Sectioned Posted November 4, 2020 Report Share Posted November 4, 2020 I'm going to reduce castor angle on my farm bike project, so...I entered current steering geometry specs into an online calculator (I'm lazy and its quicker) but the trail was a lot more than OEM spec when calculated. I fiddled around a bit and it seems the manufacturers figures are based on having no tyres on rims. This I understand would take out the tyre height variance of different products but seems its not a real world measurement. Are all motorcycles measured in this way for spec sheet? My bike is completely disassembled so can't check for sure now. I don't want to alter frame until I know I'm comparing apples to apples. Quote Link to comment Share on other sites More sharing options...
feetupfun Posted November 5, 2020 Report Share Posted November 5, 2020 Spec sheets are not a good source of geometry information because they don't specify if the bike is laden or unladen or if there is any static sag or what pressure is in the tyres or what tyres. You will get real numbers by reassembling the important bits and measuring it yourself. If you are trying to copy a proper trials bike, measure the proper trials bike yourself so you are comparing apples with apples. Be aware that modern front and rear trials tyres are a fair bit taller than front and rear trials tyres of the past and the heights of the axles are very much affected by pressure. Quote Link to comment Share on other sites More sharing options...
markparrish Posted November 5, 2020 Report Share Posted November 5, 2020 I'm just trying to get my head around all this - for a project I am planning. This thread on another site (I hope I'm allowed to post this - please let me know if not and I'll remove it) https://trials.com.au/forum/viewtopic.php?f=16&t=9348 explains the complex relationship between all the geometry you are playing with. It has helped me to realise that just changing one thing to match another bike won't necessarily make your bike better. Good luck - let us know what you decide and how it works. Quote Link to comment Share on other sites More sharing options...
Sectioned Posted November 6, 2020 Author Report Share Posted November 6, 2020 21 hours ago, feetupfun said: Spec sheets are not a good source of geometry information because they don't specify if the bike is laden or unladen or if there is any static sag or what pressure is in the tyres or what tyres. You will get real numbers by reassembling the important bits and measuring it yourself. If you are trying to copy a proper trials bike, measure the proper trials bike yourself so you are comparing apples with apples. Be aware that modern front and rear trials tyres are a fair bit taller than front and rear trials tyres of the past and the heights of the axles are very much affected by pressure. I agree about spec sheets, although if they are all created equal then it would give me peace of mind to cut 3degrees out of rake. Unfortunately I have thrown some parts away so can't reassemble. Was going to set back up from front geometry changes. I only know one guy with a trials bike so will try to measure his although he'll think I'm mad. I plan on using modern tyres and yes that further complicates things depending on psi. Quote Link to comment Share on other sites More sharing options...
Sectioned Posted November 6, 2020 Author Report Share Posted November 6, 2020 15 hours ago, markparrish said: I'm just trying to get my head around all this - for a project I am planning. This thread on another site (I hope I'm allowed to post this - please let me know if not and I'll remove it) https://trials.com.au/forum/viewtopic.php?f=16&t=9348 explains the complex relationship between all the geometry you are playing with. It has helped me to realise that just changing one thing to match another bike won't necessarily make your bike better. Good luck - let us know what you decide and how it works. What project are you planning? I have been to the thread "steer clear" a time or two and found it pretty helpful. Yes I expect there will be lots of tweaking and changing as I progress but anything is likely better than the current setup. My main problem may be that I'm 6'2 and carry most of my 105kg up high. I will keep you all up to date on progress. I should start a build thread I suppose. Next job...build frame jig. 1 Quote Link to comment Share on other sites More sharing options...
Sectioned Posted November 7, 2020 Author Report Share Posted November 7, 2020 Went round to a mate's today and took some measurements from his ty250 mono and they seemed close enough to specs. While we were talking I realised what I'd done wrong; the calculator called for fork offset but I'd forgotten to include leading axle offset (duh!) so measured and what happens? It all looks good! So I input steering stem rake angle to 23deg (from 26.2) and trail comes back to about 75mm (from 101mm) which seems like an ok place to start to me. For some reason the wheelbase didn't change on calculator but I expect it's quite minimal unfortunately as the total current measurement is 1363mm. Quote Link to comment Share on other sites More sharing options...
markparrish Posted November 7, 2020 Report Share Posted November 7, 2020 (edited) On 11/6/2020 at 5:23 AM, Sectioned said: What project are you planning? I have been to the thread "steer clear" a time or two and found it pretty helpful. Yes I expect there will be lots of tweaking and changing as I progress but anything is likely better than the current setup. My main problem may be that I'm 6'2 and carry most of my 105kg up high. I will keep you all up to date on progress. I should start a build thread I suppose. Next job...build frame jig. I've been making a full sized drawing in preparation for making a frame for my C15 - I am basing it on the "Otter" but experimenting with the bits I have and the bits I think I could make. It will be a "special" but, as it is, it wouldn't meet Scottish regulations so I'm not spoiling anything. I love making things I can use, so that's good enough for me! Good luck with yours. I'd love to see your frame jig. Edited November 7, 2020 by markparrish added last comment 1 Quote Link to comment Share on other sites More sharing options...
Sectioned Posted November 7, 2020 Author Report Share Posted November 7, 2020 3 hours ago, markparrish said: I've been making a full sized drawing in preparation for making a frame for my C15 - I am basing it on the "Otter" but experimenting with the bits I have and the bits I think I could make. It will be a "special" but, as it is, it wouldn't meet Scottish regulations so I'm not spoiling anything. I love making things I can use, so that's good enough for me! Good luck with yours. I'd love to see your frame jig. Sounds like a nice project, I'd love to find a lightweight brit bike to do this to. I have most of a b31 in pieces but the engine alone is super heavy. The frame jig I will make one day I downloaded plans from chopsource website. In the interim I will roughly cobble something together just to alter steering angle. It will only attract critisism so no pics will be taken Spoiler 1 Quote Link to comment Share on other sites More sharing options...
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