Revised aggressive rake & trail on a Rickman frame?

[Chuck78]

alright, I am building a Rickman cr900 out of a crashed and repaired former race bike. some people will say that the bikes are better for long tracks, as they are slower steering for a racer. At 28 degrees rake, I can see why.

so I have been on a quest to design into this build a revised ride height and suspension geometry to get a more modern steepened rake, and then a good trail number by running a lower-profile 110/80-18 front tire, & slightly taller 150/70-18 rear.


this will be with very high quality suspension and Brake components, in a Reynolds 531 hand-built 1970s Rickman frame, the cream of the crop for a 1970's Vintage High Performance frame for a Japanese 4
cylinder.

I am getting some custom Billet triples made to my specifications, and I am trying to figure out the offset needed to get the proper trail, but first, I need to figure out what rake I am going to arrive at. the swing arm have two different shock mounts on it, the rear laying the shocks down more and giving a stiffer ride. The forward mounts will give a softer ride with more leverage against the springs, and raise up the rear 8mm or 5/16. I got a set of Fox Factory Shox in 14.5" length, which will put me potentially around 25 degrees rake, but with a fairly tall seat height. This will also be about the maximum the swing arm angle recommended by Dave Moss I believe it was, from a suspension setup tutorial video I watched.the original Rickman Forks are as short as possible for this frame, which was built around a very short fork. My front ride height is going to be about the same as stock aside from a shorter tire to drop it down a bit and lessen the amount of offset needed in the triples.

The Fox Factory Shox have extended lower eye mounts, and I could drop up to 9/16 inch off of the ride height just from some shorter lower eye mounts from RC Manufacturing. this is what I need to look into possibly.





my big question here is, what is the final rake that I should shoot for without going over? Some people told me to try and get it down to the 24 degree range as most modern sport bikes run. My GS 750 going from 27 to 26 degrees felt pretty fantastic in the twisties. If I could get even better than that, that would be superb. I have been told around 95 mm of trail is a pretty ideal figure for a quick turning yet still stable bike. The Rake is less of a factor in the overall feel, but still plays a big part.

my concern is, some people say go for as steep as you can get, now another fellow just told me that he thought the bike would be too twitchy since is an old flexible frame. Maybe he is under estimating the rickmans?

25 or even 25.5 degrees would be most excellent out of this, and approximately 95 mm trail. I will have to do some calculations to figure out what the rate would be if I dropped 9/16 inch off of the lower shock mounts.

 

[sav0r]

Most modern bikes are somewhere around 24 degrees and around 3.8 to 4.1 inches of trail. The RD350, a bike with much loved handling, was 27.5 degrees and 4.2 inches of trail. I personally feel the RD is a little sluggish though. The RZ350, again much loved, had 26 degrees and 3.75 inches of rake (pretty close the numbers you are thinking). I guess what I am getting at here is that you should probably just pick a number that sounds good to you and go for it. It's all more or less adjustable in the end. Especially if you are doing custom triple clamps.

 

[Chuck78]

Is there a concern for taking a 1970's Vintage frame and steepening the rake considerably? Someone had mentioned to me that maybe the frame is too flexy for that steep of rake.
I get by very well on a 77 Suzuki GS750 with a 26 degree rake, 1 degree steeper than stock. The Rickman is a good bit more rigid than even a GS750 frame (best handling Japanese 4cyl frame to date in 1977) and even more so with the addional gussetting added to the front end of my Rickman. I figured 26 degrees would be real good, but shooting for 25.5 degrees.
If the swingarm angle permits, as well as my 32" inseam over an increasingly tall seat height, I may try for 25.5 or 25 degrees. I have to wrestle a gs750 loaded crankcase into the frame and throw on a stock gs750 sprocket to see how the 14.5" Fox Shox in the rear (lower ride height of the two) swingarm shock mount position will ride, or rather gauge the standover seat height and see how much if any chain clearance I will have on the swingarm pivot tube to the chain.

 

[Chuck78]

I checked geometries of a lot of modern sport bikes, most were 24 degrees or so, some 23.5 (newer models) some 25 or 25.5. Very few sport bikes were 26 or 27. Only a few sport touring or small capacity 2cyl learners bikes (ninja250) were in that range.

