a 8 or 9000 rpm red line and not much torque. I guess it depends on what you like and want. For me, for a daily driver, torque is nice, but to have fun, give me rpm's with a reasonable, but not too wide power band. Maybe not the fastest or most forgiving, but very rewarding when you get it right.

I found this forum from a post on the Birkinowners Yahoo group. Can't recall who that was though.

I live in Southern Ontario, Canada. I am 1/2 owner of a company supplying mechanical engineering services, primarily for industrial automation, but some product development as well. One of more interesting customers is Sean Hyland Motorsports were we have helped Sean with the design of some of his products. My companies web address is http://www.appliedkinetics.ca/ for those curious types. Had spent a number of years racing motorcycles in Canada and the US and managed to win the Ontario and Canadian national battle of the twins class back in 1993. Moved on to racing Superbikes in Canada and FUSA in the states, but only managed a 2nd as my best result. Enjoyed my time racing but gave it up to start my business. My current hobbies include mountain biking, trail riding dirt bikes, and snowmobiles, but am still missing the track. To rectify this, I've started a project to build up my dream Lotus 7. I've had an infactuation with them ever since high school. As times changed, so have my dreams, but I have settled on putting a Honda S2000 drivetrain into a Birkin. Everyone I've talked to said it can't be done, but by my measurements it should be possible, but will require a lot of work. The engine itself doesn't fit too bad once the right side engine bay diagonal brace is removed. It will need to be dry sumped and the valve cover cut down a bit to get it to fit with no hood bulge. The transmission is a bit more challenging, but with redoing the tunnel, it is possible. Drivers side foot well width is the biggest challenge. I've currently got the chassis stripped down and most of the design work and FEA analysis done. Wanted to engineer the solution before I start cutting metal so I'd know that it would work in the end. Hopefully the fab work can start over the winter. I'm anxious to get the car on the road, but I'd rather take the time to do it right. I guess the route I'm taking is a lot more involved then most who go with the Birkin factory recommended drivetrain, but I figure half the fun of these cars is building it and making it your own. Could have started from scratch, but that looked like a lot more work still. Also talked to Dan at Deman quite a bit, but in the end decided to take on the project myself. I guess in time we will see how I make out.

Saw a 30" wheels with a rubber band tire on it when I was at SEMA last year. Tried to pick it up, but barely could. Had to be something like 120 to 150 lbs. I can't image what that would do to the ride and handling of a car or truck. They don't even look good, at least to me. Didn't make to SEMA this year, but I wouldn't bet against someone coming up with something even bigger and more retarded. I guess to each his own.