Seeley Norton Done (or almost)

SteveA · Oct 13, 2020

springer said:
From what i know rear Seeley MK2 shocks should be 13.9 inches , anybody could help me with the front fork length from wheel center to bottom of lower fork (or top of upper fork) because i have lost my notes on that.

From what I see you have long stanchions, so you have the option of raising or lowering the yokes. I guess you are trying to get a starting point for geometry. Since you have this variability there may be a better starting point.

Headstock angle! Set the bike up on a level platform and with a suitable inclinometer measure what you have. Start out with a less steep front end, i.e. front a little raised (see below), and lower as you get confidence with the bike.

Sharp turn in may not be what you want whilst you are sorting any other issues on the bike in early testing.

You may even find that one setting for road use and one setting for track use will suit you.

What figures, well I don't have a Seeley so I don't know 100% but a friend tells me that on his Seeley, which has been successfully raced by professional level riders, he has 23 degrees, but the original geometry was more like 24 Degrees! Take with pinch of salt and start with a more gentle 24.5 to 25 Degrees maybe? Still a good bit steeper than a standard Commando!

He also told me his shocks are 13.8" But I reckon anything between 13.75" and 14" would be fine. Any longer and you are going to have chain run problems.

Chris · Oct 13, 2020

Hi Springer
I run Amal 36mm smoothbores.
Tyres 110 front & rear last time out & it was great! The old rear was pretty old. I usually run 100 front 130 650 rear as I didnt feel comfortable under braking on the 90/90. However no money! so with a k4 not being used bang! k4 sitting on its rims lol. On the fork front, I used standard Commando length forks on my Mk4 when I was waiting for Maxton to sort mine. it handled well. With Dommie length stantions fitted they are now dropped just under half an inch through the Minnovation yokes . I have Koni rear shocks but have a set of Maxtons to try. Just need to buy the right weight springs. Honestly I don't think you can upset the handling on these bikes, just work slowly until you find where everything feels sweet to you.

grandpaul · Oct 13, 2020

Awfully nice bike, @springer !

acotrel · Oct 13, 2020

It does not matter about the motor, - pretty much anything will be quick enough. The starting point is always the gearbox. Without close ratios, your bike won't go very fast. Yoke offset is important. If your bike over-steers as you accelerate when you are cranked over, you will go faster.. The springs on the back affect the rate at which the trail changes as you accelerate, and how much the bike self-steers.
Great to see you are racing your Seeley, I don't think you will be disappointed.

Chris · Oct 13, 2020

My Maxtons are just under 14 inch, they came with spherical bearings for the eyes! Instead of rubber bushes.

acotrel · Oct 14, 2020

SteveA said:
From what I see you have long stanchions, so you have the option of raising or lowering the yokes. I guess you are trying to get a starting point for geometry. Since you have this variability there may be a better starting point.

Headstock angle! Set the bike up on a level platform and with a suitable inclinometer measure what you have. Start out with a less steep front end, i.e. front a little raised (see below), and lower as you get confidence with the bike.

Sharp turn in may not be what you want whilst you are sorting any other issues on the bike in early testing.

You may even find that one setting for road use and one setting for track use will suit you.

What figures, well I don't have a Seeley so I don't know 100% but a friend tells me that on his Seeley, which has been successfully raced by professional level riders, he has 23 degrees, but the original geometry was more like 24 Degrees! Take with pinch of salt and start with a more gentle 24.5 to 25 Degrees maybe? Still a good bit steeper than a standard Commando!

He also told me his shocks are 13.8" But I reckon anything between 13.75" and 14" would be fine. Any longer and you are going to have chain run problems.

