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Marque Remarks => Yamaha => Topic started by: Slow Poke on October 13, 2015, 02:20:58 am

Title: 1974 MX-250 swing arm question.
Post by: Slow Poke on October 13, 2015, 02:20:58 am: Does anyone know what, if any, Yamaha swing arm I can bolt onto my MX-250 to get an extra inch of length?
Slow Poke
Title: Re: 1974 MX-250 swing arm question.
Post by: djr on October 13, 2015, 08:50:24 am: If you can't find a longer arm you could always extend the flat plates at the wheel end an extra inch.
I have seen this modification on several TY trials Yamaha (mostly TY175)
I think I have some photos somewhere , sorry but I don't know how to post photos here
If you search " Yamaha TY 175 swing arm extensions" on ebay.co.uk you will see what I mean
Title: Re: 1974 MX-250 swing arm question.
Post by: Andrew L on October 13, 2015, 08:59:58 am: http://www.ebay.co.uk/itm/Yamaha-Ty-175-Swing-Arm-Extensions-yamaha-trials-twinshock-/130746878557 (http://www.ebay.co.uk/itm/Yamaha-Ty-175-Swing-Arm-Extensions-yamaha-trials-twinshock-/130746878557)

I made some like these for my rt360 made a big difference

http://www.trialscentral.com/forums/topic/34503-lengthening-ty175-swingarm/ (http://www.trialscentral.com/forums/topic/34503-lengthening-ty175-swingarm/)
Title: Re: 1974 MX-250 swing arm question.
Post by: djr on October 13, 2015, 09:09:34 am: Quote from: Andrew L on October 13, 2015, 08:59:58 am
http://www.ebay.co.uk/itm/Yamaha-Ty-175-Swing-Arm-Extensions-yamaha-trials-twinshock-/130746878557 (http://www.ebay.co.uk/itm/Yamaha-Ty-175-Swing-Arm-Extensions-yamaha-trials-twinshock-/130746878557)

I made some like these for my rt360 made a big difference

Yes ,That's the ones I was thinking of, there are also some TY250 ones that are slightly different
Shouldn't be too difficult to fabricate & easier than looking for another arm
Title: Re: 1974 MX-250 swing arm question.
Post by: Andrew L on October 13, 2015, 09:27:37 am: Added another link after you posted its a trials arm but it gives you an idea of what's involved
Title: Re: 1974 MX-250 swing arm question.
Post by: vmxrider on October 13, 2015, 10:03:48 am: What's the advantage of the extra length? I race an MX250 and don't really have a problem with the way it handles, dirt track or motocross.
Title: Re: 1974 MX-250 swing arm question.
Post by: Nathan S on October 13, 2015, 10:59:26 am: More weight on the front for better steering.
Less likely to wheelie.
Less twitchy in a slide.
Slightly better geometry across the full range of suspension travel (not so convinced that this is more a theoretical improvement than a real one).

Most old Yammies handle better with a longer swing arm. Yamaha had this stupid habit of making a new model with a short swingarm, and then extending it the next year (YZ125C vs X, D vs E, etc). In every case, the later model clearly out handled the older version.
Title: Re: 1974 MX-250 swing arm question.
Post by: Husk72 on October 13, 2015, 12:35:05 pm: With longer swingarm, will it still be eligible for pre '75 racing ?
I ask, 1: as I'm not familiar with the rules.
2: I have an MX that one day I will restore, and have wondered many times if I could lengthen said part.

Cheers, Mark.
Title: Re: 1974 MX-250 swing arm question.
Post by: KTM47 on October 13, 2015, 12:54:46 pm: You would need to ask MA re eligibility. Extending the swingarm also increases wheel travel. It would need to be checked and adjusted to suit.
Title: Re: 1974 MX-250 swing arm question.
Post by: LWC82PE on October 13, 2015, 01:03:02 pm: This works well

http://www.classicmx.se/fjadringsvag/motocycle_calc_uk.htm
Title: Re: 1974 MX-250 swing arm question.
Post by: Nathan S on October 13, 2015, 01:58:35 pm: It's legal providing the travel at the wheel is kept below 4".
Title: Re: 1974 MX-250 swing arm question.
Post by: facthunt on October 13, 2015, 10:05:28 pm: A DT2/3 swingarm is longer as is a YZA .
Title: Re: 1974 MX-250 swing arm question.
Post by: Slow Poke on October 14, 2015, 03:11:21 am: I can't imagine that 1" after the shock mounts would significantly alter the travel.
Title: Re: 1974 MX-250 swing arm question.
Post by: LWC82PE on October 14, 2015, 08:27:06 am: Use the calculator in the link i posted above and see for yourself. Travel can vary depending on where the axle is in the slot but how much variation depends on the shock angle/lever ratio. With almost vertical shocks the variation will be very little.
Title: Re: 1974 MX-250 swing arm question.
Post by: KTM47 on October 14, 2015, 12:06:26 pm: The thing I found interesting about the website which can calculate travel is it listed a 30% tolerance. The MoMS does not state a tolerance. 30% of 100mm is 130mm that is over 5 inches of travel.
Title: Re: 1974 MX-250 swing arm question.
Post by: LWC82PE on October 15, 2015, 12:41:40 pm: The tolerance is for what ever country that site is from. Its not relevant here. I just ignore that. The formula is good though. I checked it against my manual measurements from compressing the rear end and the result was exactly the same as just entering the data and letting the calculator do it. I normally play around with the measurement to the axle and the actual shock stroke to see what difference it makes in wheel travel. Sometimes just a few more extra mm shaft travel can have a dramatic increase in wheel travel. I normally include ½ to ¾ compression of the bump stop depending on material/design.
Title: Re: 1974 MX-250 swing arm question.
Post by: Husk72 on October 19, 2015, 03:28:32 pm: That's a cool calculator, LWC. No longer in the dark regarding lengthening a swingarm.
Thank you for posting it up.

Cheers, Mark.