Trail was generally 3.7" to 3.9" on sport bikes.
Racers tell me around 95mm was the quickest turning without really compromising stability. So I was going to try and target 95.5mm - 96mm (3.76"-3.78") trail with whatever steepened rake figure I can work out with the chainline.
I run 1 tooth larger than stock in a 530 equivalent (stock is a 630 chain) in the rear typically. On a big cammed big bore with special carbs and maybe some head work, and a steepened swinger, I plan to try running 1+ tooth larger than stock 530 equivalent or more in front and 2 teeth larger in the rear. Maybe 3 teeth larger. To be determined. Dave Moss stresses not to exceed horizontal on the chain line on top, it should always be running uphill a bit in terms of engine force and gearing leverage fighting against the suspension when under acceleration

 

[sav0r]

Chuck, I think you are definitely on the right track. I think you can easily get away with 25.5 and 26 would be safe. I'm right around 26.5 on my RD and I could comfortably reduce it by another .5 to 1. It's completely stable and has never given a hint of head shake even during maximum-on-power corner exit. Granted the front wheel is nearly off the ground most of the time, but that's when you can find yourself in big trouble too.

Frame stiffness I think is such a misunderstood thing. Too many people attribute it to handling characteristics, but I tend to be inclined to think that the given characteristic just isn't particularly well understood. In this case, suspension and tire technology are likely the reason for greater rake than modern bikes. And while frame stiffness might play a part, those two items take precedence. If you plan to run modern tires you are instantly ahead of where the bikes were '77. And if you are planning on cartridge emulators you are likely even better off.

As far as chain pull, anti-squat, etc. that's a big topic. You definitely don't want the chain horizontal, you can end up with not just anti-squat but actual lift, and unpredictable lift at that. Also, remember that if you get too high on ride height you start getting your center of mass well above your roll center and this creates a bike that likes to wheelie, and that's true in both directions. And that effect is compounded in trailing arm suspension (motorcycle swingarms) because it reduces wheelbase, particularly in relation to the rear tire under forward acceleration.

My piece of advice on large changes like these, and let's be clear that you are making changes around what are abstract numbers, is that you should sneak up on them. Nearly every time I have attempted to tune around "what should work", i.e. picking numbers and believing they are the correct ones, I was proven wrong. Ultimately I have found it more beneficial to change variables as individually as possible (a hard task on a motorcycle) and analyze the result. Then repeat. All of that said, you aren't doing any ground breaking changes, so I say go for it.

 

[Chuck78]

Chris, I'll be running RF900R 43mm cartridge forks (adjustable versions of the Suzuki Bandit forks, very well regarded for their damping and tunability. Very highly regarded vintage Fox Piggyback shocks also.

I believe what I am going to do here really is to just mount a crankcase into the frame, with stock gs750 sprockets and chain, and check the swinger pivot tube clearance with the tall shocks as is.If it's tight, I might try to simulate +1 tooth in a 530 conversion up front and +3 teeth in rear, see where that gets me as far as chain line top angle and clearance over the swingarm pivot tube.

Then I will set the bike up on level ground and simulate my tire diameter choices by shimming the rear slightly above level from underneath. Then measure my rake angle carefully.
Then once all of that rear ride height is settled, then I will calculate what triple clamp offset I'm going to need in order to obtain 95.5mm - 96mm of trail. If I don't like it, I can always run a slightly taller front tire (simply switch Battlax BT45V to same size in Pirelli Sport Demon which is slightly taller) or put shorter eye mounts again on the rear shocks to drop them more.

I think this will work out very well if I adhere to that plan. As it has been preached, the trail measurement is far more critical than the rake. Rake can be in the ballpark, it is the trail that ultimately dictates how the steering will behave.

I'm basically going for as steep of rake as I can get up to 25 degrees while still maintaining chain to swingarm pivot clearance(may need to adjust sprocket sizes), proper chain angle, and without raising my center of gravity and seat height too terribly much.
The Rickman frames' front ends were built around the shortest forks one could possibly run on an 18" vintage tire, so the only dropping that can take place in the front is going from a stock Rickman 4.10V18 tire size down to a 110/80-18 (or 100/80-18 if you were going skinny). The rest of the rake steepening has to come from the rear, so straying far from the stock 28 degree rake down to 24 seems as if it may take too drastic of swingarm angle, so I am shooting more for 25.5 degrees for my mockup with engine and chain. Wish me luck. I don't want to induce ill handling behaviour from chain angle or high center of gravity. Thanks for the center of gravity vs roll center reality check. I need to keep things more on the modest side since I am not doing frame metalworking to adjust the rake, but rather altering geometries and ride heights in ways that will affect many other handling areas. Proceed with caution.