When Rod Tingate was in the UK, he worked for Colin Seeley. When I had a handling issue with my Mk3, he told me that all Seeley frames have 27 degree rake. Manx frames have 24.5 degree rake, but much smaller yoke offset. The Manx over-steers slightly as you accelerate when on a lean. That is why they feel so good. My Seeley has 27 degree rake and 53 mm of yoke offset and it over-steers a lot, if I accelerate when on a lean. It us unbeatable. You brake into the corner, then gas it hard. It finds it's own way around. Your head has to be around the corner before you enter it. Once you are used to it, it is great. I don't steer my bike, I aim it.

acotrel · Oct 14, 2020

acotrel said:
When Rod Tingate was in the UK, he worked for Colin Seeley. When I had a handling issue with my Mk3, he told me that all Seeley frames have 27 degree rake. Manx frames have 24.5 degree rake, but much smaller yoke offset. The Manx over-steers slightly as you accelerate when on a lean. That is why they feel so good. My Seeley has 27 degree rake and 53 mm of yoke offset and it over-steers a lot, if I accelerate when on a lean. It us unbeatable. You brake into the corner, then gas it hard. It finds it's own way around. Your head has to be around the corner before you enter it. Once you are used to it, it is great. I don't steer my bike, I aim it.

As far as I know, all Commandos have 27 degree rake, and the first model must have had minimum offset, because some guys crashed when they rode over cats'- eyes A steering damper would probably have fixed the problem, but I think the yokes were changed to give more offset. Probably the reason for the reverse-steer to tip into corners and running wide coming out ? If I reverse-steered my Mk3 going into a corner, it would tie itself into a knot. When I steer it, it goes where it is pointed.

acotrel · Oct 14, 2020

To get the starting point with the geometry, enter a hairpin bend and halfway around, gas the motor hard and see where the bike goes. Pick a corner with room on the outside, so you have somewhere to go. if your bike understeers. If you relax when you do it, you will discover your bike's natural tendency. If it runs wide fit a shorter offset set of fork yokes. The problem is yokes are expensive, so most guys don't bother to do this. I was just lucky, I had a set of Yamaha TZ yokes on a frame which was lying in my back yard. - The most important change I ever made and sheer arse.
Most early Yamaha two-strokes have 26 degree rake and the yokes are often right for a Seeley. So go to the wreckers.

acotrel · Oct 14, 2020

For the correct shocker length - when the bike is standing unloaded, the top frame tubes under the seat should be level. As you accelerate and brake, the rake on the steering head changes. The bike should become slightly more stable as you brake into corners and over-steer slightly as you accelerate out of corners. If you get into a corner too hot and start running wide, you should be able to pick a point where you are not going too fast, and accelerate out of the problem.
Like anything, it is easy when you know how. Just do it systematically and don't try too hard.

springer · Oct 14, 2020

Thank you all for the info. bike is running neutrally at this time, relatively quick to turn in but also very stable, i am just looking to fine tune and also the correct starting point for the front end length. i have a limited amount of adjustability to try.

SteveA · Oct 14, 2020

springer said:
Thank you all for the info. bike is running neutrally at this time, relatively quick to turn in but also very stable, i am just looking to fine tune and also the correct starting point for the front end length. i have a limited amount of adjustability to try.

Understood, my point is this, changing the fork leg length, like changing the shock length, changes the head angle. Don't get hooked up on fork leg length. it is the head angle (and offset, but let's assume you are not about to change that) that affects steering and stability.

For one fork leg length does not take into account any front or rear sag.

Set a target of head angle suitable for initial testing. Move up and down from there, recheck angle.

This gives you a more repeatable situation and I dare say you will understand much more than most of us about your bike when you have finished.

Actually though, it sounds like you already have a good enough start point, measure it and you will know just where that start point is.

acotrel · Oct 14, 2020

springer said:
Thank you all for the info. bike is running neutrally at this time, relatively quick to turn in but also very stable, i am just looking to fine tune and also the correct starting point for the front end length. i have a limited amount of adjustability to try.

If your bike is handling neutrally, that is good. If you practice braking up into the corners and getting on the gas earlier, you will find out whether it is biased towards running wide as you accelerate out of corners. If it ever does that - fix it, and life will become much easier for you. I think you will find that most of the after-market Seeley frames you can buy have neutral steering, but a bit of positive over-steer is better.
When you race, you are usually braking into corners when you are cranked over. If the bike tends to stand up or become difficult to tip into the corners, be careful because that is dangerous - too much yoke offset causes that and it can bite you.

Seeley Norton Done (or almost)

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